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5cdcc99eba
Since backend.ID is always a slice of constant length, use an array
instead of a slice. Mostly, arrays behave as slices, except that an
array cannot be nil, so use `*backend.ID` insteaf of `backend.ID` in
places where the absence of an ID is possible (e.g. for the Subtree of a
Node, which may not present when the node is a file node).
This change allows to directly use backend.ID as the the key for a map,
so that arbitrary data structures (e.g. a Set implemented as a
map[backend.ID]struct{}) can easily be formed.
121 lines
2.3 KiB
Go
121 lines
2.3 KiB
Go
package backend
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import (
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"crypto/sha256"
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"errors"
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"io"
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)
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const (
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MinPrefixLength = 8
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)
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var (
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ErrNoIDPrefixFound = errors.New("no ID found")
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ErrMultipleIDMatches = errors.New("multiple IDs with prefix found")
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)
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var (
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hashData = sha256.Sum256
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)
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const hashSize = sha256.Size
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// Hash returns the ID for data.
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func Hash(data []byte) ID {
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return hashData(data)
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}
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// Find loads the list of all blobs of type t and searches for names which
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// start with prefix. If none is found, nil and ErrNoIDPrefixFound is returned.
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// If more than one is found, nil and ErrMultipleIDMatches is returned.
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func Find(be Lister, t Type, prefix string) (string, error) {
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done := make(chan struct{})
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defer close(done)
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match := ""
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// TODO: optimize by sorting list etc.
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for name := range be.List(t, done) {
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if prefix == name[:len(prefix)] {
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if match == "" {
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match = name
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} else {
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return "", ErrMultipleIDMatches
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}
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}
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}
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if match != "" {
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return match, nil
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}
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return "", ErrNoIDPrefixFound
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}
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// PrefixLength returns the number of bytes required so that all prefixes of
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// all names of type t are unique.
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func PrefixLength(be Lister, t Type) (int, error) {
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done := make(chan struct{})
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defer close(done)
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// load all IDs of the given type
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list := make([]string, 0, 100)
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for name := range be.List(t, done) {
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list = append(list, name)
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}
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// select prefixes of length l, test if the last one is the same as the current one
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outer:
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for l := MinPrefixLength; l < IDSize; l++ {
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var last string
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for _, name := range list {
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if last == name[:l] {
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continue outer
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}
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last = name[:l]
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}
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return l, nil
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}
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return IDSize, nil
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}
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// wrap around io.LimitedReader that implements io.ReadCloser
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type blobReader struct {
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cl io.Closer
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rd io.Reader
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closed bool
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}
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func (l *blobReader) Read(p []byte) (int, error) {
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n, err := l.rd.Read(p)
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if err == io.EOF {
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l.Close()
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}
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return n, err
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}
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func (l *blobReader) Close() error {
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if l == nil {
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return nil
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}
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if !l.closed {
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err := l.cl.Close()
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l.closed = true
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return err
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}
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return nil
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}
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// LimitReadCloser returns a new reader wraps r in an io.LimitReader, but also
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// implements the Close() method.
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func LimitReadCloser(r io.ReadCloser, n int64) *blobReader {
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return &blobReader{cl: r, rd: io.LimitReader(r, n)}
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}
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