1 files changed, 223 insertions, 0 deletions
diff --git a/difflines/difflines.go b/difflines/difflines.go
new file mode 100644
index 00000000..db4c1a03
--- /dev/null
+++ b/difflines/difflines.go
@@ -0,0 +1,223 @@
+// Package difflines provides routines to perform line-based diffs.
+package difflines
+
+import "bytes"
+
+// DiffLines performs a line-based diff.
+// Lines are bytes up to and including '\n' (final line may lack '\n').
+func DiffLines(oldB, newB []byte) ([]LinesDiffChunk, error) {
+	type lineRef struct {
+		base  []byte
+		start int
+		end   int
+	}
+
+	split := func(b []byte) []lineRef {
+		if len(b) == 0 {
+			return nil
+		}
+		var res []lineRef
+		start := 0
+		for i := range b {
+			if b[i] == '\n' {
+				res = append(res, lineRef{base: b, start: start, end: i + 1})
+				start = i + 1
+			}
+		}
+		if start < len(b) {
+			res = append(res, lineRef{base: b, start: start, end: len(b)})
+		}
+		return res
+	}
+
+	oldLines := split(oldB)
+	newLines := split(newB)
+
+	n := len(oldLines)
+	m := len(newLines)
+	if n == 0 && m == 0 {
+		return nil, nil
+	}
+
+	idOf := make(map[string]int)
+	nextID := 0
+	oldIDs := make([]int, n)
+	for i, ln := range oldLines {
+		key := bytesToString(ln.base[ln.start:ln.end])
+		id, ok := idOf[key]
+		if !ok {
+			id = nextID
+			idOf[key] = id
+			nextID++
+		}
+		oldIDs[i] = id
+	}
+	newIDs := make([]int, m)
+	for i, ln := range newLines {
+		key := bytesToString(ln.base[ln.start:ln.end])
+		id, ok := idOf[key]
+		if !ok {
+			id = nextID
+			idOf[key] = id
+			nextID++
+		}
+		newIDs[i] = id
+	}
+
+	max := n + m
+	offset := max
+	trace := make([][]int, 0, max+1)
+
+	Vprev := make([]int, 2*max+1)
+	for i := range Vprev {
+		Vprev[i] = -1
+	}
+
+	x0 := 0
+	y0 := 0
+	for x0 < n && y0 < m && oldIDs[x0] == newIDs[y0] {
+		x0++
+		y0++
+	}
+	Vprev[offset+0] = x0
+	trace = append(trace, append([]int(nil), Vprev...))
+
+	found := x0 >= n && y0 >= m
+
+	for D := 1; D <= max && !found; D++ {
+		V := make([]int, 2*max+1)
+		for i := range V {
+			V[i] = -1
+		}
+
+		for k := -D; k <= D; k += 2 {
+			var x int
+			if k == -D || (k != D && Vprev[offset+(k-1)] < Vprev[offset+(k+1)]) {
+				x = Vprev[offset+(k+1)]
+			} else {
+				x = Vprev[offset+(k-1)] + 1
+			}
+			y := x - k
+
+			for x < n && y < m && oldIDs[x] == newIDs[y] {
+				x++
+				y++
+			}
+			V[offset+k] = x
+
+			if x >= n && y >= m {
+				trace = append(trace, V)
+				found = true
+				break
+			}
+		}
+
+		if !found {
+			trace = append(trace, V)
+			Vprev = V
+		}
+	}
+
+	type edit struct {
+		kind    LinesDiffChunkKind
+		lineref lineRef
+	}
+	revEdits := make([]edit, 0, n+m)
+
+	x := n
+	y := m
+	for D := len(trace) - 1; D >= 0; D-- {
+		k := x - y
+
+		var (
+			prevK int
+			prevX int
+			prevY int
+		)
+		if D > 0 {
+			prevV := trace[D-1]
+			if k == -D || (k != D && prevV[offset+(k-1)] < prevV[offset+(k+1)]) {
+				prevK = k + 1
+			} else {
+				prevK = k - 1
+			}
+			prevX = prevV[offset+prevK]
+			prevY = prevX - prevK
+		}
+
+		for x > prevX && y > prevY {
+			x--
+			y--
+			revEdits = append(revEdits, edit{kind: LinesDiffChunkKindUnchanged, lineref: oldLines[x]})
+		}
+
+		if D == 0 {
+			break
+		}
+
+		if x == prevX {
+			y--
+			revEdits = append(revEdits, edit{kind: LinesDiffChunkKindAdded, lineref: newLines[y]})
+		} else {
+			x--
+			revEdits = append(revEdits, edit{kind: LinesDiffChunkKindDeleted, lineref: oldLines[x]})
+		}
+	}
+
+	for i, j := 0, len(revEdits)-1; i < j; i, j = i+1, j-1 {
+		revEdits[i], revEdits[j] = revEdits[j], revEdits[i]
+	}
+
+	var out []LinesDiffChunk
+	type meta struct {
+		base  []byte
+		start int
+		end   int
+	}
+	var metas []meta
+
+	for _, e := range revEdits {
+		curBase := e.lineref.base
+		curStart := e.lineref.start
+		curEnd := e.lineref.end
+
+		if len(out) == 0 || out[len(out)-1].Kind != e.kind {
+			out = append(out, LinesDiffChunk{Kind: e.kind, Data: curBase[curStart:curEnd]})
+			metas = append(metas, meta{base: curBase, start: curStart, end: curEnd})
+			continue
+		}
+
+		lastIdx := len(out) - 1
+		lastMeta := metas[lastIdx]
+
+		if bytes.Equal(lastMeta.base, curBase) && lastMeta.end == curStart {
+			metas[lastIdx].end = curEnd
+			out[lastIdx].Data = curBase[metas[lastIdx].start:metas[lastIdx].end]
+			continue
+		}
+
+		out[lastIdx].Data = append(out[lastIdx].Data, curBase[curStart:curEnd]...)
+		metas[lastIdx] = meta{base: nil, start: 0, end: 0}
+	}
+
+	return out, nil
+}
+
+// LinesDiffChunk represents a contiguous region of lines categorized
+// as unchanged, deleted, or added.
+type LinesDiffChunk struct {
+	Kind LinesDiffChunkKind
+	Data []byte
+}
+
+// LinesDiffChunkKind enumerates the type of diff chunk.
+type LinesDiffChunkKind int
+
+const (
+	// LinesDiffChunkKindUnchanged represents an unchanged diff chunk.
+	LinesDiffChunkKindUnchanged LinesDiffChunkKind = iota
+	// LinesDiffChunkKindDeleted represents a deleted diff chunk.
+	LinesDiffChunkKindDeleted
+	// LinesDiffChunkKindAdded represents an added diff chunk.
+	LinesDiffChunkKindAdded
+)