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ts/util/unicodeSlice.ts

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+// Copyright 2024 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+let cachedSegmenter: Intl.Segmenter;
+
+/**
+ * Slice a string by bytes into a valid Unicode string.
+ *
+ * @example
+ * ```ts
+ * unicodeSlice('123456', 2, 4); // => '34'
+ * // '€' is 3 bytes, slicing it at 2 bytes would result in an invalid character
+ * unicodeSlice('€', 0, 2); // => ''
+ * // Each emoji is 4 bytes, with zero-width joiner of 3 bytes
+ * unicodeSlice('👩‍👩‍👧‍👦', 0, 18); // => '👩‍👩‍👧'
+ * ```
+ */
+export function unicodeSlice(
+  input: string,
+  begin: number,
+  end: number
+): string {
+  // Until https://chromium-review.googlesource.com/c/v8/v8/+/4190519 is merged,
+  // we should limit the input size to avoid allocating tons of memory.
+  // This should be longer than any max length we'd expect to slice.
+  const slice = input.slice(0, 5e7); // 50MB
+
+  // 'und' is the BCP 47 subtag for "undetermined"
+  // Unicode's CLDR doesn't have any special rules for granularity 'grapheme'
+  // in any language, so we don't need to rely on loading any locale data.
+  cachedSegmenter ??= new Intl.Segmenter('und', { granularity: 'grapheme' });
+
+  const graphemes = cachedSegmenter.segment(slice);
+
+  let result = '';
+  let byteOffset = 0;
+
+  for (const grapheme of graphemes) {
+    const graphemeByteLength = Buffer.byteLength(grapheme.segment);
+    const startsBefore = byteOffset < begin;
+    byteOffset += graphemeByteLength;
+    const endsAfter = byteOffset > end;
+    if (startsBefore) {
+      continue;
+    }
+    if (endsAfter) {
+      break;
+    }
+    result += grapheme.segment;
+  }
+
+  return result;
+}