diff --git a/build/builder.go b/build/builder.go
index 503b2f5c..9f0c65c9 100644
--- a/build/builder.go
+++ b/build/builder.go
@@ -21,7 +21,6 @@ import (
 	"crypto/sha1"
 	"flag"
 	"fmt"
-	"io"
 	"log"
 	"net/url"
 	"os"
@@ -334,12 +333,6 @@ func (o *Options) SetDefaults() {
 	}
 }
 
-func hashString(s string) string {
-	h := sha1.New()
-	_, _ = io.WriteString(h, s)
-	return fmt.Sprintf("%x", h.Sum(nil))
-}
-
 // ShardName returns the name the given index shard.
 func (o *Options) shardName(n int) string {
 	return o.shardNameVersion(zoekt.IndexFormatVersion, n)
@@ -347,12 +340,7 @@ func (o *Options) shardName(n int) string {
 
 func (o *Options) shardNameVersion(version, n int) string {
 	prefix := url.QueryEscape(cmp.Or(o.ShardPrefix, o.RepositoryDescription.Name))
-	if len(prefix) > 200 {
-		prefix = prefix[:200] + hashString(prefix)[:8]
-	}
-	shardName := filepath.Join(o.IndexDir,
-		fmt.Sprintf("%s_v%d.%05d.zoekt", prefix, version, n))
-	return shardName
+	return zoekt.ShardName(o.IndexDir, prefix, version, n)
 }
 
 type IndexState string
diff --git a/cmd/zoekt-sourcegraph-indexserver/cleanup_test.go b/cmd/zoekt-sourcegraph-indexserver/cleanup_test.go
index 6514b8ff..5a1fa766 100644
--- a/cmd/zoekt-sourcegraph-indexserver/cleanup_test.go
+++ b/cmd/zoekt-sourcegraph-indexserver/cleanup_test.go
@@ -2,7 +2,6 @@ package main
 
 import (
 	"fmt"
-	"net/url"
 	"os"
 	"path/filepath"
 	"reflect"
@@ -23,7 +22,7 @@ func TestCleanup(t *testing.T) {
 		return shard{
 			RepoID:        fakeID(name),
 			RepoName:      name,
-			Path:          fmt.Sprintf("%s_v%d.%05d.zoekt", url.QueryEscape(name), 15, n),
+			Path:          zoekt.ShardName("", name, 15, n),
 			ModTime:       mtime,
 			RepoTombstone: false,
 		}
diff --git a/indexbuilder.go b/indexbuilder.go
index 966a92c5..027edf9f 100644
--- a/indexbuilder.go
+++ b/indexbuilder.go
@@ -583,3 +583,13 @@ func (t *DocChecker) clearTrigrams(maxTrigramCount int) {
 		delete(t.trigrams, key)
 	}
 }
+
+// ShardName returns the name of the shard for the given prefix, version, and
+// shard number.
+func ShardName(indexDir string, prefix string, version, n int) string {
+	prefix = url.QueryEscape(prefix)
+	if len(prefix) > 200 {
+		prefix = prefix[:200] + hashString(prefix)[:8]
+	}
+	return filepath.Join(indexDir, fmt.Sprintf("%s_v%d.%05d.zoekt", prefix, version, n))
+}
diff --git a/indexbuilder_test.go b/indexbuilder_test.go
new file mode 100644
index 00000000..7487a5ad
--- /dev/null
+++ b/indexbuilder_test.go
@@ -0,0 +1,49 @@
+package zoekt
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestShardName(t *testing.T) {
+	tests := []struct {
+		name     string
+		indexDir string
+		prefix   string
+		version  int
+		shardNum int
+		expected string
+	}{
+		{
+			name:     "short prefix",
+			indexDir: "index",
+			prefix:   "short",
+			version:  1,
+			shardNum: 42,
+			expected: "index/short_v1.00042.zoekt",
+		},
+		{
+			name:     "long prefix truncated",
+			indexDir: "index",
+			prefix:   strings.Repeat("a", 300),
+			version:  2,
+			shardNum: 1,
+			expected: "index/" + strings.Repeat("a", 200) + "003ef1ba" + "_v2.00001.zoekt",
+		},
+		{
+			name:     "empty indexDir",
+			prefix:   "short",
+			version:  1,
+			expected: "short_v1.00000.zoekt",
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			actual := ShardName(test.indexDir, test.prefix, test.version, test.shardNum)
+			if actual != test.expected {
+				t.Errorf("expected %q, got %q", test.expected, actual)
+			}
+		})
+	}
+}
diff --git a/merge.go b/merge.go
index 8b2f1734..0473663e 100644
--- a/merge.go
+++ b/merge.go
@@ -5,7 +5,6 @@ import (
 	"fmt"
 	"io"
 	"log"
-	"net/url"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -170,10 +169,10 @@ func explode(dstDir string, f IndexFile, ibFuncs ...indexBuilderFunc) (map[strin
 			prefix = ib.repoList[0].Name
 		}
 
-		fn := filepath.Join(dstDir, shardName(prefix, ib.indexFormatVersion, 0))
-		fnTmp := fn + ".tmp"
-		shardNames[fnTmp] = fn
-		return builderWriteAll(fnTmp, ib)
+		shardName := ShardName(dstDir, prefix, ib.indexFormatVersion, 0)
+		shardNameTmp := shardName + ".tmp"
+		shardNames[shardNameTmp] = shardName
+		return builderWriteAll(shardNameTmp, ib)
 	}
 
 	var ib *IndexBuilder
@@ -264,12 +263,3 @@ func hashString(s string) string {
 	_, _ = io.WriteString(h, s)
 	return fmt.Sprintf("%x", h.Sum(nil))
 }
-
-// copied from builder package to avoid circular imports.
-func shardName(prefix string, version, n int) string {
-	abs := url.QueryEscape(prefix)
-	if len(abs) > 200 {
-		abs = abs[:200] + hashString(abs)[:8]
-	}
-	return fmt.Sprintf("%s_v%d.%05d.zoekt", abs, version, n)
-}
diff --git a/read_test.go b/read_test.go
index e87c643a..7b3827b1 100644
--- a/read_test.go
+++ b/read_test.go
@@ -370,10 +370,10 @@ func TestBackwardsCompat(t *testing.T) {
 			t.Fatal(err)
 		}
 
-		outname := fmt.Sprintf("testdata/backcompat/new_v%d.%05d.zoekt", IndexFormatVersion, 0)
-		t.Log("writing new file", outname)
+		outName := ShardName("testdata/backcompat", "new", IndexFormatVersion, 0)
+		t.Log("writing new file", outName)
 
-		err = os.WriteFile(outname, buf.Bytes(), 0o644)
+		err = os.WriteFile(outName, buf.Bytes(), 0o644)
 		if err != nil {
 			t.Fatalf("Creating output file: %v", err)
 		}