Optimize packed WASM base64 decoding loop (#404)

CHANGELOG.md

@@ -1,5 +1,12 @@
 # CHANGELOG
 
+## master
+
+Changes:
+
+- Optimize packed WASM base64 decoding loop
+
+
 ## 6.2.3 Jul 7, 2022
 
 Changes:

packages/wasm-util/src/base64.ts

@@ -1,11 +1,16 @@
 // Copyright 2019-2022 @polkadot/wasm-util authors & contributors
 // SPDX-License-Identifier: Apache-2.0
 
+// Use an array for our indexer - this is faster than using map access. In
+// this case we assume ASCII-only inputs, so we cannot overflow the array
 const chr = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';
-const map: Record<string, number> = {};
+const map = new Array<number>(256);
 
+// We use charCodeAt for access here and in the decoder loop - this is faster
+// on lookups (array + numbers) and also faster than accessing the specific
+// character via data[i]
 for (let i = 0; i < chr.length; i++) {
-  map[chr[i]] = i;
+  map[chr.charCodeAt(i)] = i;
 }
 
 /**
@@ -16,16 +21,18 @@ for (let i = 0; i < chr.length; i++) {
  * slightly slower, but it is platform independent.
  *
  * For our usage, since we have access to the static final size (where used), we
- * decode to a specified output buffer.
+ * decode to a specified output buffer. This also means we have applied a number
+ * of optimizations based on this - checking out output position instead of chars.
  */
 export function base64Decode (data: string, out: Uint8Array): Uint8Array {
+  const len = out.length;
   let byte = 0;
   let bits = 0;
   let pos = -1;
 
-  for (let i = 0; i < data.length && data[i] !== '='; i++) {
+  for (let i = 0; pos < len; i++) {
     // each character represents 6 bits
-    byte = (byte << 6) | map[data[i]];
+    byte = (byte << 6) | map[data.charCodeAt(i)];
 
     // each byte needs to contain 8 bits
     if ((bits += 6) >= 8) {