format/commitgraph: Add initial commit-graph support

1 files changed, 85 insertions, 0 deletions

diff --git a/format/commitgraph/commitat.go b/format/commitgraph/commitat.go
new file mode 100644
index 00000000..4d5853b1
--- /dev/null
+++ b/format/commitgraph/commitat.go
@@ -0,0 +1,85 @@
+package commitgraph
+
+import (
+	"encoding/binary"
+
+	"codeberg.org/lindenii/furgit/internal/intconv"
+	"codeberg.org/lindenii/furgit/objectid"
+)
+
+// CommitAt returns decoded commit-graph metadata at one position.
+func (reader *Reader) CommitAt(pos Position) (Commit, error) {
+	layer, err := reader.layerByPosition(pos)
+	if err != nil {
+		return Commit{}, err
+	}
+
+	hashSize := reader.algo.Size()
+	stride := hashSize + 16
+
+	strideU64, err := intconv.IntToUint64(stride)
+	if err != nil {
+		return Commit{}, err
+	}
+
+	start64 := uint64(pos.Index) * strideU64
+	end64 := start64 + strideU64
+
+	start, err := intconv.Uint64ToInt(start64)
+	if err != nil {
+		return Commit{}, err
+	}
+
+	end, err := intconv.Uint64ToInt(end64)
+	if err != nil {
+		return Commit{}, err
+	}
+
+	record := layer.chunkCommit[start:end]
+
+	treeOID, err := objectid.FromBytes(reader.algo, record[:hashSize])
+	if err != nil {
+		return Commit{}, err
+	}
+
+	oid, err := reader.OIDAt(pos)
+	if err != nil {
+		return Commit{}, err
+	}
+
+	p1 := binary.BigEndian.Uint32(record[hashSize : hashSize+4])
+	p2 := binary.BigEndian.Uint32(record[hashSize+4 : hashSize+8])
+	genAndTimeHi := binary.BigEndian.Uint32(record[hashSize+8 : hashSize+12])
+	timeLow := binary.BigEndian.Uint32(record[hashSize+12 : hashSize+16])
+
+	timeHigh := uint64(genAndTimeHi & 0x3)
+	commitTimeU64 := (timeHigh << 32) | uint64(timeLow)
+
+	commitTime, err := intconv.Uint64ToInt64(commitTimeU64)
+	if err != nil {
+		return Commit{}, err
+	}
+
+	generationV1 := genAndTimeHi >> 2
+
+	generationV2, err := reader.readGenerationV2(layer, pos.Index, commitTimeU64)
+	if err != nil {
+		return Commit{}, err
+	}
+
+	parent1, parent2, extra, err := reader.decodeParents(layer, p1, p2)
+	if err != nil {
+		return Commit{}, err
+	}
+
+	return Commit{
+		OID:            oid,
+		TreeOID:        treeOID,
+		Parent1:        parent1,
+		Parent2:        parent2,
+		ExtraParents:   extra,
+		CommitTimeUnix: commitTime,
+		GenerationV1:   generationV1,
+		GenerationV2:   generationV2,
+	}, nil
+}