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diff --git a/intl/lwbrk/Segmenter.h b/intl/lwbrk/Segmenter.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Classes to iterate over grapheme, word, sentence, or line. */
+
+#ifndef intl_components_Segmenter_h_
+#define intl_components_Segmenter_h_
+
+#include "mozilla/intl/ICUError.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Result.h"
+#include "mozilla/Span.h"
+#include "mozilla/UniquePtr.h"
+
+namespace mozilla::intl {
+
+enum class SegmenterGranularity : uint8_t {
+  Grapheme,
+  Word,
+  Sentence,
+  Line,
+};
+
+struct SegmenterOptions final {
+  SegmenterGranularity mGranularity = SegmenterGranularity::Grapheme;
+};
+
+/**
+ * Interface of segment iterators. Subclass this class to implement iterator for
+ * UTF-16 text.
+ */
+class SegmentIteratorUtf16 {
+ public:
+  virtual ~SegmentIteratorUtf16() = default;
+
+  // Disable copy or move semantics. Move semantic could be enabled in the
+  // future if needed.
+  SegmentIteratorUtf16(SegmentIteratorUtf16&&) = delete;
+  SegmentIteratorUtf16& operator=(SegmentIteratorUtf16&&) = delete;
+  SegmentIteratorUtf16(const SegmentIteratorUtf16&) = delete;
+  SegmentIteratorUtf16& operator=(const SegmentIteratorUtf16&) = delete;
+
+  /**
+   * Advance the iterator to the next break position.
+   *
+   * @return the break position. If there's no further break position, return
+   * Nothing().
+   */
+  virtual Maybe<uint32_t> Next() = 0;
+
+  /**
+   * Advance the iterator to the first break position following the specified
+   * position aPos.
+   *
+   * Note: if this iterator's current position is already >= aPos, this method
+   * behaves the same as Next().
+   */
+  virtual Maybe<uint32_t> Seek(uint32_t aPos);
+
+ protected:
+  explicit SegmentIteratorUtf16(Span<const char16_t> aText);
+
+  // The text to iterate over.
+  Span<const char16_t> mText;
+
+  // The current break position within mText.
+  uint32_t mPos = 0;
+};
+
+// Each enum value has the same meaning with respect to the `word-break`
+// property values in the CSS Text spec. See the details in
+// https://drafts.csswg.org/css-text-3/#word-break-property
+enum class WordBreakRule : uint8_t {
+  Normal = 0,
+  BreakAll,
+  KeepAll,
+};
+
+// Each enum value has the same meaning with respect to the `line-break`
+// property values in the CSS Text spec. See the details in
+// https://drafts.csswg.org/css-text-3/#line-break-property.
+enum class LineBreakRule : uint8_t {
+  Auto = 0,
+  Loose,
+  Normal,
+  Strict,
+  Anywhere,
+};
+
+// Extra options for line break iterator.
+struct LineBreakOptions final {
+  WordBreakRule mWordBreakRule = WordBreakRule::Normal;
+  LineBreakRule mLineBreakRule = LineBreakRule::Auto;
+  bool mScriptIsChineseOrJapanese = false;
+};
+
+/**
+ * Line break iterator for UTF-16 text.
+ */
+class LineBreakIteratorUtf16 final : public SegmentIteratorUtf16 {
+ public:
+  explicit LineBreakIteratorUtf16(Span<const char16_t> aText,
+                                  const LineBreakOptions& aOptions = {});
+
+  Maybe<uint32_t> Next() override;
+
+ private:
+  LineBreakOptions mOptions;
+};
+
+/**
+ * Word break iterator for UTF-16 text.
+ */
+class WordBreakIteratorUtf16 final : public SegmentIteratorUtf16 {
+ public:
+  explicit WordBreakIteratorUtf16(Span<const char16_t> aText);
+
+  Maybe<uint32_t> Next() override;
+};
+
+/**
+ * Grapheme cluster break iterator for UTF-16 text.
+ */
+class GraphemeClusterBreakIteratorUtf16 final : public SegmentIteratorUtf16 {
+ public:
+  explicit GraphemeClusterBreakIteratorUtf16(Span<const char16_t> aText);
+
+  Maybe<uint32_t> Next() override;
+};
+
+/**
+ * Grapheme cluster break reverse iterator for UTF-16 text.
+ *
+ * Note: The reverse iterator doesn't handle conjoining Jamo and emoji. Use it
+ * at your own risk.
+ */
+class GraphemeClusterBreakReverseIteratorUtf16 final
+    : public SegmentIteratorUtf16 {
+ public:
+  explicit GraphemeClusterBreakReverseIteratorUtf16(Span<const char16_t> aText);
+
+  Maybe<uint32_t> Next() override;
+  Maybe<uint32_t> Seek(uint32_t aPos) override;
+};
+
+/**
+ * This component is a Mozilla-focused API for working with segmenters in
+ * internationalization code.
+ *
+ * This is a factor class. Calling Segment() to create an iterator over a text
+ * of given granularity.
+ */
+class Segmenter final {
+ public:
+  // NOTE: aLocale is a no-op currently.
+  static Result<UniquePtr<Segmenter>, ICUError> TryCreate(
+      Span<const char> aLocale, const SegmenterOptions& aOptions);
+
+  explicit Segmenter(Span<const char> aLocale, const SegmenterOptions& aOptions)
+      : mOptions(aOptions) {}
+
+  // Creates an iterator over aText of a given granularity in mOptions.
+  UniquePtr<SegmentIteratorUtf16> Segment(Span<const char16_t> aText) const;
+
+  // TODO: Implement an iterator for Latin1 text.
+  // UniquePtr<SegmentIteratorLatin1> Segment(Span<const uint8_t> aText) const;
+
+ private:
+  SegmenterOptions mOptions;
+};
+
+}  // namespace mozilla::intl
+
+#endif