5 files changed, 84 insertions, 4 deletions
diff --git a/bug/comment.go b/bug/comment.go
index 4c9d118e..1a9ca05a 100644
--- a/bug/comment.go
+++ b/bug/comment.go
@@ -1,6 +1,8 @@
 package bug
 
 import (
+	"strings"
+
 	"github.com/dustin/go-humanize"
 
 	"github.com/MichaelMure/git-bug/entity"
@@ -31,6 +33,50 @@ func (c Comment) Id() entity.Id {
 	return c.id
 }
 
+const compiledCommentIdFormat = "BCBCBCBBBCBBBBCBBBBCBBBBCBBBBCBBBBCBBBBC"
+
+// DeriveCommentId compute a merged Id for a comment holding information from
+// both the Bug's Id and the Comment's Id. This allow to later find efficiently
+// a Comment because we can access the bug directly instead of searching for a
+// Bug that has a Comment matching the Id.
+//
+// To allow the use of an arbitrary length prefix of this merged Id, Ids from Bug
+// and Comment are interleaved with this irregular pattern to give the best chance
+// to find the Comment even with a 7 character prefix.
+//
+// A complete merged Id hold 30 characters for the Bug and 10 for the Comment,
+// which give a key space of 36^30 for the Bug (~5 * 10^46) and 36^10 for the
+// Comment (~3 * 10^15). This asymmetry assume a reasonable number of Comment
+// within a Bug, while still allowing for a vast key space for Bug (that is, a
+// globally merged bug database) with a low risk of collision.
+func DeriveCommentId(bugId entity.Id, commentId entity.Id) entity.Id {
+	var id strings.Builder
+	for _, char := range compiledCommentIdFormat {
+		if char == 'B' {
+			id.WriteByte(bugId[0])
+			bugId = bugId[1:]
+		} else {
+			id.WriteByte(commentId[0])
+			commentId = commentId[1:]
+		}
+	}
+	return entity.Id(id.String())
+}
+
+func SplitCommentId(prefix string) (bugPrefix string, commentPrefix string) {
+	var bugIdPrefix strings.Builder
+	var commentIdPrefix strings.Builder
+
+	for i, char := range prefix {
+		if compiledCommentIdFormat[i] == 'B' {
+			bugIdPrefix.WriteRune(char)
+		} else {
+			commentIdPrefix.WriteRune(char)
+		}
+	}
+	return bugIdPrefix.String(), commentIdPrefix.String()
+}
+
 // FormatTimeRel format the UnixTime of the comment for human consumption
 func (c Comment) FormatTimeRel() string {
 	return humanize.Time(c.UnixTime.Time())
diff --git a/bug/comment_test.go b/bug/comment_test.go
new file mode 100644
index 00000000..423d10d8
--- /dev/null
+++ b/bug/comment_test.go
@@ -0,0 +1,27 @@
+package bug
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/MichaelMure/git-bug/entity"
+)
+
+func TestCommentId(t *testing.T) {
+	bugId := entity.Id("abcdefghijklmnopqrstuvwxyz1234__________")
+	opId := entity.Id("ABCDEFGHIJ______________________________")
+	expectedId := entity.Id("aAbBcCdefDghijEklmnFopqrGstuvHwxyzI1234J")
+
+	mergedId := DeriveCommentId(bugId, opId)
+	require.Equal(t, expectedId, mergedId)
+
+	// full length
+	splitBugId, splitCommentId := SplitCommentId(mergedId.String())
+	require.Equal(t, string(bugId[:30]), splitBugId)
+	require.Equal(t, string(opId[:10]), splitCommentId)
+
+	splitBugId, splitCommentId = SplitCommentId(string(expectedId[:6]))
+	require.Equal(t, string(bugId[:3]), splitBugId)
+	require.Equal(t, string(opId[:3]), splitCommentId)
+}
diff --git a/bug/op_add_comment.go b/bug/op_add_comment.go
index 3f19e42e..df426ee0 100644
--- a/bug/op_add_comment.go
+++ b/bug/op_add_comment.go
@@ -36,8 +36,9 @@ func (op *AddCommentOperation) Apply(snapshot *Snapshot) {
 	snapshot.addActor(op.Author)
 	snapshot.addParticipant(op.Author)
 
+	commentId := DeriveCommentId(snapshot.Id(), op.Id())
 	comment := Comment{
-		id:       op.Id(),
+		id:       commentId,
 		Message:  op.Message,
 		Author:   op.Author,
 		Files:    op.Files,
@@ -47,7 +48,7 @@ func (op *AddCommentOperation) Apply(snapshot *Snapshot) {
 	snapshot.Comments = append(snapshot.Comments, comment)
 
 	item := &AddCommentTimelineItem{
-		CommentTimelineItem: NewCommentTimelineItem(op.Id(), comment),
+		CommentTimelineItem: NewCommentTimelineItem(commentId, comment),
 	}
 
 	snapshot.Timeline = append(snapshot.Timeline, item)
diff --git a/bug/op_create.go b/bug/op_create.go
index 41e0fca1..15fb69b5 100644
--- a/bug/op_create.go
+++ b/bug/op_create.go
@@ -59,8 +59,9 @@ func (op *CreateOperation) Apply(snapshot *Snapshot) {
 
 	snapshot.Title = op.Title
 
+	commentId := DeriveCommentId(snapshot.Id(), op.Id())
 	comment := Comment{
-		id:       op.Id(),
+		id:       commentId,
 		Message:  op.Message,
 		Author:   op.Author,
 		UnixTime: timestamp.Timestamp(op.UnixTime),
@@ -72,7 +73,7 @@ func (op *CreateOperation) Apply(snapshot *Snapshot) {
 
 	snapshot.Timeline = []TimelineItem{
 		&CreateTimelineItem{
-			CommentTimelineItem: NewCommentTimelineItem(op.Id(), comment),
+			CommentTimelineItem: NewCommentTimelineItem(commentId, comment),
 		},
 	}
 }
diff --git a/bug/snapshot.go b/bug/snapshot.go
index 11df04b2..0005b930 100644
--- a/bug/snapshot.go
+++ b/bug/snapshot.go
@@ -28,6 +28,11 @@ type Snapshot struct {
 
 // Return the Bug identifier
 func (snap *Snapshot) Id() entity.Id {
+	if snap.id == "" {
+		// simply panic as it would be a coding error
+		// (using an id of a bug not stored yet)
+		panic("no id yet")
+	}
 	return snap.id
 }