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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
pull/28/head
Ivan Tham 2019-07-06 23:24:26 +08:00
parent a585403336
commit 81404acbbd
1 changed files with 28 additions and 11 deletions
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39
src/bits.rs
View File

@ -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};
@ -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;
}
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;
}
Version::Normal((min + 1).as_i16())
// 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)]
@ -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));
})
}
//}}}
//------------------------------------------------------------------------------