text: // Copyright (c) 2012 The Chromium Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style license that can be
  // found in the LICENSE file.
  
  #include "ui/base/text/utf16_indexing.h"
  
  #include "base/logging.h"
  #include "base/third_party/icu/icu_utf.h"
  
  namespace ui {
  
  bool IsValidCodePointIndex(const string16& s, size_t index) {
    return index == 0 || index == s.length() ||
      !(CBU16_IS_TRAIL(s[index]) && CBU16_IS_LEAD(s[index - 1]));
  }
  
  ptrdiff_t UTF16IndexToOffset(const string16& s, size_t base, size_t pos) {
    // The indices point between UTF-16 words (range 0 to s.length() inclusive).
    // In order to consistently handle indices that point to the middle of a
    // surrogate pair, we count the first word in that surrogate pair and not
    // the second. The test "s[i] is not the second half of a surrogate pair" is
    // "IsValidCodePointIndex(s, i)".
    DCHECK_LE(base, s.length());
    DCHECK_LE(pos, s.length());
    ptrdiff_t delta = 0;
    while (base < pos)
      delta += IsValidCodePointIndex(s, base++) ? 1 : 0;
    while (pos < base)
      delta -= IsValidCodePointIndex(s, pos++) ? 1 : 0;
    return delta;
  }
  
  size_t UTF16OffsetToIndex(const string16& s, size_t base, ptrdiff_t offset) {
    DCHECK_LE(base, s.length());
    // As in UTF16IndexToOffset, we count the first half of a surrogate pair, not
    // the second. When stepping from pos to pos+1 we check s[pos:pos+1] == s[pos]
    // (Python syntax), hence pos++. When stepping from pos to pos-1 we check
    // s[pos-1], hence --pos.
    size_t pos = base;
    while (offset > 0 && pos < s.length())
      offset -= IsValidCodePointIndex(s, pos++) ? 1 : 0;
    while (offset < 0 && pos > 0)
      offset += IsValidCodePointIndex(s, --pos) ? 1 : 0;
    // If offset != 0 then we ran off the edge of the string, which is a contract
    // violation but is handled anyway (by clamping) in release for safety.
    DCHECK_EQ(offset, 0);
    // Since the second half of a surrogate pair has "length" zero, there is an
    // ambiguity in the returned position. Resolve it by always returning a valid
    // index.
    if (!IsValidCodePointIndex(s, pos))
      ++pos;
    return pos;
  }
  
  }  // namespace ui
  