1 files changed, 120 insertions, 0 deletions
diff --git a/cmd/explain-pack/delta.go b/cmd/explain-pack/delta.go
new file mode 100644
index 00000000..22e32195
--- /dev/null
+++ b/cmd/explain-pack/delta.go
@@ -0,0 +1,120 @@
+package main
+
+import (
+	"fmt"
+)
+
+func (explainer *explainer) walkDelta(base, payload []byte, pos int) ([]byte, bool, error) {
+	explainer.printf("\tdelta\n")
+
+	building := base != nil
+
+	var result []byte
+
+	insn := 0
+
+	for pos < len(payload) {
+		op := payload[pos]
+		pos++
+		insn++
+
+		switch {
+		case op&0x80 != 0:
+			next, seg, err := explainer.decodeCopy(base, payload, pos, op)
+			if err != nil {
+				return nil, false, err
+			}
+
+			pos = next
+
+			if building {
+				result = append(result, seg...)
+			}
+		case op != 0:
+			next, lit, err := explainer.decodeInsert(payload, pos, int(op))
+			if err != nil {
+				return nil, false, err
+			}
+
+			pos = next
+
+			if building {
+				result = append(result, lit...)
+			}
+		default:
+			explainer.printf("\t\tinvalid opcode 0x00; stopping delta decode\n")
+
+			return nil, false, nil
+		}
+	}
+
+	if !building {
+		return nil, false, nil
+	}
+
+	return result, true, nil
+}
+
+func (explainer *explainer) decodeCopy(base, payload []byte, pos int, op byte) (int, []byte, error) {
+	offset := 0
+
+	for i := range 4 {
+		if op&(1<<uint(i)) == 0 {
+			continue
+		}
+
+		if pos >= len(payload) {
+			return 0, nil, fmt.Errorf("truncated copy offset")
+		}
+
+		offset |= int(payload[pos]) << (8 * uint(i))
+		pos++
+	}
+
+	size := 0
+
+	for i := range 3 {
+		if op&(1<<uint(4+i)) == 0 {
+			continue
+		}
+
+		if pos >= len(payload) {
+			return 0, nil, fmt.Errorf("truncated copy size")
+		}
+
+		size |= int(payload[pos]) << (8 * uint(i))
+		pos++
+	}
+
+	if size == 0 {
+		size = 0x10000
+	}
+
+	explainer.printf("\t\tcpy %d from %d\n", size, offset)
+
+	if base == nil {
+		return pos, nil, nil
+	}
+
+	if offset < 0 || offset+size > len(base) {
+		return 0, nil, fmt.Errorf("copy of %d byte(s) from base offset %d exceeds base length %d", size, offset, len(base))
+	}
+
+	seg := base[offset : offset+size]
+	hexBlock(explainer.out, "\t\t\t", seg)
+
+	return pos, seg, nil
+}
+
+func (explainer *explainer) decodeInsert(payload []byte, pos, n int) (int, []byte, error) {
+	if pos+n > len(payload) {
+		return 0, nil, fmt.Errorf("truncated insert payload")
+	}
+
+	lit := payload[pos : pos+n]
+
+	explainer.printf("\t\tins %d\n", n)
+	hexBlock(explainer.out, "\t\t\t", lit)
+
+	return pos + n, lit, nil
+}