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registry/storage: more efficient path compare in catalog

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Previous component-wise path comparison is recursive and generates a
large amount of garbage. This more efficient version simply replaces the
path comparison with the zero-value to sort before everything. We do
this by replacing the byte-wise comparison that swaps a single character
inline for the separator comparison, such that separators sort first.

The resulting implementation provides component-wise path comparison
with no cost incurred for allocation or stack frame.

Direction of the comparison is also reversed to match Go style.

Signed-off-by: Stephen J Day <stephen.day@docker.com>
This commit is contained in:
Stephen J Day 2016-08-05 17:18:43 -07:00 committed by Richard Scothern
parent fdc51bb1f2
commit 2aa09ff9a8
1 changed files with 43 additions and 16 deletions
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59
registry/storage/catalog.go
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@ -38,7 +38,7 @@ func (reg *registry) Repositories(ctx context.Context, repos []string, last stri
_, file := path.Split(repoPath)
if file == "_layers" {
repoPath = strings.TrimSuffix(repoPath, "/_layers")
if pathGreaterThan(repoPath, last) {
if lessPath(last, repoPath) {
foundRepos = append(foundRepos, repoPath)
}
return ErrSkipDir
@ -100,22 +100,49 @@ func (reg *registry) Enumerate(ctx context.Context, ingester func(string) error)
}
func pathGreaterThan(pathX, pathY string) (b bool) {
splitPathX := strings.SplitN(pathX, "/", 2)
splitPathY := strings.SplitN(pathY, "/", 2)
if splitPathX[0] == splitPathY[0] {
if len(splitPathX) == 1 && len(splitPathY) == 1 {
return false
} else if len(splitPathX) == 1 && len(splitPathY) != 1 {
return false
} else if len(splitPathX) != 1 && len(splitPathY) == 1 {
return true
}
return pathGreaterThan(splitPathX[1], splitPathY[1])
// lessPath returns true if one path a is less than path b.
//
// A component-wise comparison is done, rather than the lexical comparison of
// strings.
func lessPath(a, b string) bool {
// we provide this behavior by making separator always sort first.
return compareReplaceInline(a, b, '/', '\x00') < 0
}
// compareReplaceInline modifies runtime.cmpstring to replace old with new
// during a byte-wise comparison.
func compareReplaceInline(s1, s2 string, old, new byte) int {
l := len(s1)
if len(s2) < l {
l = len(s2)
}
return splitPathX[0] > splitPathY[0]
for i := 0; i < l; i++ {
c1, c2 := s1[i], s2[i]
if c1 == old {
c1 = new
}
if c2 == old {
c2 = new
}
if c1 < c2 {
return -1
}
if c1 > c2 {
return +1
}
}
if len(s1) < len(s2) {
return -1
}
if len(s1) > len(s2) {
return +1
}
return 0
}