Add digest set implementation

Set represents a unique set of digests which allow for efficient lookup.
Dumping short codes is a function which takes in a digest set.
Any operation involving short codes may be considered secure if the list of digests added to the set is the complete list of referenceable digests.
Contains benchmarks for Add, Lookup, and Dump.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

digestset/set.go

@@ -0,0 +1,195 @@
+package digest
+
+import (
+	"errors"
+	"sort"
+	"strings"
+)
+
+var (
+	// ErrDigestNotFound is used when a matching digest
+	// could not be found in a set.
+	ErrDigestNotFound = errors.New("digest not found")
+
+	// ErrDigestAmbiguous is used when multiple digests
+	// are found in a set. None of the matching digests
+	// should be considered valid matches.
+	ErrDigestAmbiguous = errors.New("ambiguous digest string")
+)
+
+// Set is used to hold a unique set of digests which
+// may be easily referenced by easily  referenced by a string
+// representation of the digest as well as short representation.
+// The uniqueness of the short representation is based on other
+// digests in the set. If digests are ommited from this set,
+// collisions in a larger set may not be detected, therefore it
+// is important to always do short representation lookups on
+// the complete set of digests. To mitigate collisions, an
+// appropriately long short code should be used.
+type Set struct {
+	entries digestEntries
+}
+
+// NewSet creates an empty set of digests
+// which may have digests added.
+func NewSet() *Set {
+	return &Set{
+		entries: digestEntries{},
+	}
+}
+
+// checkShortMatch checks whether two digests match as either whole
+// values or short values. This function does not test equality,
+// rather whether the second value could match against the first
+// value.
+func checkShortMatch(alg, hex, shortAlg, shortHex string) bool {
+	if len(hex) == len(shortHex) {
+		if hex != shortHex {
+			return false
+		}
+		if len(shortAlg) > 0 && alg != shortAlg {
+			return false
+		}
+	} else if !strings.HasPrefix(hex, shortHex) {
+		return false
+	} else if len(shortAlg) > 0 && alg != shortAlg {
+		return false
+	}
+	return true
+}
+
+// Lookup looks for a digest matching the given string representation.
+// If no digests could be found ErrDigestNotFound will be returned
+// with an empty digest value. If multiple matches are found
+// ErrDigestAmbiguous will be returned with an empty digest value.
+func (dst *Set) Lookup(d string) (Digest, error) {
+	if len(dst.entries) == 0 {
+		return "", ErrDigestNotFound
+	}
+	var (
+		searchFunc func(int) bool
+		alg        string
+		hex        string
+	)
+	dgst, err := ParseDigest(d)
+	if err == ErrDigestInvalidFormat {
+		hex = d
+		searchFunc = func(i int) bool {
+			return dst.entries[i].val >= d
+		}
+	} else {
+		hex = dgst.Hex()
+		alg = dgst.Algorithm()
+		searchFunc = func(i int) bool {
+			if dst.entries[i].val == hex {
+				return dst.entries[i].alg >= alg
+			}
+			return dst.entries[i].val >= hex
+		}
+	}
+	idx := sort.Search(len(dst.entries), searchFunc)
+	if idx == len(dst.entries) || !checkShortMatch(dst.entries[idx].alg, dst.entries[idx].val, alg, hex) {
+		return "", ErrDigestNotFound
+	}
+	if dst.entries[idx].alg == alg && dst.entries[idx].val == hex {
+		return dst.entries[idx].digest, nil
+	}
+	if idx+1 < len(dst.entries) && checkShortMatch(dst.entries[idx+1].alg, dst.entries[idx+1].val, alg, hex) {
+		return "", ErrDigestAmbiguous
+	}
+
+	return dst.entries[idx].digest, nil
+}
+
+// Add adds the given digests to the set. An error will be returned
+// if the given digest is invalid. If the digest already exists in the
+// table, this operation will be a no-op.
+func (dst *Set) Add(d Digest) error {
+	if err := d.Validate(); err != nil {
+		return err
+	}
+	entry := &digestEntry{alg: d.Algorithm(), val: d.Hex(), digest: d}
+	searchFunc := func(i int) bool {
+		if dst.entries[i].val == entry.val {
+			return dst.entries[i].alg >= entry.alg
+		}
+		return dst.entries[i].val >= entry.val
+	}
+	idx := sort.Search(len(dst.entries), searchFunc)
+	if idx == len(dst.entries) {
+		dst.entries = append(dst.entries, entry)
+		return nil
+	} else if dst.entries[idx].digest == d {
+		return nil
+	}
+
+	entries := append(dst.entries, nil)
+	copy(entries[idx+1:], entries[idx:len(entries)-1])
+	entries[idx] = entry
+	dst.entries = entries
+	return nil
+}
+
+// ShortCodeTable returns a map of Digest to unique short codes. The
+// length represents the minimum value, the maximum length may be the
+// entire value of digest if uniqueness cannot be achieved without the
+// full value. This function will attempt to make short codes as short
+// as possible to be unique.
+func ShortCodeTable(dst *Set, length int) map[Digest]string {
+	m := make(map[Digest]string, len(dst.entries))
+	l := length
+	resetIdx := 0
+	for i := 0; i < len(dst.entries); i++ {
+		var short string
+		extended := true
+		for extended {
+			extended = false
+			if len(dst.entries[i].val) <= l {
+				short = dst.entries[i].digest.String()
+			} else {
+				short = dst.entries[i].val[:l]
+				for j := i + 1; j < len(dst.entries); j++ {
+					if checkShortMatch(dst.entries[j].alg, dst.entries[j].val, "", short) {
+						if j > resetIdx {
+							resetIdx = j
+						}
+						extended = true
+					} else {
+						break
+					}
+				}
+				if extended {
+					l++
+				}
+			}
+		}
+		m[dst.entries[i].digest] = short
+		if i >= resetIdx {
+			l = length
+		}
+	}
+	return m
+}
+
+type digestEntry struct {
+	alg    string
+	val    string
+	digest Digest
+}
+
+type digestEntries []*digestEntry
+
+func (d digestEntries) Len() int {
+	return len(d)
+}
+
+func (d digestEntries) Less(i, j int) bool {
+	if d[i].val != d[j].val {
+		return d[i].val < d[j].val
+	}
+	return d[i].alg < d[j].alg
+}
+
+func (d digestEntries) Swap(i, j int) {
+	d[i], d[j] = d[j], d[i]
+}