From f959d2995a2ee088a142d2073f415eeaa311b9c3 Mon Sep 17 00:00:00 2001
From: Paul Miller <paul@paulmillr.com>
Date: Mon, 25 Sep 2023 18:24:16 +0000
Subject: [PATCH] utils: optimize hexToBytes by 4%

---
 src/utils.ts | 40 ++++++++++++++++++----------------------
 1 file changed, 18 insertions(+), 22 deletions(-)

diff --git a/src/utils.ts b/src/utils.ts
index 8e92236..87609f6 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -37,21 +37,13 @@ export function bytesToHex(bytes: Uint8Array): string {
   return hex;
 }
 
-// We use very optimized technique to convert hex string to byte array
-const enum HexC {
-  ZERO = 48, // 0
-  NINE = 57, // 9
-  A_UP = 65, // A
-  F_UP = 70, // F
-  A_LO = 97, // a
-  F_LO = 102, // f
-}
-
-function charCodeToBase16(char: number) {
-  if (char >= HexC.ZERO && char <= HexC.NINE) return char - HexC.ZERO;
-  else if (char >= HexC.A_UP && char <= HexC.F_UP) return char - (HexC.A_UP - 10);
-  else if (char >= HexC.A_LO && char <= HexC.F_LO) return char - (HexC.A_LO - 10);
-  throw new Error('Invalid byte sequence');
+// We use optimized technique to convert hex string to byte array
+const asciis = { _0: 48, _9: 57, _A: 65, _F: 70, _a: 97, _f: 102 } as const;
+function asciiToBase16(char: number): number | undefined {
+  if (char >= asciis._0 && char <= asciis._9) return char - asciis._0;
+  if (char >= asciis._A && char <= asciis._F) return char - (asciis._A - 10);
+  if (char >= asciis._a && char <= asciis._f) return char - (asciis._a - 10);
+  return;
 }
 
 /**
@@ -59,14 +51,18 @@ function charCodeToBase16(char: number) {
  */
 export function hexToBytes(hex: string): Uint8Array {
   if (typeof hex !== 'string') throw new Error('hex string expected, got ' + typeof hex);
-  const len = hex.length;
-  if (len % 2) throw new Error('padded hex string expected, got unpadded hex of length ' + len);
-  const al = len / 2;
+  const hl = hex.length;
+  const al = hl / 2;
+  if (hl % 2) throw new Error('padded hex string expected, got unpadded hex of length ' + hl);
   const array = new Uint8Array(al);
-  for (let i = 0, j = 0; i < al; i++) {
-    const n1 = charCodeToBase16(hex.charCodeAt(j++));
-    const n2 = charCodeToBase16(hex.charCodeAt(j++));
-    array[i] = n1 * 16 + n2;
+  for (let ai = 0, hi = 0; ai < al; ai++, hi += 2) {
+    const n1 = asciiToBase16(hex.charCodeAt(hi));
+    const n2 = asciiToBase16(hex.charCodeAt(hi + 1));
+    if (n1 === undefined || n2 === undefined) {
+      const char = hex[hi] + hex[hi + 1];
+      throw new Error('hex string expected, got non-hex character "' + char + '" at index ' + hi);
+    }
+    array[ai] = n1 * 16 + n2;
   }
   return array;
 }