From bdbe716547f38e60a5d58facb64c89d1f7a34281 Mon Sep 17 00:00:00 2001
From: Timothy Flynn <trflynn89@pm.me>
Date: Mon, 19 Jul 2021 16:01:35 -0400
Subject: [PATCH] LibJS: Implement String.prototype.endsWith with UTF-16 code
 units

---
 .../LibJS/Runtime/StringPrototype.cpp         | 31 ++++++++++---------
 .../String/String.prototype.endsWith.js       |  8 +++++
 2 files changed, 25 insertions(+), 14 deletions(-)

diff --git a/Userland/Libraries/LibJS/Runtime/StringPrototype.cpp b/Userland/Libraries/LibJS/Runtime/StringPrototype.cpp
index 68ee2c3a94..8112b7d145 100644
--- a/Userland/Libraries/LibJS/Runtime/StringPrototype.cpp
+++ b/Userland/Libraries/LibJS/Runtime/StringPrototype.cpp
@@ -304,8 +304,8 @@ JS_DEFINE_NATIVE_FUNCTION(StringPrototype::starts_with)
 // 22.1.3.6 String.prototype.endsWith ( searchString [ , endPosition ] ), https://tc39.es/ecma262/#sec-string.prototype.endswith
 JS_DEFINE_NATIVE_FUNCTION(StringPrototype::ends_with)
 {
-    auto string = ak_string_from(vm, global_object);
-    if (!string.has_value())
+    auto string = utf16_string_from(vm, global_object);
+    if (vm.exception())
         return {};
 
     auto search_string_value = vm.argument(0);
@@ -318,30 +318,33 @@ JS_DEFINE_NATIVE_FUNCTION(StringPrototype::ends_with)
         return {};
     }
 
-    auto search_string = search_string_value.to_string(global_object);
+    auto search_string = search_string_value.to_utf16_string(global_object);
     if (vm.exception())
         return {};
 
-    auto string_length = string->length();
-    auto search_string_length = search_string.length();
+    Utf16View utf16_string_view { string };
+    auto string_length = utf16_string_view.length_in_code_units();
 
-    size_t pos = string_length;
+    Utf16View utf16_search_view { search_string };
+    auto search_length = utf16_search_view.length_in_code_units();
 
-    auto end_position_value = vm.argument(1);
-    if (!end_position_value.is_undefined()) {
-        double pos_as_double = end_position_value.to_integer_or_infinity(global_object);
+    size_t end = string_length;
+    if (!vm.argument(1).is_undefined()) {
+        auto position = vm.argument(1).to_integer_or_infinity(global_object);
         if (vm.exception())
             return {};
-        pos = clamp(pos_as_double, static_cast<double>(0), static_cast<double>(string_length));
+        end = clamp(position, static_cast<double>(0), static_cast<double>(string_length));
     }
 
-    if (search_string_length == 0)
+    if (search_length == 0)
         return Value(true);
-    if (pos < search_string_length)
+    if (search_length > end)
         return Value(false);
 
-    auto start = pos - search_string_length;
-    return Value(string->substring(start, search_string_length) == search_string);
+    size_t start = end - search_length;
+
+    utf16_string_view = utf16_string_view.substring_view(start, end - start);
+    return Value(utf16_string_view == utf16_search_view);
 }
 
 // 22.1.3.8 String.prototype.indexOf ( searchString [ , position ] ), https://tc39.es/ecma262/#sec-string.prototype.indexof
diff --git a/Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.endsWith.js b/Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.endsWith.js
index 286cc6b1b2..7d31172f9b 100644
--- a/Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.endsWith.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.endsWith.js
@@ -40,3 +40,11 @@ test("basic functionality", () => {
     );
     expect(s.endsWith("bar", undefined)).toBeTrue();
 });
+
+test("UTF-16", () => {
+    var s = "😀";
+    expect(s.endsWith("😀")).toBeTrue();
+    expect(s.endsWith("\ud83d")).toBeFalse();
+    expect(s.endsWith("\ude00")).toBeTrue();
+    expect(s.endsWith("a")).toBeFalse();
+});