ctxt: 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/ctxt"
 	"github.com/pkg/diff/myers"
 )
 
@@ -75,7 +76,7 @@ func main() {
 		defer cancel()
 	}
 	e := myers.Diff(ctx, ab)
-	e = diff.EditScriptWithContextSize(e, *unified) // limit amount of output context
+	e = ctxt.Size(e, *unified) // limit amount of output context
 	opts := []diff.WriteOpt{
 		diff.Names(aName, bName),
 	}

context.go → ctxt/size.go

@@ -1,18 +1,16 @@
-package diff
+// Package ctxt provides routines to reduce the amount of context in an edit script.
+package ctxt
 
 import (
-	"fmt"
-
 	"github.com/pkg/diff/edit"
 )
 
-// WithContextSize returns an edit script preserving only n common elements of context for changes.
+// Size returns an edit script preserving only n common elements of context for changes.
 // The returned edit script may alias the input.
-// If n is negative, WithContextSize panics.
-// To generate a "unified diff", use WithContextSize and then WriteUnified the resulting edit script.
-func EditScriptWithContextSize(e edit.Script, n int) edit.Script {
+// If n is negative, Size panics.
+func Size(e edit.Script, n int) edit.Script {
 	if n < 0 {
-		panic(fmt.Sprintf("EditScript.WithContextSize called with negative n: %d", n))
+		panic("ctxt.Size called with negative n")
 	}
 
 	// Handle small scripts.

example_test.go

@@ -5,6 +5,7 @@ import (
 	"os"
 
 	"github.com/pkg/diff"
+	"github.com/pkg/diff/ctxt"
 	"github.com/pkg/diff/myers"
 )
 
@@ -18,7 +19,7 @@ func Example_testHelper() {
 	if e.IsIdentity() {
 		return
 	}
-	e = diff.EditScriptWithContextSize(e, 1)
+	e = ctxt.Size(e, 1)
 	diff.WriteUnified(e, os.Stdout, ab)
 	// Output:
 	// --- a

print.go

@@ -53,6 +53,8 @@ const (
 // WriteUnified writes e to w using unified diff format.
 // ab writes the individual elements. Opts are optional write arguments.
 // WriteUnified returns the number of bytes written and the first error (if any) encountered.
+// Before writing, edit scripts usually have their context reduced,
+// such as by a call to ctxt.Size.
 func WriteUnified(e edit.Script, w io.Writer, ab WriterTo, opts ...WriteOpt) (int, error) {
 	// read opts
 	nameA := "a"

unified_test.go

@@ -7,6 +7,7 @@ import (
 	"testing"
 
 	"github.com/pkg/diff"
+	"github.com/pkg/diff/ctxt"
 	"github.com/pkg/diff/myers"
 	"github.com/sergi/go-diff/diffmatchpatch"
 )
@@ -82,7 +83,7 @@ func TestGolden(t *testing.T) {
 			// Doing it as I have done, the lazy way, mixes concerns: diff algorithm vs unification algorithm
 			// vs unified diff formatting.
 			e := myers.Diff(context.Background(), ab)
-			e = diff.EditScriptWithContextSize(e, 3)
+			e = ctxt.Size(e, 3)
 			buf := new(bytes.Buffer)
 			diff.WriteUnified(e, buf, ab, test.opts...)
 			got := buf.String()