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
10namespace gfx {
11
12bool 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
17ptrdiff_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
33size_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
58