ui/gfx/utf16_indexing.cc

   1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "ui/gfx/utf16_indexing.h"
   6
   7 #include "base/logging.h"
   8 #include "base/third_party/icu/icu_utf.h"
   9
  10 namespace gfx {
  11
  12 bool IsValidCodePointIndex(const base::string16& s, size_t index) {
  13   return index == 0 || index == s.length() ||
  14     !(CBU16_IS_TRAIL(s[index]) && CBU16_IS_LEAD(s[index - 1]));
  15 }
  16
  17 ptrdiff_t UTF16IndexToOffset(const base::string16& s, size_t base, size_t pos) {
  18   // The indices point between UTF-16 words (range 0 to s.length() inclusive).
  19   // In order to consistently handle indices that point to the middle of a
  20   // surrogate pair, we count the first word in that surrogate pair and not
  21   // the second. The test "s[i] is not the second half of a surrogate pair" is
  22   // "IsValidCodePointIndex(s, i)".
  23   DCHECK_LE(base, s.length());
  24   DCHECK_LE(pos, s.length());
  25   ptrdiff_t delta = 0;
  26   while (base < pos)
  27     delta += IsValidCodePointIndex(s, base++) ? 1 : 0;
  28   while (pos < base)
  29     delta -= IsValidCodePointIndex(s, pos++) ? 1 : 0;
  30   return delta;
  31 }
  32
  33 size_t UTF16OffsetToIndex(const base::string16& s,
  34                           size_t base,
  35                           ptrdiff_t offset) {
  36   DCHECK_LE(base, s.length());
  37   // As in UTF16IndexToOffset, we count the first half of a surrogate pair, not
  38   // the second. When stepping from pos to pos+1 we check s[pos:pos+1] == s[pos]
  39   // (Python syntax), hence pos++. When stepping from pos to pos-1 we check
  40   // s[pos-1], hence --pos.
  41   size_t pos = base;
  42   while (offset > 0 && pos < s.length())
  43     offset -= IsValidCodePointIndex(s, pos++) ? 1 : 0;
  44   while (offset < 0 && pos > 0)
  45     offset += IsValidCodePointIndex(s, --pos) ? 1 : 0;
  46   // If offset != 0 then we ran off the edge of the string, which is a contract
  47   // violation but is handled anyway (by clamping) in release for safety.
  48   DCHECK_EQ(offset, 0);
  49   // Since the second half of a surrogate pair has "length" zero, there is an
  50   // ambiguity in the returned position. Resolve it by always returning a valid
  51   // index.
  52   if (!IsValidCodePointIndex(s, pos))
  53     ++pos;
  54   return pos;
  55 }
  56
  57 }  // namespace gfx