1 files changed, 132 insertions, 0 deletions
diff --git a/plumbing/format/idxfile/writer.go b/plumbing/format/idxfile/writer.go
new file mode 100644
index 0000000..aac68b5
--- /dev/null
+++ b/plumbing/format/idxfile/writer.go
@@ -0,0 +1,132 @@
+package idxfile
+
+import (
+	"bytes"
+	"math"
+	"sort"
+
+	"gopkg.in/src-d/go-git.v4/plumbing"
+	"gopkg.in/src-d/go-git.v4/utils/binary"
+)
+
+type object struct {
+	hash   plumbing.Hash
+	offset int64
+	crc    uint32
+}
+
+type objects []object
+
+// Writer implements a packfile Observer interface and is used to generate
+// indexes.
+type Writer struct {
+	count    uint32
+	checksum plumbing.Hash
+	objects  objects
+}
+
+// Create index returns a filled MemoryIndex with the information filled by
+// the observer callbacks.
+func (w *Writer) CreateIndex() (*MemoryIndex, error) {
+	idx := new(MemoryIndex)
+	sort.Sort(w.objects)
+
+	// unmap all fans by default
+	for i := range idx.FanoutMapping {
+		idx.FanoutMapping[i] = noMapping
+	}
+
+	buf := new(bytes.Buffer)
+
+	last := -1
+	bucket := -1
+	for i, o := range w.objects {
+		fan := o.hash[0]
+
+		// fill the gaps between fans
+		for j := last + 1; j < int(fan); j++ {
+			idx.Fanout[j] = uint32(i)
+		}
+
+		// update the number of objects for this position
+		idx.Fanout[fan] = uint32(i + 1)
+
+		// we move from one bucket to another, update counters and allocate
+		// memory
+		if last != int(fan) {
+			bucket++
+			idx.FanoutMapping[fan] = bucket
+			last = int(fan)
+
+			idx.Names = append(idx.Names, make([]byte, 0))
+			idx.Offset32 = append(idx.Offset32, make([]byte, 0))
+			idx.Crc32 = append(idx.Crc32, make([]byte, 0))
+		}
+
+		idx.Names[bucket] = append(idx.Names[bucket], o.hash[:]...)
+
+		// TODO: implement 64 bit offsets
+		if o.offset > math.MaxInt32 {
+			panic("64 bit offsets not implemented")
+		}
+
+		buf.Truncate(0)
+		binary.WriteUint32(buf, uint32(o.offset))
+		idx.Offset32[bucket] = append(idx.Offset32[bucket], buf.Bytes()...)
+
+		buf.Truncate(0)
+		binary.WriteUint32(buf, uint32(o.crc))
+		idx.Crc32[bucket] = append(idx.Crc32[bucket], buf.Bytes()...)
+	}
+
+	for j := last + 1; j < 256; j++ {
+		idx.Fanout[j] = uint32(len(w.objects))
+	}
+
+	idx.PackfileChecksum = w.checksum
+	// TODO: fill IdxChecksum
+
+	return idx, nil
+}
+
+// Add appends new object data.
+func (w *Writer) Add(h plumbing.Hash, pos int64, crc uint32) {
+	w.objects = append(w.objects, object{h, pos, crc})
+}
+
+// OnHeader implements packfile.Observer interface.
+func (w *Writer) OnHeader(count uint32) error {
+	w.count = count
+	w.objects = make(objects, 0, count)
+	return nil
+}
+
+// OnInflatedObjectHeader implements packfile.Observer interface.
+func (w *Writer) OnInflatedObjectHeader(t plumbing.ObjectType, objSize int64, pos int64) error {
+	return nil
+}
+
+// OnInflatedObjectContent implements packfile.Observer interface.
+func (w *Writer) OnInflatedObjectContent(h plumbing.Hash, pos int64, crc uint32) error {
+	w.Add(h, pos, crc)
+	return nil
+}
+
+// OnFooter implements packfile.Observer interface.
+func (w *Writer) OnFooter(h plumbing.Hash) error {
+	w.checksum = h
+	return nil
+}
+
+func (o objects) Len() int {
+	return len(o)
+}
+
+func (o objects) Less(i int, j int) bool {
+	cmp := bytes.Compare(o[i].hash[:], o[j].hash[:])
+	return cmp < 0
+}
+
+func (o objects) Swap(i int, j int) {
+	o[i], o[j] = o[j], o[i]
+}