From 0e1cd59547ae7e5f62d7f948205e867d82505105 Mon Sep 17 00:00:00 2001
From: Andre Bogus <bogusandre@gmail.com>
Date: Mon, 6 Jan 2020 19:51:01 +0100
Subject: [PATCH] Galloping search for binary_search_util
---
.../binary_search_util/mod.rs | 60 ++++++++++++-------
1 file changed, 40 insertions(+), 20 deletions(-)
diff --git a/src/librustc_data_structures/binary_search_util/mod.rs b/src/librustc_data_structures/binary_search_util/mod.rs
index 6d1e1abbcef..ede5757a479 100644
--- a/src/librustc_data_structures/binary_search_util/mod.rs
+++ b/src/librustc_data_structures/binary_search_util/mod.rs
@@ -14,35 +14,55 @@ where
Ok(mid) => mid,
Err(_) => return &[],
};
+ let size = data.len();
- // We get back *some* element with the given key -- so
- // search backwards to find the *first* one.
- //
- // (It'd be more efficient to use a "galloping" search
- // here, but it's not really worth it for small-ish
- // amounts of data.)
+ // We get back *some* element with the given key -- so do
+ // a galloping search backwards to find the *first* one.
let mut start = mid;
- while start > 0 {
- if key_fn(&data[start - 1]) == *key {
- start -= 1;
- } else {
+ 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.
- //
- // (It'd be more efficient to use a "galloping" search
- // here, but it's not really worth it for small-ish
- // amounts of data.)
- let mut end = mid + 1;
- let max = data.len();
- while end < max {
- if key_fn(&data[end]) == *key {
- end += 1;
- } else {
+ let mut end = mid;
+ let mut previous = mid;
+ let mut step = 1;
+ loop {
+ end = end.saturating_add(step).min(size);
+ if end == size || key_fn(&data[end]) != *key {
break;
}
+ previous = end;
+ step *= 2;
+ }
+ step = end - previous;
+ while step > 1 {
+ let half = step / 2;
+ let mid = end - half;
+ if key_fn(&data[mid]) != *key {
+ end = mid;
+ }
+ step -= half;
}
&data[start..end]