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[rustc][data_structures] Simplify binary_search_slice. #114152

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Jul 30, 2023
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[rustc][data_structures] Simplify binary_search_slice. #114152

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38 changes: 7 additions & 31 deletions compiler/rustc_data_structures/src/binary_search_util/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -10,41 +10,17 @@ pub fn binary_search_slice<'d, E, K>(data: &'d [E], key_fn: impl Fn(&E) -> K, ke
where
K: Ord,
{
let Ok(mid) = data.binary_search_by_key(key, &key_fn) else {
let size = data.len();
let start = data.partition_point(|x| key_fn(x) < *key);
// At this point `start` either points at the first entry with equal or
// greater key or is equal to `size` in case all elements have smaller keys
if start == size || key_fn(&data[start]) != *key {
return &[];
};
let size = data.len();

// We get back *some* element with the given key -- so do
// a galloping search backwards to find the *first* one.
let mut start = mid;
let mut previous = mid;
let mut step = 1;
loop {
start = start.saturating_sub(step);
if start == 0 || key_fn(&data[start]) != *key {
break;
}
previous = start;
step *= 2;
}
step = previous - start;
while step > 1 {
let half = step / 2;
let mid = start + half;
if key_fn(&data[mid]) != *key {
start = mid;
}
step -= half;
}
// adjust by one if we have overshot
if start < size && key_fn(&data[start]) != *key {
start += 1;
}

// Now search forward to find the *last* one.
let mut end = mid;
let mut previous = mid;
let mut end = start;
let mut previous = start;
let mut step = 1;
loop {
end = end.saturating_add(step).min(size);
Expand Down