feat(text/unstable): add reverse function

Co-authored-by: Mathias Bynens <[email protected]>

text/_test_util.ts

@@ -0,0 +1,5 @@
+export function generateRandomString(min: number, max: number): string {
+  return Array.from({ length: Math.floor(Math.random() * (max - min) + min) })
+    .map(() => String.fromCharCode(Math.floor(Math.random() * 26) + 97))
+    .join("");
+}

text/_test_util_test.ts

@@ -0,0 +1,10 @@
+import { assertEquals } from "../assert/equals.ts";
+import { generateRandomString } from "./_test_util.ts";
+
+Deno.test({
+  name: "generateRandomString() generates a string of the correct length",
+  fn() {
+    assertEquals(generateRandomString(0, 0), "");
+    assertEquals(generateRandomString(10, 10).length, 10);
+  },
+});

text/deno.json

@@ -6,6 +6,7 @@
     "./closest-string": "./closest_string.ts",
     "./compare-similarity": "./compare_similarity.ts",
     "./levenshtein-distance": "./levenshtein_distance.ts",
+    "./unstable-reverse": "./unstable_reverse.ts",
     "./unstable-slugify": "./unstable_slugify.ts",
     "./to-camel-case": "./to_camel_case.ts",
     "./unstable-to-constant-case": "./unstable_to_constant_case.ts",

text/unstable_reverse.ts

@@ -0,0 +1,90 @@
+// Copyright 2018-2025 the Deno authors. MIT license.
+// This module is browser compatible.
+
+// Copyright Mathias Bynens <https://mathiasbynens.be/>
+//
+// Permission is hereby granted, free of charge, to any person obtaining
+// a copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to
+// permit persons to whom the Software is furnished to do so, subject to
+// the following conditions:
+//
+// The above copyright notice and this permission notice shall be
+// included in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+// LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+// WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+const REGEX_SYMBOL_WITH_COMBINING_MARKS =
+  /([\0-\u02FF\u0370-\u1AAF\u1B00-\u1DBF\u1E00-\u20CF\u2100-\uD7FF\uE000-\uFE1F\uFE30-\uFFFF]|[\uD800-\uDBFF][\uDC00-\uDFFF]|[\uD800-\uDBFF](?![\uDC00-\uDFFF])|(?:[^\uD800-\uDBFF]|^)[\uDC00-\uDFFF])([\u0300-\u036F\u1AB0-\u1AFF\u1DC0-\u1DFF\u20D0-\u20FF\uFE20-\uFE2F]+)/g;
+const REGEX_SURROGATE_PAIR = /([\uD800-\uDBFF])([\uDC00-\uDFFF])/g;
+
+/** Options for {@linkcode reverse}  */
+export type ReverseOptions = {
+  /**
+   * Whether to handle Unicode symbols such as 🦕 at the cost of ~60% slowdown.
+   *
+   * Check {@link ./unstable_reverse_bench.ts} for performance comparison.
+   *
+   * @default {true}
+   */
+  handleUnicode: boolean;
+};
+
+/**
+ * Performs a Unicode-aware string reversal.
+ *
+ * @experimental **UNSTABLE**: New API, yet to be vetted.
+ *
+ * @param input - The input string to be reversed.
+ * @param options The options for the reverse function.
+ * @returns The reversed string.
+ *
+ * @example Standard usage
+ * ```ts
+ * import { reverse } from "@std/text/unstable-reverse";
+ * import { assertEquals } from "@std/assert";
+ *
+ * assertEquals(reverse("Hello, world!"), "!dlrow ,olleH");
+ * assertEquals(reverse("🦕Deno♥"), "♥oneD🦕");
+ * ```
+ *
+ * @example Performance optimization with disabled Unicode handling
+ * ```ts
+ * import { reverse } from "@std/text/unstable-reverse";
+ * import { assertEquals } from "@std/assert";
+ *
+ * assertEquals(reverse("Hello, world!", { handleUnicode: false }), "!dlrow ,olleH");
+ * ```
+ */
+export function reverse(
+  input: string,
+  options?: Partial<ReverseOptions>,
+): string {
+  if (options?.handleUnicode !== false) {
+    // Step 1: deal with combining marks and astral symbols (surrogate pairs)
+    input = input
+      // Swap symbols with their combining marks so the combining marks go first
+      .replace(REGEX_SYMBOL_WITH_COMBINING_MARKS, (_, $1, $2) => {
+        // Reverse the combining marks so they will end up in the same order
+        // later on (after another round of reversing)
+        return reverse($2) + $1;
+      })
+      // Swap high and low surrogates so the low surrogates go first
+      .replace(REGEX_SURROGATE_PAIR, "$2$1");
+  }
+
+  // Step 2: reverse the code units in the string
+  let result = "";
+  for (let index = input.length; index--;) {
+    result += input.charAt(index);
+  }
+  return result;
+}

text/unstable_reverse_bench.ts

@@ -0,0 +1,104 @@
+// Copyright 2018-2025 the Deno authors. MIT license.
+import { generateRandomString } from "./_test_util.ts";
+import { reverse } from "./unstable_reverse.ts";
+
+function splitReverseJoin(str: string) {
+  return str.split("").reverse().join("");
+}
+
+function forOf(str: string) {
+  let reversed = "";
+  for (const character of str) {
+    reversed = character + reversed;
+  }
+  return reversed;
+}
+
+function reduce(str: string) {
+  return str.split("").reduce(
+    (reversed, character) => character + reversed,
+    "",
+  );
+}
+
+function spreadReverseJoin(str: string) {
+  return [...str].reverse().join("");
+}
+
+function forLoop(str: string) {
+  let x = "";
+
+  for (let i = str.length - 1; i >= 0; --i) {
+    x += str[i];
+  }
+
+  return x;
+}
+
+const strings = Array.from({ length: 10000 }).map(() =>
+  generateRandomString(0, 100)
+);
+
+Deno.bench({
+  group: "reverseString",
+  name: "splitReverseJoin",
+  fn: () => {
+    for (let i = 0; i < strings.length; i++) {
+      splitReverseJoin(strings[i]!);
+    }
+  },
+});
+Deno.bench({
+  group: "reverseString",
+  name: "forOf",
+  fn: () => {
+    for (let i = 0; i < strings.length; i++) {
+      forOf(strings[i]!);
+    }
+  },
+});
+Deno.bench({
+  group: "reverseString",
+  name: "reduce",
+  fn: () => {
+    for (let i = 0; i < strings.length; i++) {
+      reduce(strings[i]!);
+    }
+  },
+});
+Deno.bench({
+  group: "reverseString",
+  name: "spreadReverseJoin",
+  fn: () => {
+    for (let i = 0; i < strings.length; i++) {
+      spreadReverseJoin(strings[i]!);
+    }
+  },
+});
+Deno.bench({
+  group: "reverseString",
+  name: "forLoop",
+  fn: () => {
+    for (let i = 0; i < strings.length; i++) {
+      forLoop(strings[i]!);
+    }
+  },
+});
+Deno.bench({
+  group: "reverseString",
+  name: "esrever",
+  fn: () => {
+    for (let i = 0; i < strings.length; i++) {
+      reverse(strings[i]!);
+    }
+  },
+});
+Deno.bench({
+  group: "reverseString",
+  name: "esrever (no unicode)",
+  fn: () => {
+    for (let i = 0; i < strings.length; i++) {
+      reverse(strings[i]!, { handleUnicode: false });
+    }
+  },
+});

text/unstable_reverse_test.ts

@@ -0,0 +1,45 @@
+// Copyright 2018-2025 the Deno authors. MIT license.
+import { assertEquals } from "@std/assert/equals";
+import { reverse } from "./unstable_reverse.ts";
+
+function testBothAsciiAndUnicode(expected: string, input: string) {
+  testAscii(expected, input);
+  testUnicode(expected, input);
+}
+function testUnicode(expected: string, input: string) {
+  assertEquals(expected, reverse(input));
+  // check idempotency
+  assertEquals(input, reverse(reverse(input)));
+  // check empty object handling
+  assertEquals(expected, reverse(input, {}));
+}
+function testAscii(expected: string, input: string) {
+  assertEquals(expected, reverse(input, { handleUnicode: false }));
+  // check idempotency
+  assertEquals(
+    input,
+    reverse(reverse(input, { handleUnicode: false }), { handleUnicode: false }),
+  );
+  // check empty object handling
+  assertEquals(expected, reverse(input, {}));
+}
+
+Deno.test("reverse() handles empty string", () => {
+  testBothAsciiAndUnicode("", "");
+});
+
+Deno.test("reverse() reverses a string", () => {
+  testBothAsciiAndUnicode("olleh", "hello");
+  testBothAsciiAndUnicode("dlrow olleh", "hello world");
+});
+
+// CREDIT: https://github.com/mathiasbynens/esrever/blob/14b34013dad49106ca08c0e65919f1fc8fea5331/README.md
+Deno.test("reverse() handles unicode strings", () => {
+  testUnicode("Lorem ipsum 𝌆 dolor sit ameͨ͆t.", ".teͨ͆ma tis rolod 𝌆 muspi meroL");
+  testUnicode("mañana mañana", "anañam anañam");
+
+  testUnicode("H̹̙̦̮͉̩̗̗ͧ̇̏̊̾Eͨ͆͒̆ͮ̃͏̷̮̣̫̤̣ ̵̞̹̻̀̉̓ͬ͑͡ͅCͯ̂͐͏̨̛͔̦̟͈̻O̜͎͍͙͚̬̝̣̽ͮ͐͗̀ͤ̍̀͢M̴̡̲̭͍͇̼̟̯̦̉̒͠Ḛ̛̙̞̪̗ͥͤͩ̾͑̔͐ͅṮ̴̷̷̗̼͍̿̿̓̽͐H̙̙̔̄͜", "H̙̙̔̄͜Ṯ̴̷̷̗̼͍̿̿̓̽͐Ḛ̛̙̞̪̗ͥͤͩ̾͑̔͐ͅM̴̡̲̭͍͇̼̟̯̦̉̒͠O̜͎͍͙͚̬̝̣̽ͮ͐͗̀ͤ̍̀͢Cͯ̂͐͏̨̛͔̦̟͈̻ ̵̞̹̻̀̉̓ͬ͑͡ͅEͨ͆͒̆ͮ̃͏̷̮̣̫̤̣H̹̙̦̮͉̩̗̗ͧ̇̏̊̾");
+
+  testUnicode("🦕Deno♥", "♥oneD🦕");
+  testUnicode("안녕하세요", "요세하녕안");
+});