myers: create package

Part of #18.

cmd/pkg-diff-example/main.go

@@ -11,6 +11,7 @@ import (
 	"os"
 
 	"github.com/pkg/diff"
+	"github.com/pkg/diff/myers"
 )
 
 var (
@@ -73,7 +74,7 @@ func main() {
 		ctx, cancel = context.WithTimeout(ctx, *timeout)
 		defer cancel()
 	}
-	e := diff.Myers(ctx, ab)
+	e := myers.Diff(ctx, ab)
 	e = diff.EditScriptWithContextSize(e, *unified) // limit amount of output context
 	opts := []diff.WriteOpt{
 		diff.Names(aName, bName),

diff.go

@@ -1,23 +1,11 @@
 package diff
 
 import (
-	"fmt"
 	"io"
 
-	"github.com/pkg/diff/edit"
+	"github.com/pkg/diff/myers"
 )
 
-// A Pair is two things that can be diffed using the Myers diff algorithm.
-// A is the initial state; B is the final state.
-type Pair interface {
-	// LenA returns the number of initial elements.
-	LenA() int
-	// LenA returns the number of final elements.
-	LenB() int
-	// Equal reports whether the ai'th element of A is equal to the bi'th element of B.
-	Equal(ai, bi int) bool
-}
-
 // A WriterTo type supports writing a diff, element by element.
 // A is the initial state; B is the final state.
 type WriterTo interface {
@@ -29,7 +17,7 @@ type WriterTo interface {
 
 // PairWriterTo is the union of Pair and WriterTo.
 type PairWriterTo interface {
-	Pair
+	myers.Pair
 	WriterTo
 }
 
@@ -49,12 +37,6 @@ type PairWriterTo interface {
 // If you have paid the O(n) cost to intern all strings involved in both A and B,
 // then string comparisons are reduced to cheap pointer comparisons.
 
-func rangeString(r edit.Range) string {
-	// This output is helpful when hacking on a Myers diff.
-	// In other contexts it is usually more natural to group LowA, HighA and LowB, HighB.
-	return fmt.Sprintf("(%d, %d) -- %s %d --> (%d, %d)", r.LowA, r.LowB, r.Op(), r.Len(), r.HighA, r.HighB)
-}
-
 // TODO: consider adding an "it just works" test helper that accepts two slices (via interface{}),
 // diffs them using Strings or Bytes or Slices (using reflect.DeepEqual) as appropriate,
 // and calls t.Errorf with a generated diff if they're not equal.

example_test.go

@@ -5,6 +5,7 @@ import (
 	"os"
 
 	"github.com/pkg/diff"
+	"github.com/pkg/diff/myers"
 )
 
 // TODO: use a less heavyweight output format for Example_testHelper
@@ -13,7 +14,7 @@ func Example_testHelper() {
 	want := []int{1, 2, 3, 4, 5}
 	got := []int{1, 2, 4, 5}
 	ab := diff.Slices(want, got, nil)
-	e := diff.Myers(context.Background(), ab)
+	e := myers.Diff(context.Background(), ab)
 	if e.IsIdentity() {
 		return
 	}
@@ -32,7 +33,7 @@ func Example_strings() {
 	a := []string{"a", "b", "c"}
 	b := []string{"a", "c", "d"}
 	ab := diff.Strings(a, b)
-	e := diff.Myers(context.Background(), ab)
+	e := myers.Diff(context.Background(), ab)
 	diff.WriteUnified(e, os.Stdout, ab)
 	// Output:
 	// --- a
@@ -48,7 +49,7 @@ func Example_Names() {
 	a := []string{"a", "b", "c"}
 	b := []string{"a", "c", "d"}
 	ab := diff.Strings(a, b)
-	e := diff.Myers(context.Background(), ab)
+	e := myers.Diff(context.Background(), ab)
 	diff.WriteUnified(e, os.Stdout, ab, diff.Names("before", "after"))
 	// Output:
 	// --- before

myers.go → myers/myers.go

@@ -1,4 +1,5 @@
-package diff
+// Package myers implements the Myers diff algorithm.
+package myers
 
 import (
 	"context"
@@ -7,9 +8,20 @@ import (
 	"github.com/pkg/diff/edit"
 )
 
-// Myers calculates an edit.Script (diff) for ab using the Myers diff algorithm.
+// A Pair is two things that can be diffed using the Myers diff algorithm.
+// A is the initial state; B is the final state.
+type Pair interface {
+	// LenA returns the number of initial elements.
+	LenA() int
+	// LenB returns the number of final elements.
+	LenB() int
+	// Equal reports whether the aᵢ'th element of A is equal to the bᵢ'th element of B.
+	Equal(ai, bi int) bool
+}
+
+// Diff calculates an edit.Script for ab using the Myers diff algorithm.
 // Because diff calculation can be expensive, Myers supports cancellation via ctx.
-func Myers(ctx context.Context, ab Pair) edit.Script {
+func Diff(ctx context.Context, ab Pair) edit.Script {
 	aLen := ab.LenA()
 	bLen := ab.LenB()
 	if aLen == 0 {
@@ -153,3 +165,9 @@ func combineRanges(s, t edit.Range) (u edit.Range, ok bool) {
 	}
 	return s, true
 }
+
+func rangeString(r edit.Range) string {
+	// This output is helpful when hacking on a Myers diff.
+	// In other contexts it is usually more natural to group LowA, HighA and LowB, HighB.
+	return fmt.Sprintf("(%d, %d) -- %s %d --> (%d, %d)", r.LowA, r.LowB, r.Op(), r.Len(), r.HighA, r.HighB)
+}

myers_test.go → myers/myers_test.go

@@ -1,11 +1,12 @@
-package diff
+package myers_test
 
 import (
 	"context"
 	"reflect"
 	"testing"
 
 	"github.com/pkg/diff/edit"
+	"github.com/pkg/diff/myers"
 )
 
 func TestMyers(t *testing.T) {
@@ -49,7 +50,7 @@ func TestMyers(t *testing.T) {
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {
 			ab := &diffByByte{a: test.a, b: test.b}
-			got := Myers(context.Background(), ab)
+			got := myers.Diff(context.Background(), ab)
 			want := edit.Script{Ranges: test.want}
 
 			if !reflect.DeepEqual(got, want) {

unified_test.go

@@ -7,6 +7,7 @@ import (
 	"testing"
 
 	"github.com/pkg/diff"
+	"github.com/pkg/diff/myers"
 	"github.com/sergi/go-diff/diffmatchpatch"
 )
 
@@ -80,7 +81,7 @@ func TestGolden(t *testing.T) {
 			// TODO: supply an edit.Script to the tests instead doing a Myers diff here.
 			// Doing it as I have done, the lazy way, mixes concerns: diff algorithm vs unification algorithm
 			// vs unified diff formatting.
-			e := diff.Myers(context.Background(), ab)
+			e := myers.Diff(context.Background(), ab)
 			e = diff.EditScriptWithContextSize(e, 3)
 			buf := new(bytes.Buffer)
 			diff.WriteUnified(e, buf, ab, test.opts...)