Add diff package

diff/diff.go

+// Package diff implements a linewise diff algorithm.
+package diff
+
+import (
+	"bytes"
+	"fmt"
+	"strings"
+)
+
+// Chunk represents a piece of the diff.  A chunk will not have both added and
+// deleted lines.  Equal lines are always after any added or deleted lines.
+// A Chunk may or may not have any lines in it, especially for the first or last
+// chunk in a computation.
+type Chunk struct {
+	Added   []string
+	Deleted []string
+	Equal   []string
+}
+
+// Diff returns a string containing a line-by-line unified diff of the linewise
+// changes required to make A into B.  Each line is prefixed with '+', '-', or
+// ' ' to indicate if it should be added, removed, or is correct respectively.
+func Diff(A, B string) string {
+	aLines := strings.Split(A, "\n")
+	bLines := strings.Split(B, "\n")
+
+	chunks := DiffChunks(aLines, bLines)
+
+	buf := new(bytes.Buffer)
+	for _, c := range chunks {
+		for _, line := range c.Added {
+			fmt.Fprintf(buf, "+%s\n", line)
+		}
+		for _, line := range c.Deleted {
+			fmt.Fprintf(buf, "-%s\n", line)
+		}
+		for _, line := range c.Equal {
+			fmt.Fprintf(buf, " %s\n", line)
+		}
+	}
+	return buf.String()
+}
+
+// DiffChunks uses an O(D(N+M)) shortest-edit-script algorithm
+// to compute the edits required from A to B and returns the
+// edit chunks.
+func DiffChunks(A, B []string) []Chunk {
+	// algorithm: http://www.xmailserver.org/diff2.pdf
+
+	N, M := len(A), len(B)
+	MAX := N + M
+	V := make([]int, 2*MAX+1)
+	Vs := make([][]int, 0, 8)
+
+	var D int
+dLoop:
+	for D = 0; D <= MAX; D++ {
+		for k := -D; k <= D; k += 2 {
+			var x int
+			if k == -D || (k != D && V[MAX+k-1] < V[MAX+k+1]) {
+				x = V[MAX+k+1]
+			} else {
+				x = V[MAX+k-1] + 1
+			}
+			y := x - k
+			for x < N && y < M && A[x] == B[y] {
+				x++
+				y++
+			}
+			V[MAX+k] = x
+			if x >= N && y >= M {
+				Vs = append(Vs, append(make([]int, 0, len(V)), V...))
+				break dLoop
+			}
+		}
+		Vs = append(Vs, append(make([]int, 0, len(V)), V...))
+	}
+	if D == 0 {
+		return nil
+	}
+	chunks := make([]Chunk, D+1)
+
+	x, y := N, M
+	for d := D; d > 0; d-- {
+		V := Vs[d]
+		k := x - y
+		insert := k == -d || (k != d && V[MAX+k-1] < V[MAX+k+1])
+
+		x1 := V[MAX+k]
+		var x0, xM, kk int
+		if insert {
+			kk = k + 1
+			x0 = V[MAX+kk]
+			xM = x0
+		} else {
+			kk = k - 1
+			x0 = V[MAX+kk]
+			xM = x0 + 1
+		}
+		y0 := x0 - kk
+
+		var c Chunk
+		if insert {
+			c.Added = B[y0:][:1]
+		} else {
+			c.Deleted = A[x0:][:1]
+		}
+		if xM < x1 {
+			c.Equal = A[xM:][:x1-xM]
+		}
+
+		x, y = x0, y0
+		chunks[d] = c
+	}
+	if x > 0 {
+		chunks[0].Equal = A[:x]
+	}
+	return chunks
+}

diff/diff_test.go

+package diff
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestDiff(t *testing.T) {
+	tests := []struct {
+		desc   string
+		A, B   []string
+		chunks []Chunk
+	}{
+		{
+			desc: "constitution",
+			A: []string{
+				"We the People of the United States, in Order to form a more perfect Union,",
+				"establish Justice, insure domestic Tranquility, provide for the common defence,",
+				"and secure the Blessings of Liberty to ourselves",
+				"and our Posterity, do ordain and establish this Constitution for the United",
+				"States of America.",
+			},
+			B: []string{
+				"We the People of the United States, in Order to form a more perfect Union,",
+				"establish Justice, insure domestic Tranquility, provide for the common defence,",
+				"promote the general Welfare, and secure the Blessings of Liberty to ourselves",
+				"and our Posterity, do ordain and establish this Constitution for the United",
+				"States of America.",
+			},
+			chunks: []Chunk{
+				0: {
+					Equal: []string{
+						"We the People of the United States, in Order to form a more perfect Union,",
+						"establish Justice, insure domestic Tranquility, provide for the common defence,",
+					},
+				},
+				1: {
+					Deleted: []string{
+						"and secure the Blessings of Liberty to ourselves",
+					},
+				},
+				2: {
+					Added: []string{
+						"promote the general Welfare, and secure the Blessings of Liberty to ourselves",
+					},
+					Equal: []string{
+						"and our Posterity, do ordain and establish this Constitution for the United",
+						"States of America.",
+					},
+				},
+			},
+		},
+	}
+
+	for _, test := range tests {
+		got := DiffChunks(test.A, test.B)
+		if got, want := len(got), len(test.chunks); got != want {
+			t.Errorf("%s: edit distance = %v, want %v", test.desc, got-1, want-1)
+			continue
+		}
+		for i := range got {
+			got, want := got[i], test.chunks[i]
+			if got, want := got.Added, want.Added; !reflect.DeepEqual(got, want) {
+				t.Errorf("%s[%d]: Added = %v, want %v", test.desc, i, got, want)
+			}
+			if got, want := got.Deleted, want.Deleted; !reflect.DeepEqual(got, want) {
+				t.Errorf("%s[%d]: Deleted = %v, want %v", test.desc, i, got, want)
+			}
+			if got, want := got.Equal, want.Equal; !reflect.DeepEqual(got, want) {
+				t.Errorf("%s[%d]: Equal = %v, want %v", test.desc, i, got, want)
+			}
+		}
+	}
+}