normaliseSequenceDiff

src/math/normaliseSequenceDiff.test.ts

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+import { describe, expect, test } from "@jest/globals";
+import { normaliseSequenceDiff } from "./normaliseSequenceDiff";
+
+describe("normaliseSequenceDiff", () => {
+  test("simple", async () => {
+    expect(normaliseSequenceDiff([])).toStrictEqual([]);
+    expect(normaliseSequenceDiff([1])).toStrictEqual([0]);
+    expect(normaliseSequenceDiff([2])).toStrictEqual([0]);
+    expect(normaliseSequenceDiff([2], { startFrom: -1 })).toStrictEqual([-3]);
+    expect(normaliseSequenceDiff([1, 2, 3])).toStrictEqual([0, 0, 0]);
+    expect(normaliseSequenceDiff([1, 2, 4])).toStrictEqual([0, 0, -1]);
+    expect(normaliseSequenceDiff([2, 4, 6])).toStrictEqual([0, -1, -2]);
+    expect(normaliseSequenceDiff([2, 2, 6])).toStrictEqual([0, 1, -2]);
+    expect(normaliseSequenceDiff([-1, 2, 6], { startFrom: 0 })).toStrictEqual([
+      1, -1, -4,
+    ]);
+    // untidy sequence
+    expect(
+      normaliseSequenceDiff([-1, -10, 6, 4], { startFrom: 0 })
+    ).toStrictEqual([2, 10, -3, -2]);
+    expect(normaliseSequenceDiff([-1, -10, 6, 4])).toStrictEqual([
+      -8, 0, -13, -12,
+    ]);
+  });
+});

src/math/normaliseSequenceDiff.ts

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+import { min } from "./min";
+
+export const normaliseSequenceDiff = (
+  sequence: number[],
+  options: { startFrom?: number } = {}
+): number[] => {
+  const base = options?.startFrom ?? min(sequence);
+
+  const sortedIndexes = getIndexesOfSortedArray(sequence);
+
+  return sortedIndexes.reduce((diffArray, sortedIndex, index) => {
+    diffArray[sortedIndex] = base + index - sequence[sortedIndex];
+    return diffArray;
+  }, [] as number[]);
+};
+
+const getIndexesOfSortedArray = (arr: number[]): number[] => {
+  // Create an array of indexes
+  const indexes = arr.map((_, index) => index);
+
+  // Sort the indexes based on the corresponding values in the original array
+  indexes.sort((a, b) => (arr[a] < arr[b] ? -1 : arr[a] > arr[b] ? 1 : 0));
+
+  return indexes;
+};