ceremonyclient/pebble/internal/keyspan/seek.go

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2024-01-03 07:31:42 +00:00
// Copyright 2018 The LevelDB-Go and Pebble Authors. All rights reserved. Use
// of this source code is governed by a BSD-style license that can be found in
// the LICENSE file.
package keyspan
import "github.com/cockroachdb/pebble/internal/base"
// SeekLE seeks to the span that contains or is before the target key.
func SeekLE(cmp base.Compare, iter FragmentIterator, key []byte) *Span {
// NB: We use SeekLT in order to land on the proper span for a search
// key that resides in the middle of a span. Consider the scenario:
//
// a---e
// e---i
//
// The spans are indexed by their start keys `a` and `e`. If the
// search key is `c` we want to land on the span [a,e). If we were to
// use SeekGE then the search key `c` would land on the span [e,i) and
// we'd have to backtrack. The one complexity here is what happens for the
// search key `e`. In that case SeekLT will land us on the span [a,e)
// and we'll have to move forward.
iterSpan := iter.SeekLT(key)
if iterSpan == nil {
// Advance the iterator once to see if the next span has a start key
// equal to key.
iterSpan = iter.Next()
if iterSpan == nil || cmp(key, iterSpan.Start) < 0 {
// The iterator is exhausted or we've hit the next span.
return nil
}
} else {
// Invariant: key > iterSpan.Start
if cmp(key, iterSpan.End) >= 0 {
// The current span lies entirely before the search key. Check to see if
// the next span contains the search key. If it doesn't, we'll backup
// and return to our earlier candidate.
iterSpan = iter.Next()
if iterSpan == nil || cmp(key, iterSpan.Start) < 0 {
// The next span is past our search key or there is no next span. Go
// back.
iterSpan = iter.Prev()
}
}
}
return iterSpan
}