// is doing the same linear scan anyway -- no increase in complexity.
// We combine this scan with a scan for duplicates that we can merge, to reduce
// the number of elements.
+ // We stop searching after the first "block" of size 1, to avoid spending excessive
+ // amounts of time on the merging.
let mut equal_since_idx = first_idx;
+ // Once we see too many non-mergeable blocks, we stop.
+ // The initial value is chosen via... magic. Benchmarking and magic.
+ let mut successful_merge_count = 3usize;
let mut end_idx = first_idx; // when the loop is done, this is the first excluded element.
loop {
// Compute if `end` is the last element we need to look at.
end_idx += 1;
debug_assert!(done || end_idx < self.v.len(), "iter_mut: end-offset {} is out-of-bounds", offset+len);
// see if we want to merge everything in `equal_since..end` (exclusive at the end!)
- if done || self.v[end_idx].data != self.v[equal_since_idx].data {
- // Everything in `equal_since..end` was equal. Make them just one element covering
- // the entire range.
- let equal_elems = end_idx - equal_since_idx; // number of equal elements
- if equal_elems > 1 {
- // Adjust the range of the first element to cover all of them.
- let equal_until = self.v[end_idx - 1].range.end; // end of range of last of the equal elements
- self.v[equal_since_idx].range.end = equal_until;
- // Delete the rest of them.
- self.v.splice(equal_since_idx+1..end_idx, std::iter::empty());
- // Adjust `end_idx` because we made the list shorter.
- end_idx -= (equal_elems - 1);
+ if successful_merge_count > 0 {
+ if done || self.v[end_idx].data != self.v[equal_since_idx].data {
+ // Everything in `equal_since..end` was equal. Make them just one element covering
+ // the entire range.
+ let removed_elems = end_idx - equal_since_idx - 1; // number of elements that we would remove
+ if removed_elems > 0 {
+ // Adjust the range of the first element to cover all of them.
+ let equal_until = self.v[end_idx - 1].range.end; // end of range of last of the equal elements
+ self.v[equal_since_idx].range.end = equal_until;
+ // Delete the rest of them.
+ self.v.splice(equal_since_idx+1..end_idx, std::iter::empty());
+ // Adjust `end_idx` because we made the list shorter.
+ end_idx -= removed_elems;
+ // adjust the count for the cutoff
+ successful_merge_count += removed_elems;
+ } else {
+ // adjust the count for the cutoff
+ successful_merge_count -= 1;
+ }
+ // Go on scanning for the next block starting here.
+ equal_since_idx = end_idx;
}
- // Go on scanning.
- equal_since_idx = end_idx;
}
// Leave loop if this is the last element.
if done {