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// Copyright 2012 The Chromium Authors
// 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 <string_view>
#include "base/check_op.h"
#include "base/third_party/icu/icu_utf.h"
namespace gfx {
bool IsValidCodePointIndex(std::u16string_view s, size_t index) {
return index == 0 || index == s.length() ||
!(CBU16_IS_TRAIL(s[index]) && CBU16_IS_LEAD(s[index - 1]));
}
ptrdiff_t UTF16IndexToOffset(std::u16string_view 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(std::u16string_view 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
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