packfile: delta diff implementation (#159)

* packfile: delta diff implementation

- Renamed delta.go to patch_delta.go
- Added diff_delta.go file
- Added tests that creates a diff and then tries to patch it

* Requested changes

Author: ajnavarro

plumbing/format/packfile/delta_test.go

@@ -0,0 +1,93 @@
+package packfile
+
+import (
+	"fmt"
+
+	. "gopkg.in/check.v1"
+)
+
+type DeltaSuite struct {
+	testCases []deltaTest
+}
+
+var _ = Suite(&DeltaSuite{})
+
+type piece struct {
+	val   string
+	times int
+}
+
+type deltaTest struct {
+	description string
+	base        []piece
+	target      []piece
+}
+
+func (s *DeltaSuite) SetUpSuite(c *C) {
+	s.testCases = []deltaTest{{
+		description: "distinct file",
+		base:        []piece{{"0", 300}},
+		target:      []piece{{"2", 200}},
+	}, {
+		description: "same file",
+		base:        []piece{{"1", 3000}},
+		target:      []piece{{"1", 3000}},
+	}, {
+		description: "small file",
+		base:        []piece{{"1", 3}},
+		target:      []piece{{"1", 3}, {"0", 1}},
+	}, {
+		description: "big file",
+		base:        []piece{{"1", 300000}},
+		target:      []piece{{"1", 30000}, {"0", 1000000}},
+	}, {
+		description: "add elements before",
+		base:        []piece{{"0", 200}},
+		target:      []piece{{"1", 300}, {"0", 200}},
+	}, {
+		description: "add 10 times more elements at the end",
+		base:        []piece{{"1", 300}, {"0", 200}},
+		target:      []piece{{"0", 2000}},
+	}, {
+		description: "add elements between",
+		base:        []piece{{"0", 400}},
+		target:      []piece{{"0", 200}, {"1", 200}, {"0", 200}},
+	}, {
+		description: "add elements after",
+		base:        []piece{{"0", 200}},
+		target:      []piece{{"0", 200}, {"1", 200}},
+	}, {
+		description: "modify elements at the end",
+		base:        []piece{{"1", 300}, {"0", 200}},
+		target:      []piece{{"0", 100}},
+	}, {
+		description: "complex modification",
+		base: []piece{{"0", 3}, {"1", 40}, {"2", 30}, {"3", 2},
+			{"4", 400}, {"5", 23}},
+		target: []piece{{"1", 30}, {"2", 20}, {"7", 40}, {"4", 400},
+			{"5", 10}},
+	}}
+}
+
+func (s *DeltaSuite) TestAddDelta(c *C) {
+	for _, t := range s.testCases {
+		baseBuf := genBytes(t.base)
+		targetBuf := genBytes(t.target)
+		delta := DiffDelta(baseBuf, targetBuf)
+		result := PatchDelta(baseBuf, delta)
+
+		c.Log(fmt.Printf("Executing test case: %s\n", t.description))
+		c.Assert(result, DeepEquals, targetBuf)
+	}
+}
+
+func genBytes(elements []piece) []byte {
+	var result []byte
+	for _, e := range elements {
+		for i := 0; i < e.times; i++ {
+			result = append(result, e.val...)
+		}
+	}
+
+	return result
+}