add algorithm to calculate edit distance (#140)

internal/diff/diff.go

@@ -0,0 +1,67 @@
+package diff
+
+type Diff struct {
+	textLong     []rune
+	textShort    []rune
+	lenTextLong  int
+	lenTextShort int
+	isReversed   bool
+	editDistance int
+}
+
+func NewDiff(text1, text2 []rune) *Diff {
+	diff := &Diff{}
+	if len(text1) > len(text2) {
+		diff.textLong = text1
+		diff.textShort = text2
+		diff.lenTextLong = len(text1)
+		diff.lenTextShort = len(text2)
+	} else {
+		diff.textLong = text2
+		diff.textShort = text1
+		diff.lenTextLong = len(text2)
+		diff.lenTextShort = len(text1)
+		diff.isReversed = true
+	}
+
+	return diff
+}
+
+func (d *Diff) Compose() {
+	delta := d.lenTextLong - d.lenTextShort
+	offset := d.lenTextShort + 1
+	fp := make([]int, d.lenTextLong+d.lenTextShort+3)
+	for i := range fp {
+		fp[i] = -1
+	}
+
+	for p := 0; ; p++ {
+		for k := -p; k < delta; k++ {
+			fp[k+offset] = d.snake(k, max(fp[k+offset-1]+1, fp[k+offset+1]))
+		}
+		for k := delta + p; k > delta; k-- {
+			fp[k+offset] = d.snake(k, max(fp[k+offset-1]+1, fp[k+offset+1]))
+		}
+		fp[delta+offset] = d.snake(delta+offset, max(fp[delta+offset-1]+1, fp[delta+offset+1]))
+		if fp[delta+offset] == d.lenTextLong {
+			d.editDistance = p
+		}
+	}
+}
+
+func (d *Diff) snake(k, y int) int {
+	x := y - k
+	for x < d.lenTextShort && y < d.lenTextLong && d.textLong[y] == d.textShort[x] {
+		x++
+		y++
+	}
+
+	return y
+}
+
+func max(x, y int) int {
+	if x > y {
+		return x
+	}
+	return y
+}