From ced155925f4cc8fae9d17ac1953f8f80ccf70423 Mon Sep 17 00:00:00 2001
From: Ewan Breakey <ebreakey@gmail.com>
Date: Thu, 25 May 2023 13:16:25 +1000
Subject: [PATCH] Add splitArrayBy utility

---
 .changeset/fresh-elephants-hide.md |  5 +++
 lib/arrays/index.ts                |  1 +
 lib/arrays/splitArrayBy.ts         | 60 ++++++++++++++++++++++++++++++
 3 files changed, 66 insertions(+)
 create mode 100644 .changeset/fresh-elephants-hide.md
 create mode 100644 lib/arrays/splitArrayBy.ts

diff --git a/.changeset/fresh-elephants-hide.md b/.changeset/fresh-elephants-hide.md
new file mode 100644
index 0000000..5b49ba1
--- /dev/null
+++ b/.changeset/fresh-elephants-hide.md
@@ -0,0 +1,5 @@
+---
+"@giraugh/tools": minor
+---
+
+Add splitArrayBy utility
diff --git a/lib/arrays/index.ts b/lib/arrays/index.ts
index 11c1750..a3b5521 100644
--- a/lib/arrays/index.ts
+++ b/lib/arrays/index.ts
@@ -6,3 +6,4 @@ export * from './zipArrays'
 export * from './zipArraysLongest'
 export * from './partitionArray'
 export * from './rotateArray'
+export * from './splitArrayBy'
diff --git a/lib/arrays/splitArrayBy.ts b/lib/arrays/splitArrayBy.ts
new file mode 100644
index 0000000..f10d777
--- /dev/null
+++ b/lib/arrays/splitArrayBy.ts
@@ -0,0 +1,60 @@
+/**
+ * Split an array when a predicate function returns true and return all the groups
+ * @param arr the array to split
+ * @param splitFn a function over pairs of elements in the array that returns true when the elements should be in different groups
+ * @returns the array of groups split as per the `splitFn`
+ *
+ * @example
+ * const runs = splitArrayBy([1, 2, 3, 10, 11, 12], (a, b) => b - a > 1)
+ * runs // [[1, 2, 3], [10, 11, 12]]
+ */
+export const splitArrayBy = <T>(arr: T[], splitFn: (a: T, b: T) => boolean): T[][] => {
+  // Handle special case of empty array
+  if (arr.length === 0) return []
+
+  let groups: T[][] = [[]]
+  while (true) {
+    // Shift one element and peak the next element
+    if (arr.length === 0) break
+    let [x, y] = [arr.shift()!, arr.at(0)]
+
+    // Add element to current group
+    groups.at(-1)!.push(x)
+
+    // Start new group?
+    if (arr.length > 0 && splitFn(x, y!))
+      groups.push([])
+  }
+
+  return groups
+}
+
+if (import.meta.vitest) {
+  const { expect, it } = import.meta.vitest
+
+  it('works for example 1', () => {
+    const runs = splitArrayBy([1, 2, 3, 10, 11, 12], (x, y) => Math.abs(y - x) > 1)
+    expect(runs).toEqual([[1, 2, 3], [10, 11, 12]])
+  })
+
+  it('handles single group', () => {
+    const groups = splitArrayBy([1, 2, 3], () => false)
+    expect(groups).toEqual([[1, 2, 3]])
+  })
+
+  it('handles many splits', () => {
+    const groups = splitArrayBy([1, 2, 3], () => true)
+    expect(groups).toEqual([[1], [2], [3]])
+  })
+
+  it('handles special case of empty array', () => {
+    const groups = splitArrayBy([], () => true)
+    expect(groups).toEqual([])
+  })
+
+  it('handles falsy values', () => {
+    const groups = splitArrayBy([1, 1, 1, undefined, undefined, 2, 2, 2], (x, y) => !!x !== !!y)
+    expect(groups).toEqual([[1, 1, 1 ], [undefined, undefined], [2, 2, 2]])
+  })
+}
+