diff --git a/buffer.go b/buffer.go
index 67b95cf7..163a4aee 100644
--- a/buffer.go
+++ b/buffer.go
@@ -4,13 +4,14 @@ import (
 	"strings"
 
 	"github.com/elk-language/go-prompt/internal/debug"
+	istrings "github.com/elk-language/go-prompt/internal/strings"
 )
 
 // Buffer emulates the console buffer.
 type Buffer struct {
 	workingLines   []string // The working lines. Similar to history
-	workingIndex   int
-	cursorPosition int
+	workingIndex   int      // index of the current line
+	cursorPosition istrings.RuneIndex
 	cacheDocument  *Document
 	lastKeyStroke  Key
 }
@@ -36,44 +37,43 @@ func (b *Buffer) Document() (d *Document) {
 
 // DisplayCursorPosition returns the cursor position on rendered text on terminal emulators.
 // So if Document is "日本(cursor)語", DisplayedCursorPosition returns 4 because '日' and '本' are double width characters.
-func (b *Buffer) DisplayCursorPosition(columns int) Position {
+func (b *Buffer) DisplayCursorPosition(columns istrings.StringWidth) Position {
 	return b.Document().DisplayCursorPosition(columns)
 }
 
 // InsertText insert string from current line.
-func (b *Buffer) InsertText(v string, overwrite bool, moveCursor bool) {
-	or := []rune(b.Text())
-	oc := b.cursorPosition
+func (b *Buffer) InsertText(text string, overwrite bool, moveCursor bool) {
+	currentTextRunes := []rune(b.Text())
+	cursor := b.cursorPosition
 
 	if overwrite {
-		overwritten := string(or[oc:])
-		if len(overwritten) >= oc+len(v) {
-			overwritten = string(or[oc : oc+len(v)])
+		overwritten := string(currentTextRunes[cursor:])
+		if len(overwritten) >= int(cursor)+len(text) {
+			overwritten = string(currentTextRunes[cursor : cursor+istrings.RuneLen(text)])
 		}
-		if strings.Contains(overwritten, "\n") {
-			i := strings.IndexAny(overwritten, "\n")
+		if i := strings.IndexAny(overwritten, "\n"); i != -1 {
 			overwritten = overwritten[:i]
 		}
-		b.setText(string(or[:oc]) + v + string(or[oc+len(overwritten):]))
+		b.setText(string(currentTextRunes[:cursor]) + text + string(currentTextRunes[cursor+istrings.RuneLen(overwritten):]))
 	} else {
-		b.setText(string(or[:oc]) + v + string(or[oc:]))
+		b.setText(string(currentTextRunes[:cursor]) + text + string(currentTextRunes[cursor:]))
 	}
 
 	if moveCursor {
-		b.cursorPosition += len([]rune(v))
+		b.cursorPosition += istrings.RuneLen(text)
 	}
 }
 
 // SetText method to set text and update cursorPosition.
 // (When doing this, make sure that the cursor_position is valid for this text.
 // text/cursor_position should be consistent at any time, otherwise set a Document instead.)
-func (b *Buffer) setText(v string) {
-	debug.Assert(b.cursorPosition <= len([]rune(v)), "length of input should be shorter than cursor position")
-	b.workingLines[b.workingIndex] = v
+func (b *Buffer) setText(text string) {
+	debug.Assert(b.cursorPosition <= istrings.RuneLen(text), "length of input should be shorter than cursor position")
+	b.workingLines[b.workingIndex] = text
 }
 
 // Set cursor position. Return whether it changed.
-func (b *Buffer) setCursorPosition(p int) {
+func (b *Buffer) setCursorPosition(p istrings.RuneIndex) {
 	if p > 0 {
 		b.cursorPosition = p
 	} else {
@@ -88,13 +88,13 @@ func (b *Buffer) setDocument(d *Document) {
 }
 
 // CursorLeft move to left on the current line.
-func (b *Buffer) CursorLeft(count int) {
+func (b *Buffer) CursorLeft(count istrings.RuneCount) {
 	l := b.Document().GetCursorLeftPosition(count)
 	b.cursorPosition += l
 }
 
 // CursorRight move to right on the current line.
-func (b *Buffer) CursorRight(count int) {
+func (b *Buffer) CursorRight(count istrings.RuneCount) {
 	l := b.Document().GetCursorRightPosition(count)
 	b.cursorPosition += l
 }
@@ -114,7 +114,7 @@ func (b *Buffer) CursorDown(count int) {
 }
 
 // DeleteBeforeCursor delete specified number of characters before cursor and return the deleted text.
-func (b *Buffer) DeleteBeforeCursor(count int) (deleted string) {
+func (b *Buffer) DeleteBeforeCursor(count istrings.RuneCount) (deleted string) {
 	debug.Assert(count >= 0, "count should be positive")
 	r := []rune(b.Text())
 
@@ -126,7 +126,7 @@ func (b *Buffer) DeleteBeforeCursor(count int) (deleted string) {
 		deleted = string(r[start:b.cursorPosition])
 		b.setDocument(&Document{
 			Text:           string(r[:start]) + string(r[b.cursorPosition:]),
-			cursorPosition: b.cursorPosition - len([]rune(deleted)),
+			cursorPosition: b.cursorPosition - istrings.RuneIndex(len([]rune(deleted))),
 		})
 	}
 	return
@@ -142,13 +142,19 @@ func (b *Buffer) NewLine(copyMargin bool) {
 }
 
 // Delete specified number of characters and Return the deleted text.
-func (b *Buffer) Delete(count int) (deleted string) {
+func (b *Buffer) Delete(count istrings.RuneCount) string {
 	r := []rune(b.Text())
-	if b.cursorPosition < len(r) {
-		deleted = b.Document().TextAfterCursor()[:count]
-		b.setText(string(r[:b.cursorPosition]) + string(r[b.cursorPosition+len(deleted):]))
+	if b.cursorPosition < istrings.RuneIndex(len(r)) {
+		textAfterCursor := b.Document().TextAfterCursor()
+		textAfterCursorRunes := []rune(textAfterCursor)
+		deletedRunes := textAfterCursorRunes[:count]
+		b.setText(string(r[:b.cursorPosition]) + string(r[b.cursorPosition+istrings.RuneCount(len(deletedRunes)):]))
+
+		deleted := string(deletedRunes)
+		return deleted
 	}
-	return
+
+	return ""
 }
 
 // JoinNextLine joins the next line to the current one by deleting the line ending after the current line.
diff --git a/buffer_test.go b/buffer_test.go
index e6859f13..86878749 100644
--- a/buffer_test.go
+++ b/buffer_test.go
@@ -3,6 +3,8 @@ package prompt
 import (
 	"reflect"
 	"testing"
+
+	istrings "github.com/elk-language/go-prompt/internal/strings"
 )
 
 func TestNewBuffer(t *testing.T) {
@@ -23,8 +25,8 @@ func TestBuffer_InsertText(t *testing.T) {
 		t.Errorf("Text should be %#v, got %#v", "some_text", b.Text())
 	}
 
-	if b.cursorPosition != len("some_text") {
-		t.Errorf("cursorPosition should be %#v, got %#v", len("some_text"), b.cursorPosition)
+	if b.cursorPosition != istrings.RuneLen("some_text") {
+		t.Errorf("cursorPosition should be %#v, got %#v", istrings.RuneLen("some_text"), b.cursorPosition)
 	}
 }
 
@@ -36,8 +38,8 @@ func TestBuffer_InsertText_Overwrite(t *testing.T) {
 		t.Errorf("Text should be %#v, got %#v", "ABC", b.Text())
 	}
 
-	if b.cursorPosition != len("ABC") {
-		t.Errorf("cursorPosition should be %#v, got %#v", len("ABC"), b.cursorPosition)
+	if b.cursorPosition != istrings.RuneLen("ABC") {
+		t.Errorf("cursorPosition should be %#v, got %#v", istrings.RuneLen("ABC"), b.cursorPosition)
 	}
 
 	b.CursorLeft(1)
@@ -85,8 +87,8 @@ func TestBuffer_CursorMovement(t *testing.T) {
 	if b.Text() != "some_teAxt" {
 		t.Errorf("Text should be %#v, got %#v", "some_teAxt", b.Text())
 	}
-	if b.cursorPosition != len("some_teA") {
-		t.Errorf("Text should be %#v, got %#v", len("some_teA"), b.cursorPosition)
+	if b.cursorPosition != istrings.RuneLen("some_teA") {
+		t.Errorf("Text should be %#v, got %#v", istrings.RuneLen("some_teA"), b.cursorPosition)
 	}
 
 	// Moving over left character counts.
@@ -95,8 +97,8 @@ func TestBuffer_CursorMovement(t *testing.T) {
 	if b.Text() != "Asome_teAxt" {
 		t.Errorf("Text should be %#v, got %#v", "some_teAxt", b.Text())
 	}
-	if b.cursorPosition != len("A") {
-		t.Errorf("Text should be %#v, got %#v", len("some_teA"), b.cursorPosition)
+	if b.cursorPosition != istrings.RuneLen("A") {
+		t.Errorf("Text should be %#v, got %#v", istrings.RuneLen("some_teA"), b.cursorPosition)
 	}
 
 	// TODO: Going right already at right end.
@@ -109,6 +111,10 @@ func TestBuffer_CursorMovement_WithMultiByte(t *testing.T) {
 	if l := b.Document().TextAfterCursor(); l != "お" {
 		t.Errorf("Should be 'お', but got %s", l)
 	}
+	b.InsertText("żółć", true, true)
+	if b.Text() != "あいうえżółć" {
+		t.Errorf("Text should be %#v, got %#v", "あいうえżółć", b.Text())
+	}
 }
 
 func TestBuffer_CursorUp(t *testing.T) {
@@ -141,8 +147,8 @@ func TestBuffer_CursorDown(t *testing.T) {
 
 	// Normally going down
 	b.CursorDown(1)
-	if b.Document().cursorPosition != len("line1\nlin") {
-		t.Errorf("Should be %#v, got %#v", len("line1\nlin"), b.Document().cursorPosition)
+	if b.Document().cursorPosition != istrings.RuneLen("line1\nlin") {
+		t.Errorf("Should be %#v, got %#v", istrings.RuneLen("line1\nlin"), b.Document().cursorPosition)
 	}
 
 	// Going down to a line that's storter.
@@ -150,8 +156,8 @@ func TestBuffer_CursorDown(t *testing.T) {
 	b.InsertText("long line1\na\nb", false, true)
 	b.cursorPosition = 3
 	b.CursorDown(1)
-	if b.Document().cursorPosition != len("long line1\na") {
-		t.Errorf("Should be %#v, got %#v", len("long line1\na"), b.Document().cursorPosition)
+	if b.Document().cursorPosition != istrings.RuneLen("long line1\na") {
+		t.Errorf("Should be %#v, got %#v", istrings.RuneLen("long line1\na"), b.Document().cursorPosition)
 	}
 }
 
@@ -167,8 +173,8 @@ func TestBuffer_DeleteBeforeCursor(t *testing.T) {
 	if deleted != "e" {
 		t.Errorf("Should be %#v, got %#v", deleted, "e")
 	}
-	if b.cursorPosition != len("some_t") {
-		t.Errorf("Should be %#v, got %#v", len("some_t"), b.cursorPosition)
+	if b.cursorPosition != istrings.RuneLen("some_t") {
+		t.Errorf("Should be %#v, got %#v", istrings.RuneLen("some_t"), b.cursorPosition)
 	}
 
 	// Delete over the characters length before cursor.
diff --git a/completion.go b/completion.go
index fd0d01b8..5589689a 100644
--- a/completion.go
+++ b/completion.go
@@ -4,6 +4,7 @@ import (
 	"strings"
 
 	"github.com/elk-language/go-prompt/internal/debug"
+	istrings "github.com/elk-language/go-prompt/internal/strings"
 	runewidth "github.com/mattn/go-runewidth"
 )
 
@@ -155,7 +156,7 @@ func formatTexts(o []string, max int, prefix, suffix string) (new []string, widt
 	return n, lenPrefix + width + lenSuffix
 }
 
-func formatSuggestions(suggests []Suggest, max int) (new []Suggest, width int) {
+func formatSuggestions(suggests []Suggest, max int) (new []Suggest, width istrings.StringWidth) {
 	num := len(suggests)
 	new = make([]Suggest, num)
 
@@ -177,7 +178,7 @@ func formatSuggestions(suggests []Suggest, max int) (new []Suggest, width int) {
 	for i := 0; i < num; i++ {
 		new[i] = Suggest{Text: left[i], Description: right[i]}
 	}
-	return new, leftWidth + rightWidth
+	return new, istrings.StringWidth(leftWidth + rightWidth)
 }
 
 // Constructor option for CompletionManager.
diff --git a/completion_test.go b/completion_test.go
index c17c1d91..6746fafe 100644
--- a/completion_test.go
+++ b/completion_test.go
@@ -3,6 +3,8 @@ package prompt
 import (
 	"reflect"
 	"testing"
+
+	istrings "github.com/elk-language/go-prompt/internal/strings"
 )
 
 func TestFormatShortSuggestion(t *testing.T) {
@@ -10,7 +12,7 @@ func TestFormatShortSuggestion(t *testing.T) {
 		in       []Suggest
 		expected []Suggest
 		max      int
-		exWidth  int
+		exWidth  istrings.StringWidth
 	}{
 		{
 			in: []Suggest{
@@ -38,7 +40,7 @@ func TestFormatShortSuggestion(t *testing.T) {
 				{Text: " coconut ", Description: " This is coconut. "},
 			},
 			max:     100,
-			exWidth: len(" apple   " + " This is apple.   "),
+			exWidth: istrings.StringWidth(len(" apple   " + " This is apple.   ")),
 		},
 		{
 			in: []Suggest{
@@ -82,7 +84,7 @@ func TestFormatShortSuggestion(t *testing.T) {
 				{Text: " --include-extended-apis       ", Description: " --------------... "},
 			},
 			max:     50,
-			exWidth: len(" --include-extended-apis       " + " ---------------..."),
+			exWidth: istrings.StringWidth(len(" --include-extended-apis       " + " ---------------...")),
 		},
 		{
 			in: []Suggest{
@@ -102,7 +104,7 @@ func TestFormatShortSuggestion(t *testing.T) {
 				{Text: " --include-extended-apis       ", Description: " If true, include definitions of new APIs via calls to the API server. [default true]                                                            "},
 			},
 			max:     500,
-			exWidth: len(" --include-extended-apis       " + " If true, include definitions of new APIs via calls to the API server. [default true]                                                            "),
+			exWidth: istrings.StringWidth(len(" --include-extended-apis       " + " If true, include definitions of new APIs via calls to the API server. [default true]                                                            ")),
 		},
 	}
 
diff --git a/document.go b/document.go
index 84ae47fa..e01b8675 100644
--- a/document.go
+++ b/document.go
@@ -16,7 +16,7 @@ type Document struct {
 	// This represents a index in a rune array of Document.Text.
 	// So if Document is "日本(cursor)語", cursorPosition is 2.
 	// But DisplayedCursorPosition returns 4 because '日' and '本' are double width characters.
-	cursorPosition int
+	cursorPosition istrings.RuneIndex
 	lastKey        Key
 }
 
@@ -35,20 +35,20 @@ func (d *Document) LastKeyStroke() Key {
 
 // DisplayCursorPosition returns the cursor position on rendered text on terminal emulators.
 // So if Document is "日本(cursor)語", DisplayedCursorPosition returns 4 because '日' and '本' are double width characters.
-func (d *Document) DisplayCursorPosition(columns int) Position {
-	str := utf8string.NewString(d.Text).Slice(0, d.cursorPosition)
+func (d *Document) DisplayCursorPosition(columns istrings.StringWidth) Position {
+	str := utf8string.NewString(d.Text).Slice(0, int(d.cursorPosition))
 	return positionAtEndOfString(str, columns)
 }
 
 // GetCharRelativeToCursor return character relative to cursor position, or empty string
-func (d *Document) GetCharRelativeToCursor(offset int) (r rune) {
+func (d *Document) GetCharRelativeToCursor(offset istrings.RuneCount) (r rune) {
 	s := d.Text
-	cnt := 0
+	var cnt istrings.RuneIndex
 
 	for len(s) > 0 {
 		cnt++
 		r, size := utf8.DecodeRuneInString(s)
-		if cnt == d.cursorPosition+offset {
+		if cnt == d.cursorPosition+istrings.RuneIndex(offset) {
 			return r
 		}
 		s = s[size:]
@@ -124,9 +124,9 @@ func (d *Document) GetWordAfterCursorUntilSeparatorIgnoreNextToCursor(sep string
 
 // FindStartOfPreviousWord returns an index relative to the cursor position
 // pointing to the start of the previous word. Return 0 if nothing was found.
-func (d *Document) FindStartOfPreviousWord() int {
+func (d *Document) FindStartOfPreviousWord() istrings.ByteIndex {
 	x := d.TextBeforeCursor()
-	i := strings.LastIndexAny(x, " \n")
+	i := istrings.ByteIndex(strings.LastIndexAny(x, " \n"))
 	if i != -1 {
 		return i + 1
 	}
@@ -142,14 +142,14 @@ func (d *Document) FindStringWidthUntilStartOfPreviousWord() int {
 
 // FindStartOfPreviousWordWithSpace is almost the same as FindStartOfPreviousWord.
 // The only difference is to ignore contiguous spaces.
-func (d *Document) FindStartOfPreviousWordWithSpace() int {
+func (d *Document) FindStartOfPreviousWordWithSpace() istrings.ByteIndex {
 	x := d.TextBeforeCursor()
 	end := istrings.LastIndexNotByte(x, ' ')
 	if end == -1 {
 		return 0
 	}
 
-	start := strings.LastIndexByte(x[:end], ' ')
+	start := istrings.ByteIndex(strings.LastIndexByte(x[:end], ' '))
 	if start == -1 {
 		return 0
 	}
@@ -158,13 +158,13 @@ func (d *Document) FindStartOfPreviousWordWithSpace() int {
 
 // FindStartOfPreviousWordUntilSeparator is almost the same as FindStartOfPreviousWord.
 // But this can specify Separator. Return 0 if nothing was found.
-func (d *Document) FindStartOfPreviousWordUntilSeparator(sep string) int {
+func (d *Document) FindStartOfPreviousWordUntilSeparator(sep string) istrings.ByteIndex {
 	if sep == "" {
 		return d.FindStartOfPreviousWord()
 	}
 
 	x := d.TextBeforeCursor()
-	i := strings.LastIndexAny(x, sep)
+	i := istrings.ByteIndex(strings.LastIndexAny(x, sep))
 	if i != -1 {
 		return i + 1
 	}
@@ -173,7 +173,7 @@ func (d *Document) FindStartOfPreviousWordUntilSeparator(sep string) int {
 
 // FindStartOfPreviousWordUntilSeparatorIgnoreNextToCursor is almost the same as FindStartOfPreviousWordWithSpace.
 // But this can specify Separator. Return 0 if nothing was found.
-func (d *Document) FindStartOfPreviousWordUntilSeparatorIgnoreNextToCursor(sep string) int {
+func (d *Document) FindStartOfPreviousWordUntilSeparatorIgnoreNextToCursor(sep string) istrings.ByteIndex {
 	if sep == "" {
 		return d.FindStartOfPreviousWordWithSpace()
 	}
@@ -183,37 +183,37 @@ func (d *Document) FindStartOfPreviousWordUntilSeparatorIgnoreNextToCursor(sep s
 	if end == -1 {
 		return 0
 	}
-	start := strings.LastIndexAny(x[:end], sep)
+	start := istrings.ByteIndex(strings.LastIndexAny(x[:end], sep))
 	if start == -1 {
 		return 0
 	}
 	return start + 1
 }
 
-// FindEndOfCurrentWord returns an index relative to the cursor position.
+// FindEndOfCurrentWord returns a byte index relative to the cursor position.
 // pointing to the end of the current word. Return 0 if nothing was found.
-func (d *Document) FindEndOfCurrentWord() int {
+func (d *Document) FindEndOfCurrentWord() istrings.ByteIndex {
 	x := d.TextAfterCursor()
-	i := strings.IndexByte(x, ' ')
+	i := istrings.ByteIndex(strings.IndexByte(x, ' '))
 	if i != -1 {
 		return i
 	}
-	return len(x)
+	return istrings.ByteIndex(len(x))
 }
 
 // FindEndOfCurrentWordWithSpace is almost the same as FindEndOfCurrentWord.
 // The only difference is to ignore contiguous spaces.
-func (d *Document) FindEndOfCurrentWordWithSpace() int {
+func (d *Document) FindEndOfCurrentWordWithSpace() istrings.ByteIndex {
 	x := d.TextAfterCursor()
 
 	start := istrings.IndexNotByte(x, ' ')
 	if start == -1 {
-		return len(x)
+		return istrings.ByteIndex(len(x))
 	}
 
-	end := strings.IndexByte(x[start:], ' ')
+	end := istrings.ByteIndex(strings.IndexByte(x[start:], ' '))
 	if end == -1 {
-		return len(x)
+		return istrings.ByteIndex(len(x))
 	}
 
 	return start + end
@@ -221,9 +221,9 @@ func (d *Document) FindEndOfCurrentWordWithSpace() int {
 
 // Returns the string width (as visible in the terminal)
 // of the text after the cursor until the end of the current word.
-func (d *Document) FindStringWidthUntilEndOfCurrentWord() int {
+func (d *Document) FindStringWidthUntilEndOfCurrentWord() istrings.StringWidth {
 	t := d.TextAfterCursor()
-	width := 0
+	var width istrings.StringWidth
 	nonSpaceCharSeen := false
 	for _, char := range t {
 		if !nonSpaceCharSeen && char == ' ' {
@@ -236,7 +236,7 @@ func (d *Document) FindStringWidthUntilEndOfCurrentWord() int {
 		}
 
 		nonSpaceCharSeen = true
-		width += runewidth.RuneWidth(char)
+		width += istrings.StringWidth(runewidth.RuneWidth(char))
 	}
 
 	return width
@@ -244,22 +244,22 @@ func (d *Document) FindStringWidthUntilEndOfCurrentWord() int {
 
 // FindEndOfCurrentWordUntilSeparator is almost the same as FindEndOfCurrentWord.
 // But this can specify Separator. Return 0 if nothing was found.
-func (d *Document) FindEndOfCurrentWordUntilSeparator(sep string) int {
+func (d *Document) FindEndOfCurrentWordUntilSeparator(sep string) istrings.ByteIndex {
 	if sep == "" {
 		return d.FindEndOfCurrentWord()
 	}
 
 	x := d.TextAfterCursor()
-	i := strings.IndexAny(x, sep)
+	i := istrings.ByteIndex(strings.IndexAny(x, sep))
 	if i != -1 {
 		return i
 	}
-	return len(x)
+	return istrings.ByteIndex(len(x))
 }
 
 // FindEndOfCurrentWordUntilSeparatorIgnoreNextToCursor is almost the same as FindEndOfCurrentWordWithSpace.
 // But this can specify Separator. Return 0 if nothing was found.
-func (d *Document) FindEndOfCurrentWordUntilSeparatorIgnoreNextToCursor(sep string) int {
+func (d *Document) FindEndOfCurrentWordUntilSeparatorIgnoreNextToCursor(sep string) istrings.ByteIndex {
 	if sep == "" {
 		return d.FindEndOfCurrentWordWithSpace()
 	}
@@ -268,12 +268,12 @@ func (d *Document) FindEndOfCurrentWordUntilSeparatorIgnoreNextToCursor(sep stri
 
 	start := istrings.IndexNotAny(x, sep)
 	if start == -1 {
-		return len(x)
+		return istrings.ByteIndex(len(x))
 	}
 
-	end := strings.IndexAny(x[start:], sep)
+	end := istrings.ByteIndex(strings.IndexAny(x[start:], sep))
 	if end == -1 {
-		return len(x)
+		return istrings.ByteIndex(len(x))
 	}
 
 	return start + end
@@ -297,23 +297,23 @@ func (d *Document) CurrentLine() string {
 }
 
 // Array pointing to the start indices of all the lines.
-func (d *Document) lineStartIndexes() []int {
+func (d *Document) lineStartIndices() []istrings.RuneIndex {
 	// TODO: Cache, because this is often reused.
 	// (If it is used, it's often used many times.
 	// And this has to be fast for editing big documents!)
 	lc := d.LineCount()
-	lengths := make([]int, lc)
+	lengths := make([]istrings.RuneCount, lc)
 	for i, l := range d.Lines() {
-		lengths[i] = len([]rune(l))
+		lengths[i] = istrings.RuneCount(len([]rune(l)))
 	}
 
 	// Calculate cumulative sums.
-	indices := make([]int, lc+1)
+	indices := make([]istrings.RuneIndex, lc+1)
 	indices[0] = 0 // https://github.com/jonathanslenders/python-prompt-toolkit/blob/master/prompt_toolkit/document.py#L189
-	pos := 0
+	var pos istrings.RuneCount
 	for i, l := range lengths {
 		pos += l + 1
-		indices[i+1] = pos
+		indices[i+1] = istrings.RuneIndex(pos)
 	}
 	if lc > 1 {
 		// Pop the last item. (This is not a new line.)
@@ -324,33 +324,33 @@ func (d *Document) lineStartIndexes() []int {
 
 // For the index of a character at a certain line, calculate the index of
 // the first character on that line.
-func (d *Document) findLineStartIndex(index int) (pos int, lineStartIndex int) {
-	indices := d.lineStartIndexes()
+func (d *Document) findLineStartIndex(index istrings.RuneIndex) (pos, lineStartIndex istrings.RuneIndex) {
+	indices := d.lineStartIndices()
 	pos = bisect.Right(indices, index) - 1
 	lineStartIndex = indices[pos]
 	return
 }
 
 // CursorPositionRow returns the current row. (0-based.)
-func (d *Document) CursorPositionRow() (row int) {
+func (d *Document) CursorPositionRow() (row istrings.RuneIndex) {
 	row, _ = d.findLineStartIndex(d.cursorPosition)
 	return
 }
 
 // CursorPositionCol returns the current column. (0-based.)
-func (d *Document) CursorPositionCol() (col int) {
+func (d *Document) CursorPositionCol() (col istrings.RuneIndex) {
 	_, index := d.findLineStartIndex(d.cursorPosition)
 	col = d.cursorPosition - index
 	return
 }
 
 // GetCursorLeftPosition returns the relative position for cursor left.
-func (d *Document) GetCursorLeftPosition(count int) int {
+func (d *Document) GetCursorLeftPosition(count istrings.RuneCount) istrings.RuneCount {
 	if count < 0 {
 		return d.GetCursorRightPosition(-count)
 	}
 	runeSlice := []rune(d.Text)
-	counter := 0
+	var counter istrings.RuneCount
 	targetPosition := d.cursorPosition - count
 	if targetPosition < 0 {
 		targetPosition = 0
@@ -363,15 +363,15 @@ func (d *Document) GetCursorLeftPosition(count int) int {
 }
 
 // GetCursorRightPosition returns relative position for cursor right.
-func (d *Document) GetCursorRightPosition(count int) int {
+func (d *Document) GetCursorRightPosition(count istrings.RuneCount) istrings.RuneCount {
 	if count < 0 {
 		return d.GetCursorLeftPosition(-count)
 	}
 	runeSlice := []rune(d.Text)
-	counter := 0
+	var counter istrings.RuneCount
 	targetPosition := d.cursorPosition + count
-	if targetPosition > len(runeSlice) {
-		targetPosition = len(runeSlice)
+	if targetPosition > istrings.RuneCount(len(runeSlice)) {
+		targetPosition = istrings.RuneCount(len(runeSlice))
 	}
 	for range runeSlice[d.cursorPosition:targetPosition] {
 		counter++
@@ -381,26 +381,26 @@ func (d *Document) GetCursorRightPosition(count int) int {
 }
 
 // Get the current cursor position.
-func (d *Document) GetCursorPosition(columns int) Position {
+func (d *Document) GetCursorPosition(columns istrings.StringWidth) Position {
 	return positionAtEndOfString(d.TextBeforeCursor(), columns)
 }
 
 // Get the position of the end of the current text.
-func (d *Document) GetEndOfTextPosition(columns int) Position {
+func (d *Document) GetEndOfTextPosition(columns istrings.StringWidth) Position {
 	return positionAtEndOfString(d.Text, columns)
 }
 
 // GetCursorUpPosition return the relative cursor position (character index) where we would be
 // if the user pressed the arrow-up button.
-func (d *Document) GetCursorUpPosition(count int, preferredColumn int) int {
-	var col int
+func (d *Document) GetCursorUpPosition(count int, preferredColumn istrings.RuneIndex) istrings.RuneIndex {
+	var col istrings.RuneIndex
 	if preferredColumn == -1 { // -1 means nil
 		col = d.CursorPositionCol()
 	} else {
 		col = preferredColumn
 	}
 
-	row := d.CursorPositionRow() - count
+	row := int(d.CursorPositionRow()) - count
 	if row < 0 {
 		row = 0
 	}
@@ -409,14 +409,14 @@ func (d *Document) GetCursorUpPosition(count int, preferredColumn int) int {
 
 // GetCursorDownPosition return the relative cursor position (character index) where we would be if the
 // user pressed the arrow-down button.
-func (d *Document) GetCursorDownPosition(count int, preferredColumn int) int {
-	var col int
+func (d *Document) GetCursorDownPosition(count int, preferredColumn istrings.RuneIndex) istrings.RuneIndex {
+	var col istrings.RuneIndex
 	if preferredColumn == -1 { // -1 means nil
 		col = d.CursorPositionCol()
 	} else {
 		col = preferredColumn
 	}
-	row := d.CursorPositionRow() + count
+	row := int(d.CursorPositionRow()) + count
 	return d.TranslateRowColToIndex(row, col) - d.cursorPosition
 }
 
@@ -434,16 +434,16 @@ func (d *Document) LineCount() int {
 
 // TranslateIndexToPosition given an index for the text, return the corresponding (row, col) tuple.
 // (0-based. Returns (0, 0) for index=0.)
-func (d *Document) TranslateIndexToPosition(index int) (row int, col int) {
-	row, rowIndex := d.findLineStartIndex(index)
-	col = index - rowIndex
-	return
+func (d *Document) TranslateIndexToPosition(index istrings.RuneIndex) (int, int) {
+	r, rowIndex := d.findLineStartIndex(index)
+	c := index - rowIndex
+	return int(r), int(c)
 }
 
 // TranslateRowColToIndex given a (row, col), return the corresponding index.
 // (Row and col params are 0-based.)
-func (d *Document) TranslateRowColToIndex(row int, column int) (index int) {
-	indices := d.lineStartIndexes()
+func (d *Document) TranslateRowColToIndex(row int, column istrings.RuneIndex) (index istrings.RuneIndex) {
+	indices := d.lineStartIndices()
 	if row < 0 {
 		row = 0
 	} else if row > len(indices) {
@@ -454,10 +454,10 @@ func (d *Document) TranslateRowColToIndex(row int, column int) (index int) {
 
 	// python) result += max(0, min(col, len(line)))
 	if column > 0 || len(line) > 0 {
-		if column > len(line) {
-			index += len(line)
+		if column > istrings.RuneIndex(len(line)) {
+			index += istrings.RuneCount(len(line))
 		} else {
-			index += column
+			index += istrings.RuneIndex(column)
 		}
 	}
 
@@ -465,8 +465,8 @@ func (d *Document) TranslateRowColToIndex(row int, column int) (index int) {
 	// Keep in range. (len(self.text) is included, because the cursor can be
 	// right after the end of the text as well.)
 	// python) result = max(0, min(result, len(self.text)))
-	if index > len(text) {
-		index = len(text)
+	if index > istrings.RuneCount(len(text)) {
+		index = istrings.RuneIndex(len(text))
 	}
 	if index < 0 {
 		index = 0
@@ -476,12 +476,12 @@ func (d *Document) TranslateRowColToIndex(row int, column int) (index int) {
 
 // OnLastLine returns true when we are at the last line.
 func (d *Document) OnLastLine() bool {
-	return d.CursorPositionRow() == (d.LineCount() - 1)
+	return d.CursorPositionRow() == istrings.RuneIndex(d.LineCount()-1)
 }
 
 // GetEndOfLinePosition returns relative position for the end of this line.
-func (d *Document) GetEndOfLinePosition() int {
-	return len([]rune(d.CurrentLineAfterCursor()))
+func (d *Document) GetEndOfLinePosition() istrings.RuneIndex {
+	return istrings.RuneIndex(len([]rune(d.CurrentLineAfterCursor())))
 }
 
 func (d *Document) leadingWhitespaceInCurrentLine() (margin string) {
diff --git a/document_test.go b/document_test.go
index b14601fa..dbe9d59d 100644
--- a/document_test.go
+++ b/document_test.go
@@ -5,6 +5,8 @@ import (
 	"reflect"
 	"testing"
 	"unicode/utf8"
+
+	istrings "github.com/elk-language/go-prompt/internal/strings"
 )
 
 func ExampleDocument_CurrentLine() {
@@ -13,7 +15,7 @@ func ExampleDocument_CurrentLine() {
 This is a example of Document component.
 This component has texts displayed in terminal and cursor position.
 `,
-		cursorPosition: len(`Hello! my name is c-bata.
+		cursorPosition: istrings.RuneLen(`Hello! my name is c-bata.
 This is a exam`),
 	}
 	fmt.Println(d.CurrentLine())
@@ -24,7 +26,7 @@ This is a exam`),
 func ExampleDocument_DisplayCursorPosition() {
 	d := &Document{
 		Text:           `Hello! my name is c-bata.`,
-		cursorPosition: len(`Hello`),
+		cursorPosition: istrings.RuneLen(`Hello`),
 	}
 	fmt.Println("DisplayCursorPosition", d.DisplayCursorPosition(50))
 	// Output:
@@ -37,7 +39,7 @@ func ExampleDocument_CursorPositionRow() {
 This is a example of Document component.
 This component has texts displayed in terminal and cursor position.
 `,
-		cursorPosition: len(`Hello! my name is c-bata.
+		cursorPosition: istrings.RuneLen(`Hello! my name is c-bata.
 This is a exam`),
 	}
 	fmt.Println("CursorPositionRow", d.CursorPositionRow())
@@ -51,7 +53,7 @@ func ExampleDocument_CursorPositionCol() {
 This is a example of Document component.
 This component has texts displayed in terminal and cursor position.
 `,
-		cursorPosition: len(`Hello! my name is c-bata.
+		cursorPosition: istrings.RuneLen(`Hello! my name is c-bata.
 This is a exam`),
 	}
 	fmt.Println("CursorPositionCol", d.CursorPositionCol())
@@ -65,7 +67,7 @@ func ExampleDocument_TextBeforeCursor() {
 This is a example of Document component.
 This component has texts displayed in terminal and cursor position.
 `,
-		cursorPosition: len(`Hello! my name is c-bata.
+		cursorPosition: istrings.RuneLen(`Hello! my name is c-bata.
 This is a exam`),
 	}
 	fmt.Println(d.TextBeforeCursor())
@@ -80,7 +82,7 @@ func ExampleDocument_TextAfterCursor() {
 This is a example of Document component.
 This component has texts displayed in terminal and cursor position.
 `,
-		cursorPosition: len(`Hello! my name is c-bata.
+		cursorPosition: istrings.RuneLen(`Hello! my name is c-bata.
 This is a exam`),
 	}
 	fmt.Println(d.TextAfterCursor())
@@ -105,7 +107,7 @@ func ExampleDocument_CurrentLineBeforeCursor() {
 This is a example of Document component.
 This component has texts displayed in terminal and cursor position.
 `,
-		cursorPosition: len(`Hello! my name is c-bata.
+		cursorPosition: istrings.RuneLen(`Hello! my name is c-bata.
 This is a exam`),
 	}
 	fmt.Println(d.CurrentLineBeforeCursor())
@@ -119,7 +121,7 @@ func ExampleDocument_CurrentLineAfterCursor() {
 This is a example of Document component.
 This component has texts displayed in terminal and cursor position.
 `,
-		cursorPosition: len(`Hello! my name is c-bata.
+		cursorPosition: istrings.RuneLen(`Hello! my name is c-bata.
 This is a exam`),
 	}
 	fmt.Println(d.CurrentLineAfterCursor())
@@ -132,7 +134,7 @@ func ExampleDocument_GetWordBeforeCursor() {
 		Text: `Hello! my name is c-bata.
 This is a example of Document component.
 `,
-		cursorPosition: len(`Hello! my name is c-bata.
+		cursorPosition: istrings.RuneLen(`Hello! my name is c-bata.
 This is a exam`),
 	}
 	fmt.Println(d.GetWordBeforeCursor())
@@ -145,7 +147,7 @@ func ExampleDocument_GetWordAfterCursor() {
 		Text: `Hello! my name is c-bata.
 This is a example of Document component.
 `,
-		cursorPosition: len(`Hello! my name is c-bata.
+		cursorPosition: istrings.RuneLen(`Hello! my name is c-bata.
 This is a exam`),
 	}
 	fmt.Println(d.GetWordAfterCursor())
@@ -158,7 +160,7 @@ func ExampleDocument_GetWordBeforeCursorWithSpace() {
 		Text: `Hello! my name is c-bata.
 This is a example of Document component.
 `,
-		cursorPosition: len(`Hello! my name is c-bata.
+		cursorPosition: istrings.RuneLen(`Hello! my name is c-bata.
 This is a example `),
 	}
 	fmt.Println(d.GetWordBeforeCursorWithSpace())
@@ -171,7 +173,7 @@ func ExampleDocument_GetWordAfterCursorWithSpace() {
 		Text: `Hello! my name is c-bata.
 This is a example of Document component.
 `,
-		cursorPosition: len(`Hello! my name is c-bata.
+		cursorPosition: istrings.RuneLen(`Hello! my name is c-bata.
 This is a`),
 	}
 	fmt.Println(d.GetWordAfterCursorWithSpace())
@@ -182,7 +184,7 @@ This is a`),
 func ExampleDocument_GetWordBeforeCursorUntilSeparator() {
 	d := &Document{
 		Text:           `hello,i am c-bata`,
-		cursorPosition: len(`hello,i am c`),
+		cursorPosition: istrings.RuneLen(`hello,i am c`),
 	}
 	fmt.Println(d.GetWordBeforeCursorUntilSeparator(","))
 	// Output:
@@ -192,7 +194,7 @@ func ExampleDocument_GetWordBeforeCursorUntilSeparator() {
 func ExampleDocument_GetWordAfterCursorUntilSeparator() {
 	d := &Document{
 		Text:           `hello,i am c-bata,thank you for using go-prompt`,
-		cursorPosition: len(`hello,i a`),
+		cursorPosition: istrings.RuneLen(`hello,i a`),
 	}
 	fmt.Println(d.GetWordAfterCursorUntilSeparator(","))
 	// Output:
@@ -202,7 +204,7 @@ func ExampleDocument_GetWordAfterCursorUntilSeparator() {
 func ExampleDocument_GetWordBeforeCursorUntilSeparatorIgnoreNextToCursor() {
 	d := &Document{
 		Text:           `hello,i am c-bata,thank you for using go-prompt`,
-		cursorPosition: len(`hello,i am c-bata,`),
+		cursorPosition: istrings.RuneLen(`hello,i am c-bata,`),
 	}
 	fmt.Println(d.GetWordBeforeCursorUntilSeparatorIgnoreNextToCursor(","))
 	// Output:
@@ -212,7 +214,7 @@ func ExampleDocument_GetWordBeforeCursorUntilSeparatorIgnoreNextToCursor() {
 func ExampleDocument_GetWordAfterCursorUntilSeparatorIgnoreNextToCursor() {
 	d := &Document{
 		Text:           `hello,i am c-bata,thank you for using go-prompt`,
-		cursorPosition: len(`hello`),
+		cursorPosition: istrings.RuneLen(`hello`),
 	}
 	fmt.Println(d.GetWordAfterCursorUntilSeparatorIgnoreNextToCursor(","))
 	// Output:
@@ -267,7 +269,7 @@ func TestDocument_GetCharRelativeToCursor(t *testing.T) {
 		{
 			document: &Document{
 				Text:           "line 1\nline 2\nline 3\nline 4\n",
-				cursorPosition: len([]rune("line 1\n" + "lin")),
+				cursorPosition: istrings.RuneLen("line 1\n" + "lin"),
 			},
 			expected: "e",
 		},
@@ -304,7 +306,7 @@ func TestDocument_TextBeforeCursor(t *testing.T) {
 		{
 			document: &Document{
 				Text:           "line 1\nline 2\nline 3\nline 4\n",
-				cursorPosition: len("line 1\n" + "lin"),
+				cursorPosition: istrings.RuneLen("line 1\n" + "lin"),
 			},
 			expected: "line 1\nlin",
 		},
@@ -339,7 +341,7 @@ func TestDocument_TextAfterCursor(t *testing.T) {
 		{
 			document: &Document{
 				Text:           "line 1\nline 2\nline 3\nline 4\n",
-				cursorPosition: len("line 1\n" + "lin"),
+				cursorPosition: istrings.RuneLen("line 1\n" + "lin"),
 			},
 			expected: "e 2\nline 3\nline 4\n",
 		},
@@ -383,14 +385,14 @@ func TestDocument_GetWordBeforeCursor(t *testing.T) {
 		{
 			document: &Document{
 				Text:           "apple bana",
-				cursorPosition: len("apple bana"),
+				cursorPosition: istrings.RuneLen("apple bana"),
 			},
 			expected: "bana",
 		},
 		{
 			document: &Document{
 				Text:           "apply -f ./file/foo.json",
-				cursorPosition: len("apply -f ./file/foo.json"),
+				cursorPosition: istrings.RuneLen("apply -f ./file/foo.json"),
 			},
 			expected: "foo.json",
 			sep:      " /",
@@ -398,14 +400,14 @@ func TestDocument_GetWordBeforeCursor(t *testing.T) {
 		{
 			document: &Document{
 				Text:           "apple banana orange",
-				cursorPosition: len("apple ba"),
+				cursorPosition: istrings.RuneLen("apple ba"),
 			},
 			expected: "ba",
 		},
 		{
 			document: &Document{
 				Text:           "apply -f ./file/foo.json",
-				cursorPosition: len("apply -f ./fi"),
+				cursorPosition: istrings.RuneLen("apply -f ./fi"),
 			},
 			expected: "fi",
 			sep:      " /",
@@ -413,7 +415,7 @@ func TestDocument_GetWordBeforeCursor(t *testing.T) {
 		{
 			document: &Document{
 				Text:           "apple ",
-				cursorPosition: len("apple "),
+				cursorPosition: istrings.RuneLen("apple "),
 			},
 			expected: "",
 		},
@@ -461,14 +463,14 @@ func TestDocument_GetWordBeforeCursorWithSpace(t *testing.T) {
 		{
 			document: &Document{
 				Text:           "apple bana ",
-				cursorPosition: len("apple bana "),
+				cursorPosition: istrings.RuneLen("apple bana "),
 			},
 			expected: "bana ",
 		},
 		{
 			document: &Document{
 				Text:           "apply -f /path/to/file/",
-				cursorPosition: len("apply -f /path/to/file/"),
+				cursorPosition: istrings.RuneLen("apply -f /path/to/file/"),
 			},
 			expected: "file/",
 			sep:      " /",
@@ -476,14 +478,14 @@ func TestDocument_GetWordBeforeCursorWithSpace(t *testing.T) {
 		{
 			document: &Document{
 				Text:           "apple ",
-				cursorPosition: len("apple "),
+				cursorPosition: istrings.RuneLen("apple "),
 			},
 			expected: "apple ",
 		},
 		{
 			document: &Document{
 				Text:           "path/",
-				cursorPosition: len("path/"),
+				cursorPosition: istrings.RuneLen("path/"),
 			},
 			expected: "path/",
 			sep:      " /",
@@ -526,37 +528,37 @@ func TestDocument_GetWordBeforeCursorWithSpace(t *testing.T) {
 func TestDocument_FindStartOfPreviousWord(t *testing.T) {
 	pattern := []struct {
 		document *Document
-		expected int
+		expected istrings.ByteIndex
 		sep      string
 	}{
 		{
 			document: &Document{
 				Text:           "apple bana",
-				cursorPosition: len("apple bana"),
+				cursorPosition: istrings.RuneLen("apple bana"),
 			},
-			expected: len("apple "),
+			expected: istrings.Len("apple "),
 		},
 		{
 			document: &Document{
 				Text:           "apply -f ./file/foo.json",
-				cursorPosition: len("apply -f ./file/foo.json"),
+				cursorPosition: istrings.RuneLen("apply -f ./file/foo.json"),
 			},
-			expected: len("apply -f ./file/"),
+			expected: istrings.Len("apply -f ./file/"),
 			sep:      " /",
 		},
 		{
 			document: &Document{
 				Text:           "apple ",
-				cursorPosition: len("apple "),
+				cursorPosition: istrings.RuneLen("apple "),
 			},
-			expected: len("apple "),
+			expected: istrings.Len("apple "),
 		},
 		{
 			document: &Document{
 				Text:           "apply -f ./file/foo.json",
-				cursorPosition: len("apply -f ./"),
+				cursorPosition: istrings.RuneLen("apply -f ./"),
 			},
-			expected: len("apply -f ./"),
+			expected: istrings.Len("apply -f ./"),
 			sep:      " /",
 		},
 		{
@@ -564,14 +566,14 @@ func TestDocument_FindStartOfPreviousWord(t *testing.T) {
 				Text:           "あいうえお かきくけこ さしすせそ",
 				cursorPosition: 8, // between 'き' and 'く'
 			},
-			expected: len("あいうえお "), // this function returns index byte in string
+			expected: istrings.Len("あいうえお "), // this function returns index byte in string
 		},
 		{
 			document: &Document{
 				Text:           "Добрый день Добрый день",
 				cursorPosition: 9,
 			},
-			expected: len("Добрый "), // this function returns index byte in string
+			expected: istrings.Len("Добрый "), // this function returns index byte in string
 		},
 	}
 
@@ -597,37 +599,37 @@ func TestDocument_FindStartOfPreviousWord(t *testing.T) {
 func TestDocument_FindStartOfPreviousWordWithSpace(t *testing.T) {
 	pattern := []struct {
 		document *Document
-		expected int
+		expected istrings.ByteIndex
 		sep      string
 	}{
 		{
 			document: &Document{
 				Text:           "apple bana ",
-				cursorPosition: len("apple bana "),
+				cursorPosition: istrings.RuneLen("apple bana "),
 			},
-			expected: len("apple "),
+			expected: istrings.Len("apple "),
 		},
 		{
 			document: &Document{
 				Text:           "apply -f /file/foo/",
-				cursorPosition: len("apply -f /file/foo/"),
+				cursorPosition: istrings.RuneLen("apply -f /file/foo/"),
 			},
-			expected: len("apply -f /file/"),
+			expected: istrings.Len("apply -f /file/"),
 			sep:      " /",
 		},
 		{
 			document: &Document{
 				Text:           "apple ",
-				cursorPosition: len("apple "),
+				cursorPosition: istrings.RuneLen("apple "),
 			},
-			expected: len(""),
+			expected: istrings.Len(""),
 		},
 		{
 			document: &Document{
 				Text:           "file/",
-				cursorPosition: len("file/"),
+				cursorPosition: istrings.RuneLen("file/"),
 			},
-			expected: len(""),
+			expected: istrings.Len(""),
 			sep:      " /",
 		},
 		{
@@ -635,14 +637,14 @@ func TestDocument_FindStartOfPreviousWordWithSpace(t *testing.T) {
 				Text:           "あいうえお かきくけこ ",
 				cursorPosition: 12, // cursor points to last
 			},
-			expected: len("あいうえお "), // this function returns index byte in string
+			expected: istrings.Len("あいうえお "), // this function returns index byte in string
 		},
 		{
 			document: &Document{
 				Text:           "Добрый день ",
 				cursorPosition: 12,
 			},
-			expected: len("Добрый "), // this function returns index byte in string
+			expected: istrings.Len("Добрый "), // this function returns index byte in string
 		},
 	}
 
@@ -674,14 +676,14 @@ func TestDocument_GetWordAfterCursor(t *testing.T) {
 		{
 			document: &Document{
 				Text:           "apple bana",
-				cursorPosition: len("apple bana"),
+				cursorPosition: istrings.RuneLen("apple bana"),
 			},
 			expected: "",
 		},
 		{
 			document: &Document{
 				Text:           "apply -f ./file/foo.json",
-				cursorPosition: len("apply -f ./fi"),
+				cursorPosition: istrings.RuneLen("apply -f ./fi"),
 			},
 			expected: "le",
 			sep:      " /",
@@ -689,21 +691,21 @@ func TestDocument_GetWordAfterCursor(t *testing.T) {
 		{
 			document: &Document{
 				Text:           "apple bana",
-				cursorPosition: len("apple "),
+				cursorPosition: istrings.RuneLen("apple "),
 			},
 			expected: "bana",
 		},
 		{
 			document: &Document{
 				Text:           "apple bana",
-				cursorPosition: len("apple"),
+				cursorPosition: istrings.RuneLen("apple"),
 			},
 			expected: "",
 		},
 		{
 			document: &Document{
 				Text:           "apply -f ./file/foo.json",
-				cursorPosition: len("apply -f ."),
+				cursorPosition: istrings.RuneLen("apply -f ."),
 			},
 			expected: "",
 			sep:      " /",
@@ -711,7 +713,7 @@ func TestDocument_GetWordAfterCursor(t *testing.T) {
 		{
 			document: &Document{
 				Text:           "apple bana",
-				cursorPosition: len("ap"),
+				cursorPosition: istrings.RuneLen("ap"),
 			},
 			expected: "ple",
 		},
@@ -759,21 +761,21 @@ func TestDocument_GetWordAfterCursorWithSpace(t *testing.T) {
 		{
 			document: &Document{
 				Text:           "apple bana",
-				cursorPosition: len("apple bana"),
+				cursorPosition: istrings.RuneLen("apple bana"),
 			},
 			expected: "",
 		},
 		{
 			document: &Document{
 				Text:           "apple bana",
-				cursorPosition: len("apple "),
+				cursorPosition: istrings.RuneLen("apple "),
 			},
 			expected: "bana",
 		},
 		{
 			document: &Document{
 				Text:           "/path/to",
-				cursorPosition: len("/path/"),
+				cursorPosition: istrings.RuneLen("/path/"),
 			},
 			expected: "to",
 			sep:      " /",
@@ -781,7 +783,7 @@ func TestDocument_GetWordAfterCursorWithSpace(t *testing.T) {
 		{
 			document: &Document{
 				Text:           "/path/to/file",
-				cursorPosition: len("/path/"),
+				cursorPosition: istrings.RuneLen("/path/"),
 			},
 			expected: "to",
 			sep:      " /",
@@ -789,14 +791,14 @@ func TestDocument_GetWordAfterCursorWithSpace(t *testing.T) {
 		{
 			document: &Document{
 				Text:           "apple bana",
-				cursorPosition: len("apple"),
+				cursorPosition: istrings.RuneLen("apple"),
 			},
 			expected: " bana",
 		},
 		{
 			document: &Document{
 				Text:           "path/to",
-				cursorPosition: len("path"),
+				cursorPosition: istrings.RuneLen("path"),
 			},
 			expected: "/to",
 			sep:      " /",
@@ -804,7 +806,7 @@ func TestDocument_GetWordAfterCursorWithSpace(t *testing.T) {
 		{
 			document: &Document{
 				Text:           "apple bana",
-				cursorPosition: len("ap"),
+				cursorPosition: istrings.RuneLen("ap"),
 			},
 			expected: "ple",
 		},
@@ -846,52 +848,52 @@ func TestDocument_GetWordAfterCursorWithSpace(t *testing.T) {
 func TestDocument_FindEndOfCurrentWord(t *testing.T) {
 	pattern := []struct {
 		document *Document
-		expected int
+		expected istrings.ByteIndex
 		sep      string
 	}{
 		{
 			document: &Document{
 				Text:           "apple bana",
-				cursorPosition: len("apple bana"),
+				cursorPosition: istrings.RuneLen("apple bana"),
 			},
-			expected: len(""),
+			expected: istrings.Len(""),
 		},
 		{
 			document: &Document{
 				Text:           "apple bana",
-				cursorPosition: len("apple "),
+				cursorPosition: istrings.RuneLen("apple "),
 			},
-			expected: len("bana"),
+			expected: istrings.Len("bana"),
 		},
 		{
 			document: &Document{
 				Text:           "apply -f ./file/foo.json",
-				cursorPosition: len("apply -f ./"),
+				cursorPosition: istrings.RuneLen("apply -f ./"),
 			},
-			expected: len("file"),
+			expected: istrings.Len("file"),
 			sep:      " /",
 		},
 		{
 			document: &Document{
 				Text:           "apple bana",
-				cursorPosition: len("apple"),
+				cursorPosition: istrings.RuneLen("apple"),
 			},
-			expected: len(""),
+			expected: istrings.Len(""),
 		},
 		{
 			document: &Document{
 				Text:           "apply -f ./file/foo.json",
-				cursorPosition: len("apply -f ."),
+				cursorPosition: istrings.RuneLen("apply -f ."),
 			},
-			expected: len(""),
+			expected: istrings.Len(""),
 			sep:      " /",
 		},
 		{
 			document: &Document{
 				Text:           "apple bana",
-				cursorPosition: len("ap"),
+				cursorPosition: istrings.RuneLen("ap"),
 			},
-			expected: len("ple"),
+			expected: istrings.Len("ple"),
 		},
 		{
 			// りん(cursor)ご ばなな
@@ -899,7 +901,7 @@ func TestDocument_FindEndOfCurrentWord(t *testing.T) {
 				Text:           "りんご ばなな",
 				cursorPosition: 2,
 			},
-			expected: len("ご"),
+			expected: istrings.Len("ご"),
 		},
 		{
 			document: &Document{
@@ -914,7 +916,7 @@ func TestDocument_FindEndOfCurrentWord(t *testing.T) {
 				Text:           "Добрый день",
 				cursorPosition: 3,
 			},
-			expected: len("рый"),
+			expected: istrings.Len("рый"),
 		},
 	}
 
@@ -940,73 +942,73 @@ func TestDocument_FindEndOfCurrentWord(t *testing.T) {
 func TestDocument_FindEndOfCurrentWordWithSpace(t *testing.T) {
 	pattern := []struct {
 		document *Document
-		expected int
+		expected istrings.ByteIndex
 		sep      string
 	}{
 		{
 			document: &Document{
 				Text:           "apple bana",
-				cursorPosition: len("apple bana"),
+				cursorPosition: istrings.RuneLen("apple bana"),
 			},
-			expected: len(""),
+			expected: istrings.Len(""),
 		},
 		{
 			document: &Document{
 				Text:           "apple bana",
-				cursorPosition: len("apple "),
+				cursorPosition: istrings.RuneLen("apple "),
 			},
-			expected: len("bana"),
+			expected: istrings.Len("bana"),
 		},
 		{
 			document: &Document{
 				Text:           "apply -f /file/foo.json",
-				cursorPosition: len("apply -f /"),
+				cursorPosition: istrings.RuneLen("apply -f /"),
 			},
-			expected: len("file"),
+			expected: istrings.Len("file"),
 			sep:      " /",
 		},
 		{
 			document: &Document{
 				Text:           "apple bana",
-				cursorPosition: len("apple"),
+				cursorPosition: istrings.RuneLen("apple"),
 			},
-			expected: len(" bana"),
+			expected: istrings.Len(" bana"),
 		},
 		{
 			document: &Document{
 				Text:           "apply -f /path/to",
-				cursorPosition: len("apply -f /path"),
+				cursorPosition: istrings.RuneLen("apply -f /path"),
 			},
-			expected: len("/to"),
+			expected: istrings.Len("/to"),
 			sep:      " /",
 		},
 		{
 			document: &Document{
 				Text:           "apple bana",
-				cursorPosition: len("ap"),
+				cursorPosition: istrings.RuneLen("ap"),
 			},
-			expected: len("ple"),
+			expected: istrings.Len("ple"),
 		},
 		{
 			document: &Document{
 				Text:           "あいうえお かきくけこ",
 				cursorPosition: 6,
 			},
-			expected: len("かきくけこ"),
+			expected: istrings.Len("かきくけこ"),
 		},
 		{
 			document: &Document{
 				Text:           "あいうえお かきくけこ",
 				cursorPosition: 5,
 			},
-			expected: len(" かきくけこ"),
+			expected: istrings.Len(" かきくけこ"),
 		},
 		{
 			document: &Document{
 				Text:           "Добрый день",
 				cursorPosition: 6,
 			},
-			expected: len(" день"),
+			expected: istrings.Len(" день"),
 		},
 	}
 
@@ -1032,7 +1034,7 @@ func TestDocument_FindEndOfCurrentWordWithSpace(t *testing.T) {
 func TestDocument_CurrentLineBeforeCursor(t *testing.T) {
 	d := &Document{
 		Text:           "line 1\nline 2\nline 3\nline 4\n",
-		cursorPosition: len("line 1\n" + "lin"),
+		cursorPosition: istrings.RuneLen("line 1\n" + "lin"),
 	}
 	ac := d.CurrentLineBeforeCursor()
 	ex := "lin"
@@ -1044,7 +1046,7 @@ func TestDocument_CurrentLineBeforeCursor(t *testing.T) {
 func TestDocument_CurrentLineAfterCursor(t *testing.T) {
 	d := &Document{
 		Text:           "line 1\nline 2\nline 3\nline 4\n",
-		cursorPosition: len("line 1\n" + "lin"),
+		cursorPosition: istrings.RuneLen("line 1\n" + "lin"),
 	}
 	ac := d.CurrentLineAfterCursor()
 	ex := "e 2"
@@ -1056,7 +1058,7 @@ func TestDocument_CurrentLineAfterCursor(t *testing.T) {
 func TestDocument_CurrentLine(t *testing.T) {
 	d := &Document{
 		Text:           "line 1\nline 2\nline 3\nline 4\n",
-		cursorPosition: len("line 1\n" + "lin"),
+		cursorPosition: istrings.RuneLen("line 1\n" + "lin"),
 	}
 	ac := d.CurrentLine()
 	ex := "line 2"
@@ -1068,11 +1070,11 @@ func TestDocument_CurrentLine(t *testing.T) {
 func TestDocument_CursorPositionRowAndCol(t *testing.T) {
 	var cursorPositionTests = []struct {
 		document    *Document
-		expectedRow int
-		expectedCol int
+		expectedRow istrings.RuneIndex
+		expectedCol istrings.RuneIndex
 	}{
 		{
-			document:    &Document{Text: "line 1\nline 2\nline 3\n", cursorPosition: len("line 1\n" + "lin")},
+			document:    &Document{Text: "line 1\nline 2\nline 3\n", cursorPosition: istrings.RuneLen("line 1\n" + "lin")},
 			expectedRow: 1,
 			expectedCol: 3,
 		},
@@ -1097,10 +1099,10 @@ func TestDocument_CursorPositionRowAndCol(t *testing.T) {
 func TestDocument_GetCursorLeftPosition(t *testing.T) {
 	d := &Document{
 		Text:           "line 1\nline 2\nline 3\nline 4\n",
-		cursorPosition: len("line 1\n" + "line 2\n" + "lin"),
+		cursorPosition: istrings.RuneLen("line 1\n" + "line 2\n" + "lin"),
 	}
 	ac := d.GetCursorLeftPosition(2)
-	ex := -2
+	var ex istrings.RuneIndex = -2
 	if ac != ex {
 		t.Errorf("Should be %#v, got %#v", ex, ac)
 	}
@@ -1120,16 +1122,16 @@ func TestDocument_GetCursorLeftPosition(t *testing.T) {
 func TestDocument_GetCursorUpPosition(t *testing.T) {
 	d := &Document{
 		Text:           "line 1\nline 2\nline 3\nline 4\n",
-		cursorPosition: len("line 1\n" + "line 2\n" + "lin"),
+		cursorPosition: istrings.RuneLen("line 1\n" + "line 2\n" + "lin"),
 	}
 	ac := d.GetCursorUpPosition(2, -1)
-	ex := len("lin") - len("line 1\n"+"line 2\n"+"lin")
+	ex := istrings.RuneLen("lin") - istrings.RuneLen("line 1\n"+"line 2\n"+"lin")
 	if ac != ex {
 		t.Errorf("Should be %#v, got %#v", ex, ac)
 	}
 
 	ac = d.GetCursorUpPosition(100, -1)
-	ex = len("lin") - len("line 1\n"+"line 2\n"+"lin")
+	ex = istrings.RuneLen("lin") - istrings.RuneLen("line 1\n"+"line 2\n"+"lin")
 	if ac != ex {
 		t.Errorf("Should be %#v, got %#v", ex, ac)
 	}
@@ -1138,16 +1140,16 @@ func TestDocument_GetCursorUpPosition(t *testing.T) {
 func TestDocument_GetCursorDownPosition(t *testing.T) {
 	d := &Document{
 		Text:           "line 1\nline 2\nline 3\nline 4\n",
-		cursorPosition: len("lin"),
+		cursorPosition: istrings.RuneLen("lin"),
 	}
 	ac := d.GetCursorDownPosition(2, -1)
-	ex := len("line 1\n"+"line 2\n"+"lin") - len("lin")
+	ex := istrings.RuneLen("line 1\n"+"line 2\n"+"lin") - istrings.RuneLen("lin")
 	if ac != ex {
 		t.Errorf("Should be %#v, got %#v", ex, ac)
 	}
 
 	ac = d.GetCursorDownPosition(100, -1)
-	ex = len("line 1\n"+"line 2\n"+"line 3\n"+"line 4\n") - len("lin")
+	ex = istrings.RuneLen("line 1\n"+"line 2\n"+"line 3\n"+"line 4\n") - istrings.RuneLen("lin")
 	if ac != ex {
 		t.Errorf("Should be %#v, got %#v", ex, ac)
 	}
@@ -1156,10 +1158,10 @@ func TestDocument_GetCursorDownPosition(t *testing.T) {
 func TestDocument_GetCursorRightPosition(t *testing.T) {
 	d := &Document{
 		Text:           "line 1\nline 2\nline 3\nline 4\n",
-		cursorPosition: len("line 1\n" + "line 2\n" + "lin"),
+		cursorPosition: istrings.RuneLen("line 1\n" + "line 2\n" + "lin"),
 	}
 	ac := d.GetCursorRightPosition(2)
-	ex := 2
+	var ex istrings.RuneIndex = 2
 	if ac != ex {
 		t.Errorf("Should be %#v, got %#v", ex, ac)
 	}
@@ -1179,7 +1181,7 @@ func TestDocument_GetCursorRightPosition(t *testing.T) {
 func TestDocument_Lines(t *testing.T) {
 	d := &Document{
 		Text:           "line 1\nline 2\nline 3\nline 4\n",
-		cursorPosition: len("line 1\n" + "lin"),
+		cursorPosition: istrings.RuneLen("line 1\n" + "lin"),
 	}
 	ac := d.Lines()
 	ex := []string{"line 1", "line 2", "line 3", "line 4", ""}
@@ -1191,7 +1193,7 @@ func TestDocument_Lines(t *testing.T) {
 func TestDocument_LineCount(t *testing.T) {
 	d := &Document{
 		Text:           "line 1\nline 2\nline 3\nline 4\n",
-		cursorPosition: len("line 1\n" + "lin"),
+		cursorPosition: istrings.RuneLen("line 1\n" + "lin"),
 	}
 	ac := d.LineCount()
 	ex := 5
@@ -1203,9 +1205,9 @@ func TestDocument_LineCount(t *testing.T) {
 func TestDocument_TranslateIndexToPosition(t *testing.T) {
 	d := &Document{
 		Text:           "line 1\nline 2\nline 3\nline 4\n",
-		cursorPosition: len("line 1\n" + "lin"),
+		cursorPosition: istrings.RuneLen("line 1\n" + "lin"),
 	}
-	row, col := d.TranslateIndexToPosition(len("line 1\nline 2\nlin"))
+	row, col := d.TranslateIndexToPosition(istrings.RuneLen("line 1\nline 2\nlin"))
 	if row != 2 {
 		t.Errorf("Should be %#v, got %#v", 2, row)
 	}
@@ -1224,10 +1226,10 @@ func TestDocument_TranslateIndexToPosition(t *testing.T) {
 func TestDocument_TranslateRowColToIndex(t *testing.T) {
 	d := &Document{
 		Text:           "line 1\nline 2\nline 3\nline 4\n",
-		cursorPosition: len("line 1\n" + "lin"),
+		cursorPosition: istrings.RuneLen("line 1\n" + "lin"),
 	}
 	ac := d.TranslateRowColToIndex(2, 3)
-	ex := len("line 1\nline 2\nlin")
+	ex := istrings.RuneLen("line 1\nline 2\nlin")
 	if ac != ex {
 		t.Errorf("Should be %#v, got %#v", ex, ac)
 	}
@@ -1241,13 +1243,13 @@ func TestDocument_TranslateRowColToIndex(t *testing.T) {
 func TestDocument_OnLastLine(t *testing.T) {
 	d := &Document{
 		Text:           "line 1\nline 2\nline 3",
-		cursorPosition: len("line 1\nline"),
+		cursorPosition: istrings.RuneLen("line 1\nline"),
 	}
 	ac := d.OnLastLine()
 	if ac {
 		t.Errorf("Should be %#v, got %#v", false, ac)
 	}
-	d.cursorPosition = len("line 1\nline 2\nline")
+	d.cursorPosition = istrings.RuneLen("line 1\nline 2\nline")
 	ac = d.OnLastLine()
 	if !ac {
 		t.Errorf("Should be %#v, got %#v", true, ac)
@@ -1257,10 +1259,10 @@ func TestDocument_OnLastLine(t *testing.T) {
 func TestDocument_GetEndOfLinePosition(t *testing.T) {
 	d := &Document{
 		Text:           "line 1\nline 2\nline 3",
-		cursorPosition: len("line 1\nli"),
+		cursorPosition: istrings.RuneLen("line 1\nli"),
 	}
 	ac := d.GetEndOfLinePosition()
-	ex := len("ne 2")
+	ex := istrings.RuneLen("ne 2")
 	if ac != ex {
 		t.Errorf("Should be %#v, got %#v", ex, ac)
 	}
diff --git a/emacs.go b/emacs.go
index be34b6c4..2123cb52 100644
--- a/emacs.go
+++ b/emacs.go
@@ -1,6 +1,9 @@
 package prompt
 
-import "github.com/elk-language/go-prompt/internal/debug"
+import (
+	"github.com/elk-language/go-prompt/internal/debug"
+	istrings "github.com/elk-language/go-prompt/internal/strings"
+)
 
 /*
 
@@ -44,7 +47,7 @@ var emacsKeyBindings = []KeyBind{
 		Key: ControlE,
 		Fn: func(buf *Buffer) {
 			x := []rune(buf.Document().TextAfterCursor())
-			buf.CursorRight(len(x))
+			buf.CursorRight(istrings.RuneCount(len(x)))
 		},
 	},
 	// Go to the beginning of the line
@@ -52,7 +55,7 @@ var emacsKeyBindings = []KeyBind{
 		Key: ControlA,
 		Fn: func(buf *Buffer) {
 			x := []rune(buf.Document().TextBeforeCursor())
-			buf.CursorLeft(len(x))
+			buf.CursorLeft(istrings.RuneCount(len(x)))
 		},
 	},
 	// Cut the Line after the cursor
@@ -60,7 +63,7 @@ var emacsKeyBindings = []KeyBind{
 		Key: ControlK,
 		Fn: func(buf *Buffer) {
 			x := []rune(buf.Document().TextAfterCursor())
-			buf.Delete(len(x))
+			buf.Delete(istrings.RuneCount(len(x)))
 		},
 	},
 	// Cut/delete the Line before the cursor
@@ -68,7 +71,7 @@ var emacsKeyBindings = []KeyBind{
 		Key: ControlU,
 		Fn: func(buf *Buffer) {
 			x := []rune(buf.Document().TextBeforeCursor())
-			buf.DeleteBeforeCursor(len(x))
+			buf.DeleteBeforeCursor(istrings.RuneCount(len(x)))
 		},
 	},
 	// Delete character under the cursor
@@ -98,7 +101,7 @@ var emacsKeyBindings = []KeyBind{
 	{
 		Key: AltRight,
 		Fn: func(buf *Buffer) {
-			buf.CursorRight(buf.Document().FindStringWidthUntilEndOfCurrentWord())
+			buf.CursorRight(istrings.RuneIndex(buf.Document().FindStringWidthUntilEndOfCurrentWord())) // WARN
 		},
 	},
 	// Left allow: Backward one character
@@ -112,20 +115,20 @@ var emacsKeyBindings = []KeyBind{
 	{
 		Key: AltLeft,
 		Fn: func(buf *Buffer) {
-			buf.CursorLeft(buf.Document().FindStringWidthUntilStartOfPreviousWord())
+			buf.CursorLeft(istrings.RuneIndex(buf.Document().FindStringWidthUntilStartOfPreviousWord())) // WARN
 		},
 	},
 	// Cut the Word before the cursor.
 	{
 		Key: ControlW,
 		Fn: func(buf *Buffer) {
-			buf.DeleteBeforeCursor(len([]rune(buf.Document().GetWordBeforeCursorWithSpace())))
+			buf.DeleteBeforeCursor(istrings.RuneIndex(len([]rune(buf.Document().GetWordBeforeCursorWithSpace()))))
 		},
 	},
 	{
 		Key: AltBackspace,
 		Fn: func(buf *Buffer) {
-			buf.DeleteBeforeCursor(len([]rune(buf.Document().GetWordBeforeCursorWithSpace())))
+			buf.DeleteBeforeCursor(istrings.RuneIndex(len([]rune(buf.Document().GetWordBeforeCursorWithSpace()))))
 		},
 	},
 	// Clear the Screen, similar to the clear command
diff --git a/emacs_test.go b/emacs_test.go
index bfda59e6..20e9a0e4 100644
--- a/emacs_test.go
+++ b/emacs_test.go
@@ -1,11 +1,15 @@
 package prompt
 
-import "testing"
+import (
+	"testing"
+
+	istrings "github.com/elk-language/go-prompt/internal/strings"
+)
 
 func TestEmacsKeyBindings(t *testing.T) {
 	buf := NewBuffer()
 	buf.InsertText("abcde", false, true)
-	if buf.cursorPosition != len("abcde") {
+	if buf.cursorPosition != istrings.RuneIndex(len("abcde")) {
 		t.Errorf("Want %d, but got %d", len("abcde"), buf.cursorPosition)
 	}
 
@@ -17,7 +21,7 @@ func TestEmacsKeyBindings(t *testing.T) {
 
 	// Go to the end of the line
 	applyEmacsKeyBind(buf, ControlE)
-	if buf.cursorPosition != len("abcde") {
+	if buf.cursorPosition != istrings.RuneIndex(len("abcde")) {
 		t.Errorf("Want %d, but got %d", len("abcde"), buf.cursorPosition)
 	}
 }
diff --git a/internal/bisect/bisect.go b/internal/bisect/bisect.go
index efe162fa..e8f25d74 100644
--- a/internal/bisect/bisect.go
+++ b/internal/bisect/bisect.go
@@ -3,13 +3,13 @@ package bisect
 import "sort"
 
 // Right to locate the insertion point for v in a to maintain sorted order.
-func Right(a []int, v int) int {
+func Right[T ~int](a []T, v T) T {
 	return bisectRightRange(a, v, 0, len(a))
 }
 
-func bisectRightRange(a []int, v int, lo, hi int) int {
+func bisectRightRange[T ~int](a []T, v T, lo, hi int) T {
 	s := a[lo:hi]
-	return sort.Search(len(s), func(i int) bool {
+	return T(sort.Search(len(s), func(i int) bool {
 		return s[i] > v
-	})
+	}))
 }
diff --git a/internal/strings/index.go b/internal/strings/index.go
new file mode 100644
index 00000000..e711af0f
--- /dev/null
+++ b/internal/strings/index.go
@@ -0,0 +1,21 @@
+package strings
+
+// Numeric type that represents an index
+// of a single byte in a string, array or slice.
+type ByteIndex int
+
+// Numeric type that represents an index
+// of a single rune in a string, array or slice.
+type RuneIndex int
+
+// Numeric type that represents the visible
+// width of characters in a string as seen in a terminal emulator.
+type StringWidth int
+
+// Numeric type that represents the amount
+// of bytes in a string, array or slice.
+type ByteCount = ByteIndex
+
+// Numeric type that represents the amount
+// of runes in a string, array or slice.
+type RuneCount = RuneIndex
diff --git a/internal/strings/strings.go b/internal/strings/strings.go
index f7d1886a..7800becf 100644
--- a/internal/strings/strings.go
+++ b/internal/strings/strings.go
@@ -1,23 +1,35 @@
 package strings
 
-import "unicode/utf8"
+import (
+	"unicode/utf8"
+)
+
+// Get the length of the string in bytes.
+func Len(s string) ByteCount {
+	return ByteCount(len(s))
+}
+
+// Get the length of the string in runes.
+func RuneLen(s string) RuneCount {
+	return RuneCount(utf8.RuneCountInString(s))
+}
 
 // IndexNotByte is similar with strings.IndexByte but showing the opposite behavior.
-func IndexNotByte(s string, c byte) int {
+func IndexNotByte(s string, c byte) ByteIndex {
 	n := len(s)
 	for i := 0; i < n; i++ {
 		if s[i] != c {
-			return i
+			return ByteIndex(i)
 		}
 	}
 	return -1
 }
 
 // LastIndexNotByte is similar with strings.LastIndexByte but showing the opposite behavior.
-func LastIndexNotByte(s string, c byte) int {
+func LastIndexNotByte(s string, c byte) ByteIndex {
 	for i := len(s) - 1; i >= 0; i-- {
 		if s[i] != c {
-			return i
+			return ByteIndex(i)
 		}
 	}
 	return -1
@@ -41,13 +53,13 @@ func makeASCIISet(chars string) (as asciiSet, ok bool) {
 }
 
 // IndexNotAny is similar with strings.IndexAny but showing the opposite behavior.
-func IndexNotAny(s, chars string) int {
+func IndexNotAny(s, chars string) ByteIndex {
 	if len(chars) > 0 {
 		if len(s) > 8 {
 			if as, isASCII := makeASCIISet(chars); isASCII {
 				for i := 0; i < len(s); i++ {
 					if as.notContains(s[i]) {
-						return i
+						return ByteIndex(i)
 					}
 				}
 				return -1
@@ -58,7 +70,7 @@ func IndexNotAny(s, chars string) int {
 		for i, c := range s {
 			for j, m := range chars {
 				if c != m && j == len(chars)-1 {
-					return i
+					return ByteIndex(i)
 				} else if c != m {
 					continue
 				} else {
@@ -71,13 +83,13 @@ func IndexNotAny(s, chars string) int {
 }
 
 // LastIndexNotAny is similar with strings.LastIndexAny but showing the opposite behavior.
-func LastIndexNotAny(s, chars string) int {
+func LastIndexNotAny(s, chars string) ByteIndex {
 	if len(chars) > 0 {
 		if len(s) > 8 {
 			if as, isASCII := makeASCIISet(chars); isASCII {
 				for i := len(s) - 1; i >= 0; i-- {
 					if as.notContains(s[i]) {
-						return i
+						return ByteIndex(i)
 					}
 				}
 				return -1
@@ -89,7 +101,7 @@ func LastIndexNotAny(s, chars string) int {
 			i -= size
 			for j, m := range chars {
 				if r != m && j == len(chars)-1 {
-					return i
+					return ByteIndex(i)
 				} else if r != m {
 					continue
 				} else {
diff --git a/key_bind_func.go b/key_bind_func.go
index 7b2ecdf6..00bd3066 100644
--- a/key_bind_func.go
+++ b/key_bind_func.go
@@ -1,15 +1,19 @@
 package prompt
 
+import (
+	istrings "github.com/elk-language/go-prompt/internal/strings"
+)
+
 // GoLineEnd Go to the End of the line
 func GoLineEnd(buf *Buffer) {
 	x := []rune(buf.Document().TextAfterCursor())
-	buf.CursorRight(len(x))
+	buf.CursorRight(istrings.RuneCount(len(x)))
 }
 
 // GoLineBeginning Go to the beginning of the line
 func GoLineBeginning(buf *Buffer) {
 	x := []rune(buf.Document().TextBeforeCursor())
-	buf.CursorLeft(len(x))
+	buf.CursorLeft(istrings.RuneCount(len(x)))
 }
 
 // DeleteChar Delete character under the cursor
@@ -19,7 +23,7 @@ func DeleteChar(buf *Buffer) {
 
 // DeleteWord Delete word before the cursor
 func DeleteWord(buf *Buffer) {
-	buf.DeleteBeforeCursor(len([]rune(buf.Document().TextBeforeCursor())) - buf.Document().FindStartOfPreviousWordWithSpace())
+	buf.DeleteBeforeCursor(istrings.RuneCount(len([]rune(buf.Document().TextBeforeCursor()))) - istrings.RuneCount(buf.Document().FindStartOfPreviousWordWithSpace())) // WARN
 }
 
 // DeleteBeforeChar Go to Backspace
@@ -39,10 +43,10 @@ func GoLeftChar(buf *Buffer) {
 
 // GoRightWord Forward one word
 func GoRightWord(buf *Buffer) {
-	buf.CursorRight(buf.Document().FindEndOfCurrentWordWithSpace())
+	buf.CursorRight(istrings.RuneCount(buf.Document().FindEndOfCurrentWordWithSpace())) // WARN
 }
 
 // GoLeftWord Backward one word
 func GoLeftWord(buf *Buffer) {
-	buf.CursorLeft(len([]rune(buf.Document().TextBeforeCursor())) - buf.Document().FindStartOfPreviousWordWithSpace())
+	buf.CursorLeft(istrings.RuneCount(len([]rune(buf.Document().TextBeforeCursor()))) - istrings.RuneCount(buf.Document().FindStartOfPreviousWordWithSpace())) // WARN
 }
diff --git a/position.go b/position.go
index 58365669..ac6d834b 100644
--- a/position.go
+++ b/position.go
@@ -4,6 +4,7 @@ import (
 	"io"
 	"strings"
 
+	istrings "github.com/elk-language/go-prompt/internal/strings"
 	"github.com/mattn/go-runewidth"
 )
 
@@ -13,7 +14,8 @@ import (
 // (0, 0) represents the top-left corner of the prompt,
 // while (n, n) the bottom-right corner.
 type Position struct {
-	X, Y int
+	X istrings.StringWidth
+	Y int
 }
 
 // Join two positions and return a new position.
@@ -45,16 +47,16 @@ func (p Position) Subtract(other Position) Position {
 
 // positionAtEndOfString calculates the position of the
 // p at the end of the given string.
-func positionAtEndOfString(str string, columns int) Position {
-	// fmt.Printf("%q\n", str)
+func positionAtEndOfString(str string, columns istrings.StringWidth) Position {
 	pos := positionAtEndOfReader(strings.NewReader(str), columns)
 	return pos
 }
 
 // positionAtEndOfReader calculates the position of the
 // p at the end of the given io.Reader.
-func positionAtEndOfReader(reader io.RuneReader, columns int) Position {
-	var down, right int
+func positionAtEndOfReader(reader io.RuneReader, columns istrings.StringWidth) Position {
+	var down int
+	var right istrings.StringWidth
 
 charLoop:
 	for {
@@ -78,7 +80,7 @@ charLoop:
 			down++
 			right = 0
 		default:
-			right += runewidth.RuneWidth(char)
+			right += istrings.StringWidth(runewidth.RuneWidth(char))
 			if right == columns {
 				right = 0
 				down++
diff --git a/position_test.go b/position_test.go
index 1de7defb..44d14567 100644
--- a/position_test.go
+++ b/position_test.go
@@ -6,13 +6,14 @@ package prompt
 import (
 	"testing"
 
+	istrings "github.com/elk-language/go-prompt/internal/strings"
 	"github.com/google/go-cmp/cmp"
 )
 
 func TestPositionAtEndOfString(t *testing.T) {
 	tests := map[string]struct {
 		input   string
-		columns int
+		columns istrings.StringWidth
 		want    Position
 	}{
 		"empty": {
diff --git a/prompt.go b/prompt.go
index ecc43ed9..19c3d135 100644
--- a/prompt.go
+++ b/prompt.go
@@ -8,6 +8,7 @@ import (
 	"unicode/utf8"
 
 	"github.com/elk-language/go-prompt/internal/debug"
+	istrings "github.com/elk-language/go-prompt/internal/strings"
 )
 
 const inputBufferSize = 1024
@@ -161,7 +162,7 @@ func (p *Prompt) feed(b []byte) (shouldExit bool, userInput *UserInput) {
 		p.buf = NewBuffer()
 		p.history.Clear()
 	case Up, ControlP:
-		cursor := p.buf.Document().GetCursorPosition(int(p.renderer.col))
+		cursor := p.buf.Document().GetCursorPosition(p.renderer.col)
 		if cursor.Y != 0 {
 			p.buf.CursorUp(1)
 			break
@@ -175,8 +176,8 @@ func (p *Prompt) feed(b []byte) (shouldExit bool, userInput *UserInput) {
 		}
 
 	case Down, ControlN:
-		endOfTextCursor := p.buf.Document().GetEndOfTextPosition(int(p.renderer.col))
-		cursor := p.buf.Document().GetCursorPosition(int(p.renderer.col))
+		endOfTextCursor := p.buf.Document().GetEndOfTextPosition(p.renderer.col)
+		cursor := p.buf.Document().GetCursorPosition(p.renderer.col)
 		if endOfTextCursor.Y > cursor.Y {
 			p.buf.CursorDown(1)
 			break
@@ -229,7 +230,7 @@ func (p *Prompt) handleCompletionKeyBinding(key Key, completing bool) {
 		if s, ok := p.completion.GetSelectedSuggestion(); ok {
 			w := p.buf.Document().GetWordBeforeCursorUntilSeparator(p.completion.wordSeparator)
 			if w != "" {
-				p.buf.DeleteBeforeCursor(len([]rune(w)))
+				p.buf.DeleteBeforeCursor(istrings.RuneCount(len([]rune(w))))
 			}
 			p.buf.InsertText(s.Text, false, true)
 		}
diff --git a/render.go b/render.go
index 97b9d6c4..f68298da 100644
--- a/render.go
+++ b/render.go
@@ -5,6 +5,7 @@ import (
 	"strings"
 
 	"github.com/elk-language/go-prompt/internal/debug"
+	istrings "github.com/elk-language/go-prompt/internal/strings"
 	runewidth "github.com/mattn/go-runewidth"
 )
 
@@ -15,7 +16,7 @@ type Render struct {
 	breakLineCallback func(*Document)
 	title             string
 	row               uint16
-	col               uint16
+	col               istrings.StringWidth
 
 	previousCursor Position
 
@@ -78,7 +79,7 @@ func (r *Render) prepareArea(lines int) {
 // UpdateWinSize called when window size is changed.
 func (r *Render) UpdateWinSize(ws *WinSize) {
 	r.row = ws.Row
-	r.col = ws.Col
+	r.col = istrings.StringWidth(ws.Col)
 }
 
 func (r *Render) renderWindowTooSmall() {
@@ -108,10 +109,10 @@ func (r *Render) renderCompletion(buf *Buffer, completions *CompletionManager) {
 	formatted = formatted[completions.verticalScroll : completions.verticalScroll+windowHeight]
 	r.prepareArea(windowHeight)
 
-	cursor := positionAtEndOfString(prefix+buf.Document().TextBeforeCursor(), int(r.col))
+	cursor := positionAtEndOfString(prefix+buf.Document().TextBeforeCursor(), r.col)
 	x := cursor.X
-	if x+width >= int(r.col) {
-		cursor = r.backward(cursor, x+width-int(r.col))
+	if x+width >= r.col {
+		cursor = r.backward(cursor, x+width-r.col)
 	}
 
 	contentHeight := len(completions.tmp)
@@ -160,8 +161,8 @@ func (r *Render) renderCompletion(buf *Buffer, completions *CompletionManager) {
 		r.backward(c, width)
 	}
 
-	if x+width >= int(r.col) {
-		r.out.CursorForward(x + width - int(r.col))
+	if x+width >= r.col {
+		r.out.CursorForward(int(x + width - r.col))
 	}
 
 	r.out.CursorUp(windowHeight)
@@ -180,8 +181,8 @@ func (r *Render) Render(buffer *Buffer, completion *CompletionManager, lexer Lex
 
 	line := buffer.Text()
 	prefix := r.getCurrentPrefix()
-	prefixWidth := runewidth.StringWidth(prefix)
-	cursor := positionAtEndOfString(prefix+line, int(r.col))
+	prefixWidth := istrings.StringWidth(runewidth.StringWidth(prefix))
+	cursor := positionAtEndOfString(prefix+line, r.col)
 
 	// prepare area
 	y := cursor.Y
@@ -209,7 +210,7 @@ func (r *Render) Render(buffer *Buffer, completion *CompletionManager, lexer Lex
 
 	r.lineWrap(&cursor)
 
-	targetCursor := buffer.DisplayCursorPosition(int(r.col))
+	targetCursor := buffer.DisplayCursorPosition(r.col)
 	if targetCursor.Y == 0 {
 		targetCursor.X += prefixWidth
 	}
@@ -217,12 +218,12 @@ func (r *Render) Render(buffer *Buffer, completion *CompletionManager, lexer Lex
 
 	r.renderCompletion(buffer, completion)
 	if suggest, ok := completion.GetSelectedSuggestion(); ok {
-		cursor = r.backward(cursor, runewidth.StringWidth(buffer.Document().GetWordBeforeCursorUntilSeparator(completion.wordSeparator)))
+		cursor = r.backward(cursor, istrings.StringWidth(runewidth.StringWidth(buffer.Document().GetWordBeforeCursorUntilSeparator(completion.wordSeparator))))
 
 		r.out.SetColor(r.previewSuggestionTextColor, r.previewSuggestionBGColor, false)
 		r.out.WriteString(suggest.Text)
 		r.out.SetColor(DefaultColor, DefaultColor, false)
-		cursor.X += runewidth.StringWidth(suggest.Text)
+		cursor.X += istrings.StringWidth(runewidth.StringWidth(suggest.Text))
 		endOfSuggestionPos := cursor
 
 		rest := buffer.Document().TextAfterCursor()
@@ -235,7 +236,7 @@ func (r *Render) Render(buffer *Buffer, completion *CompletionManager, lexer Lex
 
 		r.out.SetColor(DefaultColor, DefaultColor, false)
 
-		cursor = cursor.Join(positionAtEndOfString(rest, int(r.col)))
+		cursor = cursor.Join(positionAtEndOfString(rest, r.col))
 
 		r.lineWrap(&cursor)
 
@@ -267,7 +268,7 @@ func (r *Render) lex(lexer Lexer, input string) {
 // BreakLine to break line.
 func (r *Render) BreakLine(buffer *Buffer, lexer Lexer) {
 	// Erasing and Render
-	cursor := positionAtEndOfString(buffer.Document().TextBeforeCursor()+r.getCurrentPrefix(), int(r.col))
+	cursor := positionAtEndOfString(buffer.Document().TextBeforeCursor()+r.getCurrentPrefix(), r.col)
 	r.clear(cursor)
 
 	r.renderPrefix()
@@ -298,7 +299,7 @@ func (r *Render) clear(cursor Position) {
 
 // backward moves cursor to backward from a current cursor position
 // regardless there is a line break.
-func (r *Render) backward(from Position, n int) Position {
+func (r *Render) backward(from Position, n istrings.StringWidth) Position {
 	return r.move(from, Position{X: from.X - n, Y: from.Y})
 }
 
@@ -307,12 +308,12 @@ func (r *Render) backward(from Position, n int) Position {
 func (r *Render) move(from, to Position) Position {
 	newPosition := from.Subtract(to)
 	r.out.CursorUp(newPosition.Y)
-	r.out.CursorBackward(newPosition.X)
+	r.out.CursorBackward(int(newPosition.X))
 	return to
 }
 
 func (r *Render) lineWrap(cursor *Position) {
-	if runtime.GOOS != "windows" && cursor.X > 0 && cursor.X%int(r.col) == 0 {
+	if runtime.GOOS != "windows" && cursor.X > 0 && cursor.X%r.col == 0 {
 		cursor.X = 0
 		cursor.Y += 1
 		r.out.WriteRaw([]byte{'\n'})
@@ -330,8 +331,8 @@ func clamp(high, low, x float64) float64 {
 	}
 }
 
-func alignNextLine(r *Render, col int) {
+func alignNextLine(r *Render, col istrings.StringWidth) {
 	r.out.CursorDown(1)
 	r.out.WriteString("\r")
-	r.out.CursorForward(col)
+	r.out.CursorForward(int(col))
 }
diff --git a/render_test.go b/render_test.go
index 81e0f643..ab349520 100644
--- a/render_test.go
+++ b/render_test.go
@@ -7,6 +7,8 @@ import (
 	"reflect"
 	"syscall"
 	"testing"
+
+	istrings "github.com/elk-language/go-prompt/internal/strings"
 )
 
 func TestFormatCompletion(t *testing.T) {
@@ -17,7 +19,7 @@ func TestFormatCompletion(t *testing.T) {
 		suffix        string
 		expected      []Suggest
 		maxWidth      int
-		expectedWidth int
+		expectedWidth istrings.StringWidth
 	}{
 		{
 			scenario: "",