Fix array decoder empty check

.changeset/quick-arrays-decode.md

@@ -0,0 +1,7 @@
+---
+'@solana/codecs-data-structures': patch
+---
+
+Avoid copying the remaining buffer in `getArrayDecoder`'s emptiness check
+
+`getArrayDecoder`'s `read()` tested for an empty byte array with `bytes.slice(offset).length === 0`, which allocates and copies every byte from `offset` to the end just to read `.length` off the result. On large accounts containing many prefixed arrays, maps, or sets this made decoding quadratic in account size. The check is now the equivalent O(1) comparison `offset >= bytes.length`. `getMapDecoder` and `getSetDecoder` delegate to `getArrayDecoder` and benefit as well.

packages/codecs-data-structures/src/array.ts

@@ -178,7 +178,7 @@ export function getArrayDecoder<TTo>(item: Decoder<TTo>, config: ArrayCodecConfi
         ...(fixedSize !== null ? { fixedSize } : { maxSize }),
         read: (bytes: ReadonlyUint8Array | Uint8Array, offset) => {
             const array: TTo[] = [];
-            if (typeof size === 'object' && bytes.slice(offset).length === 0) {
+            if (typeof size === 'object' && offset >= bytes.length) {
                 return [array, offset];
             }