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Use branchless binary search to find_min_version #28

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Jul 6, 2019
Merged

Use branchless binary search to find_min_version #28

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41 changes: 29 additions & 12 deletions src/bits.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,7 +3,7 @@
use std::cmp::min;

#[cfg(feature = "bench")]
use test::Bencher;
use test::{black_box, Bencher};

use cast::{As, Truncate};
use optimize::{total_encoded_len, Optimizer, Parser, Segment};
Expand Down Expand Up @@ -859,17 +859,20 @@ pub fn encode_auto(data: &[u8], ec_level: EcLevel) -> QrResult<Bits> {
/// Finds the smallest version (QR code only) that can store N bits of data
/// in the given error correction level.
fn find_min_version(length: usize, ec_level: EcLevel) -> Version {
let mut min = 0;
let mut max = 39;
while min < max {
let half = (min + max) / 2;
if DATA_LENGTHS[half][ec_level as usize] < length {
min = half + 1;
} else {
max = half;
}
}
Version::Normal((min + 1).as_i16())
let mut base = 0usize;
let mut size = 39;
while size > 1 {
let half = size / 2;
let mid = base + half;
// mid is always in [0, size).
// mid >= 0: by definition
// mid < size: mid = size / 2 + size / 4 + size / 8 ...
base = if DATA_LENGTHS[mid][ec_level as usize] > length { base } else { mid };
size -= half;
}
// base is always in [0, mid) because base <= mid.
base = if DATA_LENGTHS[base][ec_level as usize] >= length { base } else { base + 1 };
Version::Normal((base + 1).as_i16())
}

#[cfg(test)]
Expand Down Expand Up @@ -907,5 +910,19 @@ mod encode_auto_tests {
}
}

#[cfg(feature = "bench")]
#[bench]
fn bench_find_min_version(bencher: &mut Bencher) {
bencher.iter(|| {
black_box(find_min_version(60, EcLevel::L));
black_box(find_min_version(200, EcLevel::L));
black_box(find_min_version(200, EcLevel::H));
black_box(find_min_version(20000, EcLevel::L));
black_box(find_min_version(640, EcLevel::L));
black_box(find_min_version(641, EcLevel::L));
black_box(find_min_version(999999, EcLevel::H));
})
}

//}}}
//------------------------------------------------------------------------------