3 files changed, 105 insertions, 1 deletions
diff --git a/entity/id.go b/entity/id.go
index 9e724012..b602452e 100644
--- a/entity/id.go
+++ b/entity/id.go
@@ -65,7 +65,7 @@ func (i Id) MarshalGQL(w io.Writer) {
 
 // IsValid tell if the Id is valid
 func (i Id) Validate() error {
-	// Special case to
+	// Special case to detect outdated repo
 	if len(i) == 40 {
 		return fmt.Errorf("outdated repository format, please use https://github.com/MichaelMure/git-bug-migration to upgrade")
 	}
diff --git a/entity/id_interleaved.go b/entity/id_interleaved.go
new file mode 100644
index 00000000..5423afee
--- /dev/null
+++ b/entity/id_interleaved.go
@@ -0,0 +1,68 @@
+package entity
+
+import (
+	"strings"
+)
+
+// CombineIds compute a merged Id holding information from both the primary Id
+// and the secondary Id.
+//
+// This allow to later find efficiently a secondary element because we can access
+// the primary one directly instead of searching for a primary that has a
+// secondary matching the Id.
+//
+// An example usage is Comment in a Bug. The interleaved Id will hold part of the
+// Bug Id and part of the Comment Id.
+//
+// To allow the use of an arbitrary length prefix of this Id, Ids from primary
+// and secondary are interleaved with this irregular pattern to give the
+// best chance to find the secondary even with a 7 character prefix.
+//
+// Format is: PSPSPSPPPSPPPPSPPPPSPPPPSPPPPSPPPPSPPPPSPPPPSPPPPSPPPPSPPPPSPPPP
+//
+// A complete interleaved Id hold 50 characters for the primary and 14 for the
+// secondary, which give a key space of 36^50 for the primary (~6 * 10^77) and
+// 36^14 for the secondary (~6 * 10^21). This asymmetry assume a reasonable number
+// of secondary within a primary Entity, while still allowing for a vast key space
+// for the primary (that is, a globally merged database) with a low risk of collision.
+//
+// Here is the breakdown of several common prefix length:
+//
+// 5:    3P, 2S
+// 7:    4P, 3S
+// 10:   6P, 4S
+// 16:  11P, 5S
+func CombineIds(primary Id, secondary Id) Id {
+	var id strings.Builder
+
+	for i := 0; i < idLength; i++ {
+		switch {
+		default:
+			id.WriteByte(primary[0])
+			primary = primary[1:]
+		case i == 1, i == 3, i == 5, i == 9, i >= 10 && i%5 == 4:
+			id.WriteByte(secondary[0])
+			secondary = secondary[1:]
+		}
+	}
+
+	return Id(id.String())
+}
+
+// SeparateIds extract primary and secondary prefix from an arbitrary length prefix
+// of an Id created with CombineIds.
+func SeparateIds(prefix string) (primaryPrefix string, secondaryPrefix string) {
+	var primary strings.Builder
+	var secondary strings.Builder
+
+	for i, r := range prefix {
+		switch {
+		default:
+			primary.WriteRune(r)
+		case i == 1, i == 3, i == 5, i == 9, i >= 10 && i%5 == 4:
+			secondary.WriteRune(r)
+		}
+	}
+
+	return primary.String(), secondary.String()
+}
diff --git a/entity/id_interleaved_test.go b/entity/id_interleaved_test.go
new file mode 100644
index 00000000..ef9218c9
--- /dev/null
+++ b/entity/id_interleaved_test.go
@@ -0,0 +1,36 @@
+package entity
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestInterleaved(t *testing.T) {
+	primary := Id("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWX______________")
+	secondary := Id("YZ0123456789+/________________________________________________")
+	expectedId := Id("aYbZc0def1ghij2klmn3opqr4stuv5wxyz6ABCD7EFGH8IJKL9MNOP+QRST/UVWX")
+
+	interleaved := CombineIds(primary, secondary)
+	require.Equal(t, expectedId, interleaved)
+
+	// full length
+	splitPrimary, splitSecondary := SeparateIds(interleaved.String())
+	require.Equal(t, string(primary[:50]), splitPrimary)
+	require.Equal(t, string(secondary[:14]), splitSecondary)
+
+	// partial
+	splitPrimary, splitSecondary = SeparateIds(string(expectedId[:7]))
+	require.Equal(t, string(primary[:4]), splitPrimary)
+	require.Equal(t, string(secondary[:3]), splitSecondary)
+
+	// partial
+	splitPrimary, splitSecondary = SeparateIds(string(expectedId[:10]))
+	require.Equal(t, string(primary[:6]), splitPrimary)
+	require.Equal(t, string(secondary[:4]), splitSecondary)
+
+	// partial
+	splitPrimary, splitSecondary = SeparateIds(string(expectedId[:16]))
+	require.Equal(t, string(primary[:11]), splitPrimary)
+	require.Equal(t, string(secondary[:5]), splitSecondary)
+}