add support for bank switches in data sequences

internal/program/program.go

@@ -7,13 +7,17 @@ import (
 	"github.com/retroenv/retrogolib/arch/nes/cartridge"
 )
 
+// WriteCallbackFunc is a custom callback function that gets called before the offset is written.
+type WriteCallbackFunc func(writer io.Writer) error
+
 // Offset defines the content of an offset in a program that can represent data or code.
 type Offset struct {
 	// data byte or all opcode bytes that are part of the instruction
 	OpcodeBytes []byte
-	// custom callback function that gets called before the offset is written, this
-	// allows custom bank switch code to be written at specific offsets
-	WriteCallback func(writer io.Writer) error
+	// WriteCallback is a custom callback function that gets called before the offset is
+	// written, this allows custom bank switch code to be written at specific offsets.
+	// The output of bundled data bytes will be interrupted.
+	WriteCallback WriteCallbackFunc
 
 	Type OffsetType
 

internal/writer/writer.go

@@ -273,6 +273,7 @@ func getPrgData(bank *program.PRGBank, startIndex, endIndex int) []byte {
 
 	for i := startIndex; i < endIndex; i++ {
 		offset := bank.PRG[i]
+
 		// opcode bytes can be nil if data bytes have been combined for an unofficial nop
 		if !offset.IsType(program.DataOffset) || len(offset.OpcodeBytes) == 0 {
 			break
@@ -281,6 +282,12 @@ func getPrgData(bank *program.PRGBank, startIndex, endIndex int) []byte {
 		if i > startIndex && (offset.IsType(program.CodeOffset|program.CodeAsData) || offset.Label != "") {
 			break
 		}
+		// break at potential bank switch, do ignore callback on first iteration as it has been handled
+		// by the caller already.
+		if offset.WriteCallback != nil && i != startIndex {
+			break
+		}
+
 		data = append(data, offset.OpcodeBytes...)
 	}