More work on porting diff kitten

tools/cmd/diff/patch.go

@@ -0,0 +1,84 @@
+// License: GPLv3 Copyright: 2023, Kovid Goyal, <kovid at kovidgoyal.net>
+
+package diff
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"kitty/tools/utils"
+	"kitty/tools/utils/shlex"
+	"os/exec"
+	"path/filepath"
+	"strconv"
+	"strings"
+)
+
+var _ = fmt.Print
+
+const GIT_DIFF = `git diff --no-color --no-ext-diff --exit-code -U_CONTEXT_ --no-index --`
+const DIFF_DIFF = `diff -p -U _CONTEXT_ --`
+
+var diff_cmd []string
+
+var GitExe = (&utils.Once[string]{Run: func() string {
+	return utils.FindExe("git")
+}}).Get
+
+var DiffExe = (&utils.Once[string]{Run: func() string {
+	return utils.FindExe("diff")
+}}).Get
+
+func find_differ() error {
+	if GitExe() != "git" && exec.Command(GitExe(), "--help").Run() == nil {
+		diff_cmd, _ = shlex.Split(GIT_DIFF)
+		return nil
+	}
+	if DiffExe() != "diff" && exec.Command(DiffExe(), "--help").Run() == nil {
+		diff_cmd, _ = shlex.Split(DIFF_DIFF)
+		return nil
+	}
+	return fmt.Errorf("Neither the git nor the diff programs were found in PATH")
+}
+
+func set_diff_command(q string) error {
+	if q == "auto" {
+		return find_differ()
+	}
+	c, err := shlex.Split(q)
+	if err == nil {
+		diff_cmd = c
+	}
+	return err
+}
+
+func run_diff(file1, file2 string, num_of_context_lines int) (ok, is_different bool, patch string, err error) {
+	context := strconv.Itoa(num_of_context_lines)
+	cmd := utils.Map(func(x string) string {
+		return strings.ReplaceAll(x, "_CONTEXT_", context)
+	}, diff_cmd)
+	// we resolve symlinks because git diff does not follow symlinks, while diff
+	// does. We want consistent behavior, also for integration with git difftool
+	// we always want symlinks to be followed.
+	path1, err := filepath.EvalSymlinks(file1)
+	if err != nil {
+		return
+	}
+	path2, err := filepath.EvalSymlinks(file2)
+	if err != nil {
+		return
+	}
+	cmd = append(cmd, path1, path2)
+	c := exec.Command(cmd[0], cmd[1:]...)
+	stdout, stderr := bytes.Buffer{}, bytes.Buffer{}
+	c.Stdout, c.Stderr = &stdout, &stderr
+	err = c.Run()
+	if err != nil {
+		var e *exec.ExitError
+		if errors.As(err, &e) && e.ExitCode() == 1 {
+			return true, true, stdout.String(), nil
+		}
+		return false, false, stderr.String(), err
+	}
+	return true, false, stdout.String(), nil
+}