Add tests of canonicalStringify and helper lookupSortedKeys.

The lookupSortedKeys function is not intended to be used directly, but
seemed worth unit testing nevertheless.

src/utilities/common/__tests__/canonicalStringify.ts

@@ -0,0 +1,132 @@
+import {
+  canonicalStringify,
+  lookupSortedKeys,
+} from "../canonicalStringify";
+
+function forEachPermutation(
+  keys: string[],
+  callback: (permutation: string[]) => void,
+) {
+  if (keys.length <= 1) {
+    callback(keys);
+    return;
+  }
+  const first = keys[0];
+  const rest = keys.slice(1);
+  forEachPermutation(rest, (permutation) => {
+    for (let i = 0; i <= permutation.length; ++i) {
+      callback([
+        ...permutation.slice(0, i),
+        first,
+        ...permutation.slice(i),
+      ]);
+    }
+  });
+}
+
+function allObjectPermutations<T extends Record<string, any>>(obj: T) {
+  const keys = Object.keys(obj);
+  const permutations: T[] = [];
+  forEachPermutation(keys, permutation => {
+    const permutationObj =
+      Object.create(Object.getPrototypeOf(obj));
+    permutation.forEach(key => {
+      permutationObj[key] = obj[key];
+    });
+    permutations.push(permutationObj);
+  });
+  return permutations;
+}
+
+describe("canonicalStringify", () => {
+  beforeEach(() => {
+    canonicalStringify.reset();
+  });
+
+  it("should not modify original object", () => {
+    const obj = { c: 3, a: 1, b: 2 };
+    expect(canonicalStringify(obj)).toBe('{"a":1,"b":2,"c":3}');
+    expect(Object.keys(obj)).toEqual(["c", "a", "b"]);
+  });
+
+  it("forEachPermutation should work", () => {
+    const permutations: string[][] = [];
+    forEachPermutation(["a", "b", "c"], (permutation) => {
+      permutations.push(permutation);
+    });
+    expect(permutations).toEqual([
+      ["a", "b", "c"],
+      ["b", "a", "c"],
+      ["b", "c", "a"],
+      ["a", "c", "b"],
+      ["c", "a", "b"],
+      ["c", "b", "a"],
+    ]);
+  });
+
+  it("canonicalStringify should stably stringify all permutations of an object", () => {
+    const unstableStrings = new Set<string>();
+    const stableStrings = new Set<string>();
+
+    allObjectPermutations({
+      c: 3,
+      a: 1,
+      b: 2,
+    }).forEach(obj => {
+      unstableStrings.add(JSON.stringify(obj));
+      stableStrings.add(canonicalStringify(obj));
+
+      expect(canonicalStringify(obj)).toBe('{"a":1,"b":2,"c":3}');
+
+      allObjectPermutations({
+        z: "z",
+        y: ["y", obj, "why"],
+        x: "x",
+      }).forEach(parent => {
+        expect(canonicalStringify(parent)).toBe(
+          '{"x":"x","y":["y",{"a":1,"b":2,"c":3},"why"],"z":"z"}',
+        );
+      });
+    });
+
+    expect(unstableStrings.size).toBe(6);
+    expect(stableStrings.size).toBe(1);
+  });
+
+  it("lookupSortedKeys(keys, false) should reuse same sorted array for all permutations", () => {
+    const keys = ["z", "a", "c", "b"];
+    const sorted = lookupSortedKeys(["z", "a", "b", "c"], false);
+    expect(sorted).toEqual(["a", "b", "c", "z"]);
+    forEachPermutation(keys, permutation => {
+      expect(lookupSortedKeys(permutation, false)).toBe(sorted);
+    });
+  });
+
+  it("lookupSortedKeys(keys, true) should return same array if already sorted", () => {
+    const keys = ["a", "b", "c", "x", "y", "z"].sort();
+    const sorted = lookupSortedKeys(keys, true);
+    expect(sorted).toBe(keys);
+
+    forEachPermutation(keys, permutation => {
+      const sortedTrue = lookupSortedKeys(permutation, true);
+      const sortedFalse = lookupSortedKeys(permutation, false);
+
+      expect(sortedTrue).toEqual(sorted);
+      expect(sortedFalse).toEqual(sorted);
+
+      const wasPermutationSorted =
+        permutation.every((key, i) => key === keys[i]);
+
+      if (wasPermutationSorted) {
+        expect(sortedTrue).toBe(permutation);
+        expect(sortedTrue).not.toBe(sorted);
+      } else {
+        expect(sortedTrue).not.toBe(permutation);
+        expect(sortedTrue).toBe(sorted);
+      }
+
+      expect(sortedFalse).not.toBe(permutation);
+      expect(sortedFalse).toBe(sorted);
+    });
+  });
+});

src/utilities/common/canonicalStringify.ts

@@ -70,7 +70,7 @@ const sortingTrieRoot: SortingTrie = new Map;
 // Sort the given keys using a lookup trie, with an option to return the same
 // (===) array in case it was already sorted, so we can avoid always creating a
 // new object in the replacer function above.
-function lookupSortedKeys(
+export function lookupSortedKeys(
   keys: readonly string[],
   returnKeysIfAlreadySorted: boolean,
 ): readonly string[] {

src/utilities/index.ts

@@ -120,8 +120,8 @@ export * from "./common/makeUniqueId.js";
 export * from "./common/stringifyForDisplay.js";
 export * from "./common/mergeOptions.js";
 export * from "./common/incrementalResult.js";
-export * from "./common/canonicalStringify.js";
 
+export { canonicalStringify } from "./common/canonicalStringify.js";
 export { omitDeep } from "./common/omitDeep.js";
 export { stripTypename } from "./common/stripTypename.js";