From f4ce6b4ee7c629b100c071e179af1bf22db73d70 Mon Sep 17 00:00:00 2001
From: Runxi Yu
Date: Thu, 5 Mar 2026 21:05:56 +0800
Subject: objectid, format/pack/ingest: Pack hash ID in algo

---
 format/pack/ingest/rev_write.go | 16 +---------------
 objectid/algorithms.go          | 26 ++++++++++++++++++--------
 2 files changed, 19 insertions(+), 23 deletions(-)

diff --git a/format/pack/ingest/rev_write.go b/format/pack/ingest/rev_write.go
index d7eab31f..afa9b4a9 100644
--- a/format/pack/ingest/rev_write.go
+++ b/format/pack/ingest/rev_write.go
@@ -3,8 +3,6 @@ package ingest
 import (
 	"encoding/binary"
 	"slices"
-
-	"codeberg.org/lindenii/furgit/objectid"
 )
 
 const (
@@ -47,7 +45,7 @@ func writeRev(state *ingestState) error {
 		return err
 	}
 
-	binary.BigEndian.PutUint32(scratch[:4], hashID(state.algo))
+	binary.BigEndian.PutUint32(scratch[:4], state.algo.PackHashID())
 
 	err = writeAndHash(state.revFile, hashImpl, scratch[:4])
 	if err != nil {
@@ -101,15 +99,3 @@ func buildPackOrder(state *ingestState) []int {
 
 	return out
 }
-
-// hashID converts object algorithm to pack hash-id encoding.
-func hashID(algo objectid.Algorithm) uint32 {
-	switch algo {
-	case objectid.AlgorithmSHA1:
-		return 1
-	case objectid.AlgorithmSHA256:
-		return 2
-	default:
-		return 0
-	}
-}
diff --git a/objectid/algorithms.go b/objectid/algorithms.go
index 0a8d4517..a07edf7b 100644
--- a/objectid/algorithms.go
+++ b/objectid/algorithms.go
@@ -19,17 +19,19 @@ const (
 )
 
 type algorithmDetails struct {
-	name string
-	size int
-	sum  func([]byte) ObjectID
-	new  func() hash.Hash
+	name       string
+	size       int
+	packHashID uint32
+	sum        func([]byte) ObjectID
+	new        func() hash.Hash
 }
 
 var algorithmTable = [...]algorithmDetails{
 	AlgorithmUnknown: {},
 	AlgorithmSHA1: {
-		name: "sha1",
-		size: sha1.Size,
+		name:       "sha1",
+		size:       sha1.Size,
+		packHashID: 1,
 		sum: func(data []byte) ObjectID {
 			sum := sha1.Sum(data) //#nosec G401
 
@@ -42,8 +44,9 @@ var algorithmTable = [...]algorithmDetails{
 		new: sha1.New,
 	},
 	AlgorithmSHA256: {
-		name: "sha256",
-		size: sha256.Size,
+		name:       "sha256",
+		size:       sha256.Size,
+		packHashID: 2,
 		sum: func(data []byte) ObjectID {
 			sum := sha256.Sum256(data)
 
@@ -107,6 +110,13 @@ func (algo Algorithm) HexLen() int {
 	return algo.Size() * 2
 }
 
+// PackHashID returns the Git pack/rev hash-id encoding for this algorithm.
+//
+// Unknown algorithms return 0.
+func (algo Algorithm) PackHashID() uint32 {
+	return algo.info().packHashID
+}
+
 // Sum computes an object ID from raw data using the selected algorithm.
 func (algo Algorithm) Sum(data []byte) ObjectID {
 	return algo.info().sum(data)
-- 
cgit v1.3.1-10-gc9f91