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Using linear search

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This commit is contained in:
Paul Masurel
2019-01-29 15:59:24 +01:00
parent 4c93b096eb
commit 259ce567d1
1 changed files with 8 additions and 30 deletions
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38
src/postings/segment_postings.rs
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@@ -123,32 +123,10 @@ impl SegmentPostings {
}
}
// target is assumed to be <= to the last element of arr.
//
// returns i such that
// - arr[res - 1] < target
// - arr[res] >= target
fn binary_search(arr: &[u32], mut base: usize, mut len: usize, target: u32) -> usize {
while len > 1 {
let half = len / 2;
let mid = base + half;
let pivot = *unsafe { arr.get_unchecked(mid) };
base = if pivot > target { base } else { mid };
// Unfortunately, rustc does not compiles this to a conditional mov.
// since rustc 1.25.
//
// See https://github.com/rust-lang/rust/issues/53823 for detail
//
// unsafe {
// let pivot: u32 = *arr.get_unchecked(mid);
// asm!("cmpl $2, $1\ncmovge $3, $0"
// : "+r"(base)
// : "r"(target), "r"(pivot), "r"(mid))
// ;
// }
len -= half;
}
base + ((*unsafe { arr.get_unchecked(base) } < target) as usize)
fn linear_search(arr: &[u32], target: u32) -> usize {
arr.iter()
.map(|&el| if el < target { 1 } else { 0 })
.sum()
}
fn exponential_search(arr: &[u32], target: u32) -> (usize, usize) {
@@ -175,7 +153,7 @@ fn exponential_search(arr: &[u32], target: u32) -> (usize, usize) {
/// The target is assumed smaller or equal to the last element.
fn search_within_block(block_docs: &[u32], target: u32) -> usize {
let (start, end) = exponential_search(block_docs, target);
binary_search(&block_docs, start, end - start, target)
start + linear_search(&block_docs[start..end], target)
}
impl DocSet for SegmentPostings {
@@ -641,7 +619,7 @@ impl<'b> Streamer<'b> for BlockSegmentPostings {
#[cfg(test)]
mod tests {
use super::binary_search;
use super::linear_search;
use super::exponential_search;
use super::search_within_block;
use super::BlockSegmentPostings;
@@ -659,11 +637,11 @@ mod tests {
use SkipResult;
#[test]
fn test_binary_search() {
fn test_linear_search() {
let len: usize = 50;
let arr: Vec<u32> = (0..len).map(|el| 1u32 + (el as u32) * 2).collect();
for target in 1..*arr.last().unwrap() {
let res = binary_search(&arr[..], 0, len, target);
let res = linear_search(&arr[..], target);
if res > 0 {
assert!(arr[res - 1] < target);
}