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
IsValidCodePointIndex(const base::string16 & s,size_t index)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
UTF16IndexToOffset(const base::string16 & s,size_t base,size_t pos)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
UTF16OffsetToIndex(const base::string16 & s,size_t base,ptrdiff_t offset)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
58