1 files changed, 91 insertions, 0 deletions
diff --git a/object/store/packed/idx_lookup.go b/object/store/packed/idx_lookup.go
new file mode 100644
index 00000000..0bd11d1b
--- /dev/null
+++ b/object/store/packed/idx_lookup.go
@@ -0,0 +1,91 @@
+package packed
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+
+	objectid "codeberg.org/lindenii/furgit/object/id"
+)
+
+// lookup resolves one object ID to its pack offset within this index.
+func (index *idxFile) lookup(id objectid.ObjectID) (uint64, bool, error) {
+	if id.Algorithm() != index.algo {
+		return 0, false, fmt.Errorf("objectstore/packed: object id algorithm mismatch")
+	}
+
+	idBytes := (&id).RawBytes()
+
+	hashSize := len(idBytes)
+	if hashSize != index.algo.Size() {
+		return 0, false, fmt.Errorf("objectstore/packed: unexpected object id length")
+	}
+
+	first := int(idBytes[0])
+
+	lo := 0
+	if first > 0 {
+		lo = int(index.fanout[first-1])
+	}
+
+	hi := int(index.fanout[first])
+	if lo < 0 || hi < 0 || lo > hi || hi > index.numObjects {
+		return 0, false, fmt.Errorf("objectstore/packed: idx %q has invalid fanout bounds", index.idxName)
+	}
+
+	for lo < hi {
+		mid := lo + (hi-lo)/2
+
+		nameOffset := index.namesOffset + mid*hashSize
+		if nameOffset < 0 || nameOffset+hashSize > len(index.data) {
+			return 0, false, fmt.Errorf("objectstore/packed: idx %q truncated name table", index.idxName)
+		}
+
+		cmp := bytes.Compare(index.data[nameOffset:nameOffset+hashSize], idBytes)
+		if cmp == 0 {
+			offset, err := index.offsetAt(mid)
+			if err != nil {
+				return 0, false, err
+			}
+
+			return offset, true, nil
+		}
+
+		if cmp < 0 {
+			lo = mid + 1
+		} else {
+			hi = mid
+		}
+	}
+
+	return 0, false, nil
+}
+
+// offsetAt resolves the pack offset for one object index entry.
+func (index *idxFile) offsetAt(objectIndex int) (uint64, error) {
+	if objectIndex < 0 || objectIndex >= index.numObjects {
+		return 0, fmt.Errorf("objectstore/packed: idx %q offset index out of bounds", index.idxName)
+	}
+
+	wordOffset := index.offset32Offset + objectIndex*4
+	if wordOffset < 0 || wordOffset+4 > len(index.data) {
+		return 0, fmt.Errorf("objectstore/packed: idx %q truncated 32-bit offset table", index.idxName)
+	}
+
+	word := binary.BigEndian.Uint32(index.data[wordOffset : wordOffset+4])
+	if word&0x80000000 == 0 {
+		return uint64(word), nil
+	}
+
+	pos := int(word & 0x7fffffff)
+	if pos < 0 || pos >= index.offset64Count {
+		return 0, fmt.Errorf("objectstore/packed: idx %q invalid 64-bit offset position", index.idxName)
+	}
+
+	offOffset := index.offset64Offset + pos*8
+	if offOffset < 0 || offOffset+8 > len(index.data)-2*index.algo.Size() {
+		return 0, fmt.Errorf("objectstore/packed: idx %q truncated 64-bit offset table", index.idxName)
+	}
+
+	return binary.BigEndian.Uint64(index.data[offOffset : offOffset+8]), nil
+}