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util: improve streams.h:FindByte() performance

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Avoid use of the expensive mod operator (%) when calculating the
buffer offset. No functional difference.

Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>
This commit is contained in:
Larry Ruane
2020-08-10 15:23:03 -06:00
parent 604df63f6c
commit 72efc26439
5 changed files with 19 additions and 9 deletions
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20
src/streams.h
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@@ -756,15 +756,25 @@ public:
}
//! search for a given byte in the stream, and remain positioned on it
void FindByte(uint8_t ch)
void FindByte(std::byte byte)
{
// For best performance, avoid mod operation within the loop.
size_t buf_offset{size_t(m_read_pos % uint64_t(vchBuf.size()))};
while (true) {
if (m_read_pos == nSrcPos)
if (m_read_pos == nSrcPos) {
// No more bytes available; read from the file into the buffer,
// setting nSrcPos to one beyond the end of the new data.
// Throws exception if end-of-file reached.
Fill();
if (vchBuf[m_read_pos % vchBuf.size()] == std::byte{ch}) {
break;
}
m_read_pos++;
const size_t len{std::min<size_t>(vchBuf.size() - buf_offset, nSrcPos - m_read_pos)};
const auto it_start{vchBuf.begin() + buf_offset};
const auto it_find{std::find(it_start, it_start + len, byte)};
const size_t inc{size_t(std::distance(it_start, it_find))};
m_read_pos += inc;
if (inc < len) break;
buf_offset += inc;
if (buf_offset >= vchBuf.size()) buf_offset = 0;
}
}
};