objectstore/packed: Add initial pack reading support

1 files changed, 81 insertions, 0 deletions

diff --git a/objectstore/packed/delta_plan.go b/objectstore/packed/delta_plan.go
new file mode 100644
index 00000000..e55400aa
--- /dev/null
+++ b/objectstore/packed/delta_plan.go
@@ -0,0 +1,81 @@
+package packed
+
+import (
+	"fmt"
+
+	"codeberg.org/lindenii/furgit/objecttype"
+)
+
+// deltaFrame describes one delta payload to apply during reconstruction.
+type deltaFrame struct {
+	// packName identifies where the delta payload lives.
+	packName string
+	// dataOffset points to the start of the delta zlib payload in pack.
+	dataOffset int
+}
+
+// deltaPlan describes how to reconstruct one requested object.
+type deltaPlan struct {
+	// declaredSize is the target object's declared content size.
+	declaredSize int64
+	// baseLoc points to the innermost base object.
+	baseLoc location
+	// baseType is the canonical object type resolved from baseLoc.
+	baseType objecttype.Type
+	// frames contains deltas from target down toward base.
+	frames []deltaFrame
+}
+
+// deltaPlanFor walks one object's chain and builds a delta reconstruction plan.
+func (store *Store) deltaPlanFor(start location) (deltaPlan, error) {
+	visited := make(map[location]struct{})
+	current := start
+
+	var plan deltaPlan
+	plan.declaredSize = -1
+
+	for {
+		if _, ok := visited[current]; ok {
+			return deltaPlan{}, fmt.Errorf("objectstore/packed: delta cycle while resolving object")
+		}
+		visited[current] = struct{}{}
+
+		_, meta, err := store.entryMetaAt(current)
+		if err != nil {
+			return deltaPlan{}, err
+		}
+		if plan.declaredSize < 0 {
+			plan.declaredSize = meta.size
+		}
+
+		if isBaseObjectType(meta.ty) {
+			plan.baseLoc = current
+			plan.baseType = meta.ty
+			return plan, nil
+		}
+
+		switch meta.ty {
+		case objecttype.TypeRefDelta:
+			plan.frames = append(plan.frames, deltaFrame{
+				packName:   current.packName,
+				dataOffset: meta.dataOffset,
+			})
+			next, err := store.lookup(meta.baseRefID)
+			if err != nil {
+				return deltaPlan{}, err
+			}
+			current = next
+		case objecttype.TypeOfsDelta:
+			plan.frames = append(plan.frames, deltaFrame{
+				packName:   current.packName,
+				dataOffset: meta.dataOffset,
+			})
+			current = location{
+				packName: current.packName,
+				offset:   meta.baseOfs,
+			}
+		default:
+			return deltaPlan{}, fmt.Errorf("objectstore/packed: unsupported pack type %d", meta.ty)
+		}
+	}
+}