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Diffstat (limited to 'ui/gfx/utf16_indexing.cc')
-rw-r--r-- | ui/gfx/utf16_indexing.cc | 55 |
1 files changed, 55 insertions, 0 deletions
diff --git a/ui/gfx/utf16_indexing.cc b/ui/gfx/utf16_indexing.cc new file mode 100644 index 0000000..6af10c1 --- /dev/null +++ b/ui/gfx/utf16_indexing.cc @@ -0,0 +1,55 @@ +// 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/gfx/utf16_indexing.h" + +#include "base/logging.h" +#include "base/third_party/icu/icu_utf.h" + +namespace gfx { + +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 gfx |