feat: Add diff mode

README.md

@@ -128,6 +128,11 @@ bar = [
 
    If the file is `-`, the tool will read from stdin and write to stdout.
 
+   You can select a mode with the `--mode` flag:
+   - `fix` (default) sorts files in place.
+   - `lint` prints JSON findings and exits non-zero if anything is out of order.
+   - `diff` prints a unified diff instead of rewriting files and exits non-zero if anything would change.
+
 #### pre-commit
 
 You can run keep-sorted automatically by adding this repository to your

cmd/cmd.go

@@ -25,6 +25,7 @@ import (
 	"strconv"
 	"strings"
 
+	"github.com/aymanbagabas/go-udiff"
 	"github.com/google/keep-sorted/keepsorted"
 	"github.com/rs/zerolog/log"
 	flag "github.com/spf13/pflag"
@@ -84,6 +85,7 @@ var (
 	operations = map[string]operation{
 		"lint": lint,
 		"fix":  fix,
+		"diff": diff,
 	}
 )
 
@@ -228,18 +230,9 @@ func fix(fixer *keepsorted.Fixer, filenames []string, modifiedLines []keepsorted
 			}
 		}
 
-		for _, warn := range warnings {
+		if len(warnings) > 0 {
 			ok = false
-			log := log.Warn()
-			if warn.Path != stdin {
-				log = log.Str("file", warn.Path)
-			}
-			if warn.Lines.Start == warn.Lines.End {
-				log = log.Int("line", warn.Lines.Start)
-			} else {
-				log = log.Ints("[start,end]", []int{warn.Lines.Start, warn.Lines.End})
-			}
-			log.Msg(warn.Message)
+			logWarnings(warnings)
 		}
 	}
 
@@ -269,6 +262,52 @@ func lint(fixer *keepsorted.Fixer, filenames []string, modifiedLines []keepsorte
 	return false, nil
 }
 
+func diff(fixer *keepsorted.Fixer, filenames []string, modifiedLines []keepsorted.LineRange) (ok bool, err error) {
+	ok = true
+
+	for _, fn := range filenames {
+		contents, err := read(fn)
+		if err != nil {
+			return false, err
+		}
+		want, alreadyFixed, warnings := fixer.Fix(fn, contents, modifiedLines)
+		if !alreadyFixed {
+			ok = false
+			inName := fn
+			outName := fn
+			if fn == stdin {
+				inName = "stdin"
+				outName = "stdout"
+			}
+			if _, err := os.Stdout.WriteString(udiff.Unified(inName, outName, contents, want)); err != nil {
+				return false, err
+			}
+		}
+
+		if len(warnings) > 0 {
+			ok = false
+			logWarnings(warnings)
+		}
+	}
+
+	return ok, nil
+}
+
+func logWarnings(warnings []*keepsorted.Finding) {
+	for _, warn := range warnings {
+		log := log.Warn()
+		if warn.Path != stdin {
+			log = log.Str("file", warn.Path)
+		}
+		if warn.Lines.Start == warn.Lines.End {
+			log = log.Int("line", warn.Lines.Start)
+		} else {
+			log = log.Ints("[start,end]", []int{warn.Lines.Start, warn.Lines.End})
+		}
+		log.Msg(warn.Message)
+	}
+}
+
 func read(fn string) (string, error) {
 	if fn == stdin {
 		b, err := io.ReadAll(os.Stdin)

go.mod

@@ -6,6 +6,8 @@ go 1.23.1
 
 require (
 	github.com/Workiva/go-datastructures v1.0.53
+	github.com/aymanbagabas/go-udiff v0.3.1
+	github.com/bluekeyes/go-gitdiff v0.8.1
 	github.com/google/go-cmp v0.5.8
 	github.com/mattn/go-isatty v0.0.20
 	github.com/rs/zerolog v1.31.0

go.sum

@@ -1,5 +1,9 @@
 github.com/Workiva/go-datastructures v1.0.53 h1:J6Y/52yX10Xc5JjXmGtWoSSxs3mZnGSaq37xZZh7Yig=
 github.com/Workiva/go-datastructures v1.0.53/go.mod h1:1yZL+zfsztete+ePzZz/Zb1/t5BnDuE2Ya2MMGhzP6A=
+github.com/aymanbagabas/go-udiff v0.3.1 h1:LV+qyBQ2pqe0u42ZsUEtPiCaUoqgA9gYRDs3vj1nolY=
+github.com/aymanbagabas/go-udiff v0.3.1/go.mod h1:G0fsKmG+P6ylD0r6N/KgQD/nWzgfnl8ZBcNLgcbrw8E=
+github.com/bluekeyes/go-gitdiff v0.8.1 h1:lL1GofKMywO17c0lgQmJYcKek5+s8X6tXVNOLxy4smI=
+github.com/bluekeyes/go-gitdiff v0.8.1/go.mod h1:WWAk1Mc6EgWarCrPFO+xeYlujPu98VuLW3Tu+B/85AE=
 github.com/coreos/go-systemd/v22 v22.5.0/go.mod h1:Y58oyj3AT4RCenI/lSvhwexgC+NSVTIJ3seZv2GcEnc=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=

goldens/golden_test.go

@@ -15,6 +15,7 @@
 package golden_test
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"io"
@@ -27,6 +28,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/bluekeyes/go-gitdiff/gitdiff"
 	"github.com/google/go-cmp/cmp"
 )
 
@@ -68,7 +70,7 @@ func TestGoldens(t *testing.T) {
 			t.Run(tc, func(t *testing.T) {
 				t.Parallel()
 				inFile := filepath.Join(dir, tc+".in")
-				in, err := os.Open(inFile)
+				in, err := os.ReadFile(inFile)
 				if err != nil {
 					t.Fatalf("Could not open .in file: %v", err)
 				}
@@ -91,7 +93,7 @@ func TestGoldens(t *testing.T) {
 				wantErr = []byte(strings.ReplaceAll(string(wantErr), "\r\n", "\n"))
 				wantErr = []byte(strings.ReplaceAll(string(wantErr), "\r", "\n"))
 
-				gotOut, gotErr, exitCode, err := runKeepSorted(in)
+				gotOut, gotErr, exitCode, err := runKeepSorted(bytes.NewReader(in), "fix")
 				if err != nil {
 					t.Errorf("Had trouble running keep-sorted: %v", err)
 				}
@@ -104,7 +106,9 @@ func TestGoldens(t *testing.T) {
 					needsRegen <- inFile
 				}
 
-				gotOut2, _, exitCode2, err := runKeepSorted(strings.NewReader(gotOut))
+				testDiffMode(t, in, wantOut)
+
+				gotOut2, _, exitCode2, err := runKeepSorted(strings.NewReader(gotOut), "fix")
 				if err != nil {
 					t.Errorf("Had trouble running keep-sorted on keep-sorted output: %v", err)
 				}
@@ -129,13 +133,48 @@ func TestGoldens(t *testing.T) {
 	}
 }
 
+func testDiffMode(t *testing.T, in []byte, wantOut []byte) {
+	t.Run("diff", func(t *testing.T) {
+		t.Parallel()
+		gotDiff, _, _, err := runKeepSorted(bytes.NewReader(in), "diff")
+		if err != nil {
+			t.Fatalf("Had trouble running keep-sorted --mode diff: %v", err)
+		}
+		files, _, err := gitdiff.Parse(strings.NewReader(gotDiff))
+		if err != nil {
+			t.Fatalf("Had trouble parsing diff: %v", err)
+		}
+		if len(files) != 1 {
+			t.Fatalf("Exactly one file is expected in diff, got %d", len(files))
+		}
+		var b strings.Builder
+		err = gitdiff.Apply(&b, bytes.NewReader(in), files[0])
+		if err != nil {
+			t.Fatalf("Had trouble applying diff: %v", err)
+		}
+		if diff := cmp.Diff(string(wantOut), b.String()); diff != "" {
+			t.Fatalf("Diff applied to the input didn't match expected out:\n%s", diff)
+		}
+	})
+	t.Run("diff after fix", func(t *testing.T) {
+		t.Parallel()
+		gotDiff, _, _, err := runKeepSorted(bytes.NewReader(wantOut), "diff")
+		if err != nil {
+			t.Fatalf("Had trouble running keep-sorted --mode diff: %v", err)
+		}
+		if gotDiff != "" {
+			t.Errorf("Non-empty diff produced:\n%s", gotDiff)
+		}
+	})
+}
+
 func showTopLevel(dir string) (string, error) {
 	b, err := exec.Command("git", "-C", dir, "rev-parse", "--show-toplevel").Output()
 	return strings.TrimSpace(string(b)), err
 }
 
-func runKeepSorted(stdin io.Reader) (stdout, stderr string, exitCode int, err error) {
-	cmd := exec.Command("go", "run", gitDir, "--id=keep-sorted-test", "--omit-timestamps", "-")
+func runKeepSorted(stdin io.Reader, mode string) (stdout, stderr string, exitCode int, err error) {
+	cmd := exec.Command("go", "run", gitDir, "--id=keep-sorted-test", "--mode="+mode, "--omit-timestamps", "-")
 	cmd.Stdin = stdin
 	outPipe, err := cmd.StdoutPipe()
 	if err != nil {