Use branchless binary search to find_min_version
Constant search time and no unpredictable conditional branches Benchmark results: 39 ns/iter -> 2 ns/iter (including multiple call) Based on https://github.com/rust-lang/rust/pull/45333 Inspired by https://raw.githubusercontent.com/RustStudy/rust_daily_news/master/img/%E4%BA%8C%E7%BB%B4%E7%A0%81%E7%94%9F%E6%88%90.png
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39
src/bits.rs
39
src/bits.rs
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@ -3,7 +3,7 @@
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use std::cmp::min;
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#[cfg(feature = "bench")]
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use test::Bencher;
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use test::{black_box, Bencher};
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use cast::{As, Truncate};
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use optimize::{total_encoded_len, Optimizer, Parser, Segment};
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@ -859,17 +859,20 @@ pub fn encode_auto(data: &[u8], ec_level: EcLevel) -> QrResult<Bits> {
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/// Finds the smallest version (QR code only) that can store N bits of data
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/// in the given error correction level.
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fn find_min_version(length: usize, ec_level: EcLevel) -> Version {
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let mut min = 0;
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let mut max = 39;
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while min < max {
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let half = (min + max) / 2;
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if DATA_LENGTHS[half][ec_level as usize] < length {
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min = half + 1;
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} else {
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max = half;
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}
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let mut base = 0usize;
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let mut size = 39;
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while size > 1 {
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let half = size / 2;
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let mid = base + half;
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// mid is always in [0, size).
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// mid >= 0: by definition
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// mid < size: mid = size / 2 + size / 4 + size / 8 ...
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base = if DATA_LENGTHS[mid][ec_level as usize] > length { base } else { mid };
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size -= half;
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}
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Version::Normal((min + 1).as_i16())
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// base is always in [0, mid) because base <= mid.
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base = if DATA_LENGTHS[base][ec_level as usize] >= length { base } else { base + 1 };
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Version::Normal((base + 1).as_i16())
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}
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#[cfg(test)]
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@ -907,5 +910,19 @@ mod encode_auto_tests {
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}
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}
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#[cfg(feature = "bench")]
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#[bench]
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fn bench_find_min_version(bencher: &mut Bencher) {
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bencher.iter(|| {
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black_box(find_min_version(60, EcLevel::L));
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black_box(find_min_version(200, EcLevel::L));
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black_box(find_min_version(200, EcLevel::H));
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black_box(find_min_version(20000, EcLevel::L));
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black_box(find_min_version(640, EcLevel::L));
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black_box(find_min_version(641, EcLevel::L));
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black_box(find_min_version(999999, EcLevel::H));
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})
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}
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//}}}
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//------------------------------------------------------------------------------
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