Add support for config inheritance

internal/templates/templates.go

@@ -5,6 +5,7 @@ import (
 	"embed"
 	"encoding/json"
 	"fmt"
+	"os"
 	"path/filepath"
 	"sort"
 	"strings"
@@ -13,6 +14,15 @@ import (
 	"github.com/tidwall/jsonc"
 )
 
+// maxExtendsDepth limits inheritance chain depth to prevent infinite loops.
+const maxExtendsDepth = 10
+
+// isPath returns true if the extends value looks like a file path rather than a template name.
+// A value is considered a path if it contains a path separator or starts with ".".
+func isPath(s string) bool {
+	return strings.ContainsAny(s, "/\\") || strings.HasPrefix(s, ".")
+}
+
 //go:embed *.json
 var templatesFS embed.FS
 
@@ -63,11 +73,30 @@ func List() []Template {
 }
 
 // Load loads a template by name and returns the parsed config.
+// If the template uses "extends", the inheritance chain is resolved.
 func Load(name string) (*config.Config, error) {
+	return loadWithDepth(name, 0, nil)
+}
+
+// loadWithDepth loads a template with cycle and depth tracking.
+func loadWithDepth(name string, depth int, seen map[string]bool) (*config.Config, error) {
+	if depth > maxExtendsDepth {
+		return nil, fmt.Errorf("extends chain too deep (max %d)", maxExtendsDepth)
+	}
+
 	// Normalize name (remove .json if present)
 	name = strings.TrimSuffix(name, ".json")
-	filename := name + ".json"
 
+	// Check for cycles
+	if seen == nil {
+		seen = make(map[string]bool)
+	}
+	if seen[name] {
+		return nil, fmt.Errorf("circular extends detected: %q", name)
+	}
+	seen[name] = true
+
+	filename := name + ".json"
 	data, err := templatesFS.ReadFile(filename)
 	if err != nil {
 		return nil, fmt.Errorf("template %q not found", name)
@@ -78,6 +107,15 @@ func Load(name string) (*config.Config, error) {
 		return nil, fmt.Errorf("failed to parse template %q: %w", name, err)
 	}
 
+	// If this template extends another, resolve the chain
+	if cfg.Extends != "" {
+		baseCfg, err := loadWithDepth(cfg.Extends, depth+1, seen)
+		if err != nil {
+			return nil, fmt.Errorf("failed to load base template %q: %w", cfg.Extends, err)
+		}
+		return config.Merge(baseCfg, &cfg), nil
+	}
+
 	return &cfg, nil
 }
 
@@ -95,3 +133,120 @@ func GetPath(name string) string {
 	name = strings.TrimSuffix(name, ".json")
 	return filepath.Join("internal/templates", name+".json")
 }
+
+// ResolveExtends resolves the extends field in a config by loading and merging
+// the base template or config file. If the config has no extends field, it is returned as-is.
+// Relative paths are resolved relative to the current working directory.
+// Use ResolveExtendsWithBaseDir if you need to resolve relative to a specific directory.
+func ResolveExtends(cfg *config.Config) (*config.Config, error) {
+	return ResolveExtendsWithBaseDir(cfg, "")
+}
+
+// ResolveExtendsWithBaseDir resolves the extends field in a config.
+// The baseDir is used to resolve relative paths in the extends field.
+// If baseDir is empty, relative paths will be resolved relative to the current working directory.
+//
+// The extends field can be:
+//   - A template name (e.g., "code", "npm-install")
+//   - An absolute path (e.g., "/path/to/base.json")
+//   - A relative path (e.g., "./base.json", "../shared/base.json")
+//
+// Paths are detected by the presence of "/" or "\" or a leading ".".
+func ResolveExtendsWithBaseDir(cfg *config.Config, baseDir string) (*config.Config, error) {
+	if cfg == nil || cfg.Extends == "" {
+		return cfg, nil
+	}
+
+	return resolveExtendsWithDepth(cfg, baseDir, 0, nil)
+}
+
+// resolveExtendsWithDepth resolves extends with cycle and depth tracking.
+func resolveExtendsWithDepth(cfg *config.Config, baseDir string, depth int, seen map[string]bool) (*config.Config, error) {
+	if cfg == nil || cfg.Extends == "" {
+		return cfg, nil
+	}
+
+	if depth > maxExtendsDepth {
+		return nil, fmt.Errorf("extends chain too deep (max %d)", maxExtendsDepth)
+	}
+
+	if seen == nil {
+		seen = make(map[string]bool)
+	}
+
+	var baseCfg *config.Config
+	var newBaseDir string
+	var err error
+
+	// Handle file path or template name extends
+	if isPath(cfg.Extends) {
+		baseCfg, newBaseDir, err = loadConfigFile(cfg.Extends, baseDir, seen)
+	} else {
+		baseCfg, err = loadWithDepth(cfg.Extends, depth+1, seen)
+		newBaseDir = ""
+	}
+
+	if err != nil {
+		return nil, err
+	}
+
+	// If the base config also has extends, resolve it recursively
+	if baseCfg.Extends != "" {
+		baseCfg, err = resolveExtendsWithDepth(baseCfg, newBaseDir, depth+1, seen)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return config.Merge(baseCfg, cfg), nil
+}
+
+// loadConfigFile loads a config from a file path with cycle detection.
+// Returns the loaded config, the directory of the loaded file (for resolving nested extends), and any error.
+func loadConfigFile(path, baseDir string, seen map[string]bool) (*config.Config, string, error) {
+	var resolvedPath string
+	switch {
+	case filepath.IsAbs(path):
+		resolvedPath = path
+	case baseDir != "":
+		resolvedPath = filepath.Join(baseDir, path)
+	default:
+		var err error
+		resolvedPath, err = filepath.Abs(path)
+		if err != nil {
+			return nil, "", fmt.Errorf("failed to resolve path %q: %w", path, err)
+		}
+	}
+
+	// Clean and normalize the path for cycle detection
+	resolvedPath = filepath.Clean(resolvedPath)
+
+	if seen[resolvedPath] {
+		return nil, "", fmt.Errorf("circular extends detected: %q", path)
+	}
+	seen[resolvedPath] = true
+
+	data, err := os.ReadFile(resolvedPath) //nolint:gosec // user-provided config path - intentional
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, "", fmt.Errorf("extends file not found: %q", path)
+		}
+		return nil, "", fmt.Errorf("failed to read extends file %q: %w", path, err)
+	}
+
+	// Handle empty file
+	if len(strings.TrimSpace(string(data))) == 0 {
+		return nil, "", fmt.Errorf("extends file is empty: %q", path)
+	}
+
+	var cfg config.Config
+	if err := json.Unmarshal(jsonc.ToJSON(data), &cfg); err != nil {
+		return nil, "", fmt.Errorf("invalid JSON in extends file %q: %w", path, err)
+	}
+
+	if err := cfg.Validate(); err != nil {
+		return nil, "", fmt.Errorf("invalid configuration in extends file %q: %w", path, err)
+	}
+
+	return &cfg, filepath.Dir(resolvedPath), nil
+}