feat(go): implement link with subdir and force handling

internal/homesick/core/core.go

@@ -18,6 +18,7 @@ type App struct {
 	Stdout   io.Writer
 	Stderr   io.Writer
 	Verbose  bool
+	Force    bool
 }
 
 func New(stdout io.Writer, stderr io.Writer) (*App, error) {
@@ -126,8 +127,44 @@ func (a *App) Diff(castle string) error {
 	return runGit(filepath.Join(a.ReposDir, castle), "diff")
 }
 
-func (a *App) Link(string) error {
-	return errors.New("link is not implemented in Go yet")
+func (a *App) Link(castle string) error {
+	if strings.TrimSpace(castle) == "" {
+		castle = "dotfiles"
+	}
+	return a.LinkCastle(castle)
+}
+
+func (a *App) LinkCastle(castle string) error {
+	castleHome := filepath.Join(a.ReposDir, castle, "home")
+	info, err := os.Stat(castleHome)
+	if err != nil || !info.IsDir() {
+		return fmt.Errorf("could not symlink %s, expected %s to exist and contain dotfiles", castle, castleHome)
+	}
+
+	subdirs, err := readSubdirs(filepath.Join(a.ReposDir, castle, ".homesick_subdir"))
+	if err != nil {
+		return err
+	}
+
+	if err := a.linkEach(castleHome, castleHome, subdirs); err != nil {
+		return err
+	}
+
+	for _, subdir := range subdirs {
+		base := filepath.Join(castleHome, subdir)
+		if _, err := os.Stat(base); err != nil {
+			if errors.Is(err, os.ErrNotExist) {
+				continue
+			}
+			return err
+		}
+
+		if err := a.linkEach(castleHome, base, subdirs); err != nil {
+			return err
+		}
+	}
+
+	return nil
 }
 
 func (a *App) Unlink(string) error {
@@ -138,6 +175,127 @@ func (a *App) Track(string, string) error {
 	return errors.New("track is not implemented in Go yet")
 }
 
+func (a *App) linkEach(castleHome string, baseDir string, subdirs []string) error {
+	entries, err := os.ReadDir(baseDir)
+	if err != nil {
+		return err
+	}
+
+	for _, entry := range entries {
+		name := entry.Name()
+		if name == "." || name == ".." {
+			continue
+		}
+
+		source := filepath.Join(baseDir, name)
+		ignore, err := matchesIgnoredDir(castleHome, source, subdirs)
+		if err != nil {
+			return err
+		}
+		if ignore {
+			continue
+		}
+
+		relDir, err := filepath.Rel(castleHome, baseDir)
+		if err != nil {
+			return err
+		}
+
+		destination := filepath.Join(a.HomeDir, relDir, name)
+		if relDir == "." {
+			destination = filepath.Join(a.HomeDir, name)
+		}
+
+		if err := a.linkPath(source, destination); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func (a *App) linkPath(source string, destination string) error {
+	absSource, err := filepath.Abs(source)
+	if err != nil {
+		return err
+	}
+
+	if err := os.MkdirAll(filepath.Dir(destination), 0o755); err != nil {
+		return err
+	}
+
+	info, err := os.Lstat(destination)
+	if err == nil {
+		if info.Mode()&os.ModeSymlink != 0 {
+			target, readErr := os.Readlink(destination)
+			if readErr == nil && target == absSource {
+				return nil
+			}
+		}
+
+		if !a.Force {
+			return fmt.Errorf("%s exists", destination)
+		}
+
+		if rmErr := os.RemoveAll(destination); rmErr != nil {
+			return rmErr
+		}
+	} else if !errors.Is(err, os.ErrNotExist) {
+		return err
+	}
+
+	if err := os.Symlink(absSource, destination); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func readSubdirs(path string) ([]string, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return []string{}, nil
+		}
+		return nil, err
+	}
+
+	lines := strings.Split(string(data), "\n")
+	result := make([]string, 0, len(lines))
+	for _, line := range lines {
+		trimmed := strings.TrimSpace(line)
+		if trimmed == "" {
+			continue
+		}
+		result = append(result, filepath.Clean(trimmed))
+	}
+
+	return result, nil
+}
+
+func matchesIgnoredDir(castleHome string, candidate string, subdirs []string) (bool, error) {
+	absCandidate, err := filepath.Abs(candidate)
+	if err != nil {
+		return false, err
+	}
+
+	ignoreSet := map[string]struct{}{}
+	for _, subdir := range subdirs {
+		clean := filepath.Clean(subdir)
+		for clean != "." && clean != string(filepath.Separator) {
+			ignoreSet[filepath.Join(castleHome, clean)] = struct{}{}
+			next := filepath.Dir(clean)
+			if next == clean {
+				break
+			}
+			clean = next
+		}
+	}
+
+	_, ok := ignoreSet[absCandidate]
+	return ok, nil
+}
+
 func runGit(dir string, args ...string) error {
 	cmd := exec.Command("git", args...)
 	cmd.Dir = dir