feat(arrays): add isSorted()

packages/arrays/README.md

@@ -51,6 +51,7 @@ import * as a from "@thi.ng/arrays";
 - [ensureArray()](https://github.com/thi-ng/umbrella/tree/master/packages/arrays/src/ensure-array.ts)
 - [ensureIterable()](https://github.com/thi-ng/umbrella/tree/master/packages/arrays/src/ensure-iterable.ts)
 - [fuzzyMatch()](https://github.com/thi-ng/umbrella/tree/master/packages/arrays/src/fuzzy-match.ts)
+- [isSorted()](https://github.com/thi-ng/umbrella/tree/master/packages/arrays/src/is-sorted.ts)
 - [multiSwap()](https://github.com/thi-ng/umbrella/tree/master/packages/arrays/src/swap.ts)
 - [peek()](https://github.com/thi-ng/umbrella/tree/master/packages/arrays/src/peek.ts)
 - [quickSort()](https://github.com/thi-ng/umbrella/tree/master/packages/arrays/src/quicksort.ts)

packages/arrays/src/index.ts

@@ -4,6 +4,7 @@ export * from "./ensure-array";
 export * from "./ensure-iterable";
 export * from "./find";
 export * from "./fuzzy-match";
+export * from "./is-sorted";
 export * from "./peek";
 export * from "./quicksort";
 export * from "./shuffle";

packages/arrays/src/is-sorted.ts

@@ -0,0 +1,38 @@
+import { Comparator } from "@thi.ng/api";
+import { compare } from "@thi.ng/compare";
+
+/**
+ * Returns true if the given array and its elements in the selected
+ * index range (entire array, by default) are in the order defined by
+ * the given comparator (thi.ng/compare by default). Always returns
+ * true, if effective index range (or array length) has less than two
+ * elements. No bounds checking.
+ *
+ * ```
+ * isSorted([3, 2, 1])
+ * // false
+ *
+ * // w/ custom comparator
+ * isSorted([3, 2, 1], (a, b) => b - a)
+ * // true
+ * ```
+ *
+ * @param arr
+ * @param cmp
+ * @param start
+ * @param end
+ */
+export const isSorted = <T>(
+    arr: ArrayLike<T>,
+    cmp: Comparator<T> = compare,
+    start = 0,
+    end = arr.length
+) => {
+    let prev = arr[start];
+    while (++start < end) {
+        const curr = arr[start];
+        if (cmp(prev, curr) > 0) return false;
+        prev = curr;
+    }
+    return true;
+};