wip: add a git-mergetool sub command

Partial implementation of #523.

Signed-off-by: Brian McGee <brian@bmcgee.ie>

Author: brianmcgee

cmd/git_mergetool.go

@@ -0,0 +1,72 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+
+	"github.com/numtide/treefmt/v2/cmd/format"
+	"github.com/numtide/treefmt/v2/stats"
+	"github.com/spf13/cobra"
+	"github.com/spf13/viper"
+)
+
+// gitMergetool handles a 3-way merge using `git merge-file` and formats the resulting merged file.
+// It expects 4 arguments: current, base, other, and merged filenames.
+// Returns an error if the process fails or if arguments are invalid.
+func gitMergetool(
+	v *viper.Viper,
+	statz *stats.Stats,
+	cmd *cobra.Command,
+	args []string,
+) error {
+	if len(args) != 4 {
+		return fmt.Errorf("expected 4 arguments, got %d", len(args))
+	}
+
+	current := args[0]
+	base := args[1]
+	other := args[2]
+	merged := args[3]
+
+	// run treefmt on the first three arguments: current, base and other
+	_, _ = fmt.Fprintf(os.Stderr, "formatting: %s, %s, %s\n\n", current, base, other)
+
+	//nolint:wrapcheck
+	if err := format.Run(v, statz, cmd, args[:3]); err != nil {
+		return err
+	}
+
+	// open merge file
+	mergeFile, err := os.OpenFile(merged, os.O_WRONLY|os.O_CREATE, 0o600)
+	if err != nil {
+		return fmt.Errorf("failed to open merge file: %w", err)
+	}
+
+	// merge current base and other
+	merge := exec.Command("git", "merge-file", "--stdout", current, base, other)
+	_, _ = fmt.Fprintf(os.Stderr, "\n%s\n", merge.String())
+
+	// redirect stdout to the merge file
+	merge.Stdout = mergeFile
+	// capture stderr
+	merge.Stderr = os.Stderr
+
+	if err = merge.Run(); err != nil {
+		return fmt.Errorf("failed to run git merge-file: %w", err)
+	}
+
+	// close the merge file
+	if err = mergeFile.Close(); err != nil {
+		return fmt.Errorf("failed to close temporary merge file: %w", err)
+	}
+
+	// format the merge file
+	_, _ = fmt.Fprintf(os.Stderr, "formatting: %s\n\n", mergeFile.Name())
+
+	if err = format.Run(v, stats.New(), cmd, []string{mergeFile.Name()}); err != nil {
+		return fmt.Errorf("failed to format merged file: %w", err)
+	}
+
+	return nil
+}

cmd/root.go

@@ -34,7 +34,7 @@ func NewRoot() (*cobra.Command, *stats.Stats) {
 			DisableDefaultCmd: true,
 		},
 		RunE: func(cmd *cobra.Command, args []string) error {
-			return runE(v, &statz, cmd, args)
+			return runE(v, statz, cmd, args)
 		},
 	}
 
@@ -70,6 +70,9 @@ func NewRoot() (*cobra.Command, *stats.Stats) {
 		"[bash|zsh|fish] Generate shell completion scripts for the specified shell.",
 	)
 
+	// add a flag for git merge tool sub command
+	fs.Bool("git-mergetool", false, "Use treefmt as a git merge tool. Accepts four arguments: current base other merged.")
+
 	// bind our command's flags to viper
 	if err := v.BindPFlags(fs); err != nil {
 		cobra.CheckErr(fmt.Errorf("failed to bind global config to viper: %w", err))
@@ -79,7 +82,7 @@ func NewRoot() (*cobra.Command, *stats.Stats) {
 	// conforms with https://github.com/numtide/prj-spec/blob/main/PRJ_SPEC.md
 	cobra.CheckErr(v.BindPFlag("prj_root", fs.Lookup("tree-root")))
 
-	return cmd, &statz
+	return cmd, statz
 }
 
 func runE(v *viper.Viper, statz *stats.Stats, cmd *cobra.Command, args []string) error {
@@ -175,6 +178,13 @@ func runE(v *viper.Viper, statz *stats.Stats, cmd *cobra.Command, args []string)
 		}
 	}
 
+	// git mergetool
+	if merge, err := flags.GetBool("git-mergetool"); err != nil {
+		cobra.CheckErr(fmt.Errorf("failed to read git-mergetool flag: %w", err))
+	} else if merge {
+		return gitMergetool(v, statz, cmd, args)
+	}
+
 	// format
 	return format.Run(v, statz, cmd, args) //nolint:wrapcheck
 }

format/formatter_test.go

@@ -24,7 +24,7 @@ func TestInvalidFormatterName(t *testing.T) {
 	statz := stats.New()
 
 	// simple "empty" config
-	_, err := NewCompositeFormatter(cfg, &statz, batchSize)
+	_, err := NewCompositeFormatter(cfg, statz, batchSize)
 	as.NoError(err)
 
 	// valid name using all the acceptable characters
@@ -35,7 +35,7 @@ func TestInvalidFormatterName(t *testing.T) {
 		},
 	}
 
-	_, err = NewCompositeFormatter(cfg, &statz, batchSize)
+	_, err = NewCompositeFormatter(cfg, statz, batchSize)
 	as.NoError(err)
 
 	// test with some bad examples
@@ -48,7 +48,7 @@ func TestInvalidFormatterName(t *testing.T) {
 			},
 		}
 
-		_, err = NewCompositeFormatter(cfg, &statz, batchSize)
+		_, err = NewCompositeFormatter(cfg, statz, batchSize)
 		as.ErrorIs(err, ErrInvalidName)
 	}
 }
@@ -108,7 +108,7 @@ func TestFormatSignature(t *testing.T) {
 	})
 
 	t.Run("modify formatter options", func(_ *testing.T) {
-		f, err := NewCompositeFormatter(cfg, &statz, batchSize)
+		f, err := NewCompositeFormatter(cfg, statz, batchSize)
 		as.NoError(err)
 
 		oldSignature = assertSignatureChangedAndStable(t, as, cfg, nil)
@@ -169,7 +169,7 @@ func assertSignatureChangedAndStable(
 	t.Helper()
 
 	statz := stats.New()
-	f, err := NewCompositeFormatter(cfg, &statz, 1024)
+	f, err := NewCompositeFormatter(cfg, statz, 1024)
 	as.NoError(err)
 
 	newHash, err := f.signature()

stats/stats.go

@@ -55,14 +55,14 @@ func (s *Stats) PrintToStderr() {
 	)
 }
 
-func New() Stats {
+func New() *Stats {
 	counters := make(map[Type]*atomic.Int64)
 	counters[Traversed] = &atomic.Int64{}
 	counters[Matched] = &atomic.Int64{}
 	counters[Formatted] = &atomic.Int64{}
 	counters[Changed] = &atomic.Int64{}
 
-	return Stats{
+	return &Stats{
 		start:    time.Now(),
 		counters: counters,
 	}

walk/filesystem_test.go

@@ -54,7 +54,7 @@ func TestFilesystemReader(t *testing.T) {
 	tempDir := test.TempExamples(t)
 	statz := stats.New()
 
-	r := walk.NewFilesystemReader(tempDir, "", &statz, 1024)
+	r := walk.NewFilesystemReader(tempDir, "", statz, 1024)
 
 	count := 0
 

walk/git_test.go

@@ -26,7 +26,7 @@ func TestGitReader(t *testing.T) {
 
 	// read empty worktree
 	statz := stats.New()
-	reader, err := walk.NewGitReader(tempDir, "", &statz)
+	reader, err := walk.NewGitReader(tempDir, "", statz)
 	as.NoError(err)
 
 	files := make([]*walk.File, 8)
@@ -42,7 +42,7 @@ func TestGitReader(t *testing.T) {
 	cmd.Dir = tempDir
 	as.NoError(cmd.Run(), "failed to add everything to the index")
 
-	reader, err = walk.NewGitReader(tempDir, "", &statz)
+	reader, err = walk.NewGitReader(tempDir, "", statz)
 	as.NoError(err)
 
 	count := 0