From 59057eec03f0fdce834b2c01abe61381b6e7256b Mon Sep 17 00:00:00 2001
From: Pejman Nikram <Pejman.Nikram@sievo.com>
Date: Wed, 19 Feb 2025 22:17:21 +0200
Subject: [PATCH] adding support for nonstandard map key in the decoder

fix #255
---
 README.md                | 25 ++++++++++++++-----------
 src/Decoder.ts           | 21 +++++++++++++++------
 test/decodeAsync.test.ts | 15 +++++++++++++++
 3 files changed, 44 insertions(+), 17 deletions(-)

diff --git a/README.md b/README.md
index 6ba5681..f7c36cc 100644
--- a/README.md
+++ b/README.md
@@ -145,17 +145,20 @@ NodeJS `Buffer` is also acceptable because it is a subclass of `Uint8Array`.
 
 #### `DecoderOptions`
 
-| Name           | Type           | Default                       |
-| -------------- | -------------- | ----------------------------- |
-| extensionCodec | ExtensionCodec | `ExtensionCodec.defaultCodec` |
-| context        | user-defined   | -                             |
-| useBigInt64    | boolean        | false                         |
-| rawStrings     | boolean        | false                         |
-| maxStrLength   | number         | `4_294_967_295` (UINT32_MAX)  |
-| maxBinLength   | number         | `4_294_967_295` (UINT32_MAX)  |
-| maxArrayLength | number         | `4_294_967_295` (UINT32_MAX)  |
-| maxMapLength   | number         | `4_294_967_295` (UINT32_MAX)  |
-| maxExtLength   | number         | `4_294_967_295` (UINT32_MAX)  |
+| Name            | Type                | Default                                        |
+| --------------- | ------------------- | ---------------------------------------------- |
+| extensionCodec  | ExtensionCodec      | `ExtensionCodec.defaultCodec`                  |
+| context         | user-defined        | -                                              |
+| useBigInt64     | boolean             | false                                          |
+| rawStrings      | boolean             | false                                          |
+| maxStrLength    | number              | `4_294_967_295` (UINT32_MAX)                   |
+| maxBinLength    | number              | `4_294_967_295` (UINT32_MAX)                   |
+| maxArrayLength  | number              | `4_294_967_295` (UINT32_MAX)                   |
+| maxMapLength    | number              | `4_294_967_295` (UINT32_MAX)                   |
+| maxExtLength    | number              | `4_294_967_295` (UINT32_MAX)                   |
+| mapKeyConverter | MapKeyConverterType | throw exception if key is not string or number |
+
+`MapKeyConverterType` is defined as `(key: unknown) => string | number`.
 
 To skip UTF-8 decoding of strings, `rawStrings` can be set to `true`. In this case, strings are decoded into `Uint8Array`.
 
diff --git a/src/Decoder.ts b/src/Decoder.ts
index 0bf74de..e9257a9 100644
--- a/src/Decoder.ts
+++ b/src/Decoder.ts
@@ -68,6 +68,13 @@ export type DecoderOptions<ContextType = undefined> = Readonly<
      * `null` is a special value to disable the use of the key decoder at all.
      */
     keyDecoder: KeyDecoder | null;
+
+    /**
+     * A function to convert decoded map key to a valid JS key type.
+     * 
+     * Defaults to a function that throws an error if the key is not a string or a number.
+     */
+    mapKeyConverter: (key: unknown) => MapKeyType;
   }>
 > &
   ContextOf<ContextType>;
@@ -78,8 +85,11 @@ const STATE_MAP_VALUE = "map_value";
 
 type MapKeyType = string | number;
 
-const isValidMapKeyType = (key: unknown): key is MapKeyType => {
-  return typeof key === "string" || typeof key === "number";
+const mapKeyConverter = (key: unknown): MapKeyType => {
+  if (typeof key === "string" || typeof key === "number") {
+    return key;
+  }
+  throw new DecodeError("The type of key must be string or number but " + typeof key);
 };
 
 type StackMapState = {
@@ -213,6 +223,7 @@ export class Decoder<ContextType = undefined> {
   private readonly maxMapLength: number;
   private readonly maxExtLength: number;
   private readonly keyDecoder: KeyDecoder | null;
+  private readonly mapKeyConverter: (key: unknown) => MapKeyType;
 
   private totalPos = 0;
   private pos = 0;
@@ -236,6 +247,7 @@ export class Decoder<ContextType = undefined> {
     this.maxMapLength = options?.maxMapLength ?? UINT32_MAX;
     this.maxExtLength = options?.maxExtLength ?? UINT32_MAX;
     this.keyDecoder = options?.keyDecoder !== undefined ? options.keyDecoder : sharedCachedKeyDecoder;
+    this.mapKeyConverter = options?.mapKeyConverter ?? mapKeyConverter;
   }
 
   private clone(): Decoder<ContextType> {
@@ -631,14 +643,11 @@ export class Decoder<ContextType = undefined> {
             continue DECODE;
           }
         } else if (state.type === STATE_MAP_KEY) {
-          if (!isValidMapKeyType(object)) {
-            throw new DecodeError("The type of key must be string or number but " + typeof object);
-          }
           if (object === "__proto__") {
             throw new DecodeError("The key __proto__ is not allowed");
           }
 
-          state.key = object;
+          state.key = this.mapKeyConverter(object);
           state.type = STATE_MAP_VALUE;
           continue DECODE;
         } else {
diff --git a/test/decodeAsync.test.ts b/test/decodeAsync.test.ts
index 9687370..6af6c19 100644
--- a/test/decodeAsync.test.ts
+++ b/test/decodeAsync.test.ts
@@ -36,6 +36,21 @@ describe("decodeAsync", () => {
     assert.deepStrictEqual(object, { "foo": "bar" });
   });
 
+  it("decodes fixmap {'[1, 2]': 'baz'} with custom map key converter", async () => {
+    const createStream = async function* () {
+      yield [0x81]; // fixmap size=1
+      yield encode([1, 2]);
+      yield encode("baz");
+    };
+
+    const object = await decodeAsync(createStream(), {
+      mapKeyConverter: (key) => JSON.stringify(key),
+    });
+
+    const key = JSON.stringify([1, 2]);
+    assert.deepStrictEqual(object, { [key]: "baz" });
+  });
+
   it("decodes multi-byte integer byte-by-byte", async () => {
     const createStream = async function* () {
       yield [0xcd]; // uint 16