From 3a5fd27ee830155dc0c7c4e7cb643ff68252a4d4 Mon Sep 17 00:00:00 2001
From: Runxi Yu
Date: Thu, 11 Jun 2026 13:31:29 +0000
Subject: internal/format/packidx: Add lookup by object ID

---
 internal/format/packidx/lookup.go | 73 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 73 insertions(+)
 create mode 100644 internal/format/packidx/lookup.go

(limited to 'internal/format')

diff --git a/internal/format/packidx/lookup.go b/internal/format/packidx/lookup.go
new file mode 100644
index 00000000..5135a5de
--- /dev/null
+++ b/internal/format/packidx/lookup.go
@@ -0,0 +1,73 @@
+package packidx
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+)
+
+// Lookup searches the index for one object ID
+// and returns its pack file offset.
+//
+// oid must be exactly the index's hash size;
+// Lookup panics otherwise.
+func (idx *Packidx) Lookup(oid []byte) (offset uint64, found bool, err error) {
+	if len(oid) != idx.hashSize {
+		panic("internal/format/packidx: invalid object ID length")
+	}
+
+	lo, hi := idx.fanoutRange(oid[0])
+
+	for lo < hi {
+		mid := lo + (hi-lo)/2
+
+		cmp := bytes.Compare(oid, idx.OIDAt(mid))
+
+		switch {
+		case cmp == 0:
+			offset, err = idx.OffsetAt(mid)
+			if err != nil {
+				return 0, false, err
+			}
+
+			return offset, true, nil
+		case cmp < 0:
+			hi = mid
+		default:
+			lo = mid + 1
+		}
+	}
+
+	return 0, false, nil
+}
+
+// OffsetAt returns the pack file offset at one index position.
+//
+// OffsetAt panics when pos is out of range.
+func (idx *Packidx) OffsetAt(pos int) (uint64, error) {
+	idx.checkPos(pos)
+
+	raw := binary.BigEndian.Uint32(idx.data[idx.off32Off+4*pos:])
+	if raw&largeOffsetFlag == 0 {
+		return uint64(raw), nil
+	}
+
+	slot := raw &^ largeOffsetFlag
+	if slot >= idx.off64Count {
+		return 0, fmt.Errorf("%w: 64-bit offset reference out of range", ErrMalformedPackIndex)
+	}
+
+	return binary.BigEndian.Uint64(idx.data[idx.off64Off+8*int(slot):]), nil
+}
+
+// fanoutRange returns the index position range [lo, hi)
+// of object IDs beginning with first.
+func (idx *Packidx) fanoutRange(first byte) (lo, hi int) {
+	hi = int(binary.BigEndian.Uint32(idx.data[headerLen+4*int(first):]))
+
+	if first > 0 {
+		lo = int(binary.BigEndian.Uint32(idx.data[headerLen+4*(int(first)-1):]))
+	}
+
+	return lo, hi
+}
-- 
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