3 files changed, 78 insertions, 8 deletions
diff --git a/skate/schema.go b/skate/schema.go
index 10504fe..6b335a3 100644
--- a/skate/schema.go
+++ b/skate/schema.go
@@ -1,6 +1,9 @@
 package skate
 
 import (
+	"bytes"
+	"crypto/sha1"
+	"encoding/gob"
 	"fmt"
 	"regexp"
 	"strconv"
@@ -350,6 +353,34 @@ type BiblioRef struct {
 	// } `json:"extra,omitempty"`
 }
 
+// Hash returns a string that will be the same, if source and target are
+// equals; different otherwise.
+func (b *BiblioRef) LinkHash() string {
+	switch {
+	case b.SourceReleaseIdent != "" && b.TargetReleaseIdent != "":
+		return fmt.Sprintf("fc:%s--fc:%s", b.SourceReleaseIdent, b.TargetReleaseIdent)
+	case b.SourceReleaseIdent != "" && b.TargetOpenLibraryWork != "":
+		return fmt.Sprintf("fc:%s--ol:%s", b.SourceReleaseIdent, b.TargetOpenLibraryWork)
+	case b.SourceReleaseIdent != "" && b.TargetURL != "":
+		return fmt.Sprintf("fc:%s--wb:%s", b.SourceReleaseIdent, b.TargetURL)
+	case b.SourceReleaseIdent != "" && b.TargetURLSurt != "":
+		return fmt.Sprintf("fc:%s--wb:%s", b.SourceReleaseIdent, b.TargetURLSurt)
+	default:
+		var (
+			buf bytes.Buffer
+			enc = gob.NewEncoder(&buf)
+			h   = sha1.New()
+		)
+		if err := enc.Encode(b); err != nil {
+			return ""
+		}
+		if _, err := buf.WriteTo(h); err != nil {
+			return ""
+		}
+		return fmt.Sprintf("%x", h.Sum(nil))
+	}
+}
+
 // ReleaseCluster, a list of match candidates. This is typically serialized as a
 // single JSON line containing the match key and a list of release documents.
 //
diff --git a/skate/schema_test.go b/skate/schema_test.go
index 6cf4559..1301184 100644
--- a/skate/schema_test.go
+++ b/skate/schema_test.go
@@ -213,6 +213,49 @@ func TestParseIsbn(t *testing.T) {
 	}
 }
 
+func TestLinkHash(t *testing.T) {
+	var cases = []struct {
+		bref     BiblioRef
+		linkHash string
+	}{
+		{
+			bref:     BiblioRef{},
+			linkHash: "e39b4dd927dbfb19c8aa78b10f684b0a5c35fcca",
+		},
+		{
+			bref:     BiblioRef{SourceReleaseIdent: "123"},
+			linkHash: "6d244189489b57c8f70ac7a835f1bacfee00c42a",
+		},
+		{
+			bref: BiblioRef{
+				SourceReleaseIdent: "123",
+				TargetReleaseIdent: "456",
+			},
+			linkHash: "fc:123--fc:456",
+		},
+		{
+			bref: BiblioRef{
+				SourceReleaseIdent:    "123",
+				TargetOpenLibraryWork: "/works/OL456M",
+			},
+			linkHash: "fc:123--ol:/works/OL456M",
+		},
+		{
+			bref: BiblioRef{
+				SourceReleaseIdent: "123",
+				TargetURL:          "http://fatcat.wiki",
+			},
+			linkHash: "fc:123--wb:http://fatcat.wiki",
+		},
+	}
+	for _, c := range cases {
+		result := c.bref.LinkHash()
+		if result != c.linkHash {
+			t.Fatalf("got %v, want %v", result, c.linkHash)
+		}
+	}
+}
+
 func BenchmarkParseIsbn(b *testing.B) {
 	for n := 0; n < b.N; n++ {
 		ParseIsbn("House Pvt. Limited., (2006), ISBN 9788183561426. Date accessed: August 2015.")
diff --git a/skate/zippy.go b/skate/zippy.go
index a6c5083..d71d57e 100644
--- a/skate/zippy.go
+++ b/skate/zippy.go
@@ -411,20 +411,16 @@ func uniqueMatches(docs []string, stats *statsAugment) (result []*BiblioRef, err
 	sort.Slice(brefs, func(i, j int) bool {
 		return brefs[i].MatchStatus == StatusExact.Short()
 	})
-	// We consider a match unique, if source and target match.
-	hash := func(bref *BiblioRef) string {
-		return bref.SourceReleaseIdent + "-" + bref.TargetReleaseIdent
-	}
 	seen := set.New()
 	for _, doc := range brefs {
-		v := hash(doc)
-		if seen.Contains(v) {
+		h := doc.LinkHash()
+		if seen.Contains(h) {
 			stats.skipDuplicatedBref++
 			log.Printf("skip-dup-bref [%d]: hash=%v source=%v status=%v reason=%v",
-				stats.skipDuplicatedBref, v, doc.SourceReleaseIdent, doc.MatchStatus, doc.MatchReason)
+				stats.skipDuplicatedBref, h, doc.SourceReleaseIdent, doc.MatchStatus, doc.MatchReason)
 			continue
 		}
-		seen.Add(v)
+		seen.Add(h)
 		result = append(result, doc)
 	}
 	return result, nil