package config

import (
	"fmt"
	"os"
	"path/filepath"
	"runtime"

	"gopkg.in/yaml.v3"
)

// CurrentConfigSchemaVersion is the highest config-file schema version
// this binary understands. A future schema version bump must add the
// matching parser support alongside this constant.
const CurrentConfigSchemaVersion = 1

// ConfigFile is the strict YAML schema for the global ctask config file.
// All fields are optional; missing fields fall back to built-in defaults
// via the Resolver. Unknown keys invalidate the entire file (no partial
// apply) per the v0.6 spec.
type ConfigFile struct {
	SchemaVersion   int    `yaml:"schema_version"`
	CtaskRoot       string `yaml:"ctask_root"`
	ProjectRoot     string `yaml:"project_root"`
	SeedDir         string `yaml:"seed_dir"`
	DefaultAgent    string `yaml:"default_agent"`
	DefaultCategory string `yaml:"default_category"`
	Editor          string `yaml:"editor"`
	SessionMode     string `yaml:"session_mode"`
}

// configFileKnownKeys returns the set of YAML keys recognized by the
// ConfigFile struct. Kept in lockstep with the yaml tags above.
func configFileKnownKeys() map[string]struct{} {
	return map[string]struct{}{
		"schema_version":   {},
		"ctask_root":       {},
		"project_root":     {},
		"seed_dir":         {},
		"default_agent":    {},
		"default_category": {},
		"editor":           {},
		"session_mode":     {},
	}
}

// LoadConfigFile reads and parses the config file at path. Returns:
//   - (nil, nil)  the file does not exist (caller falls back to defaults)
//   - (nil, err)  any I/O, parse, unknown-key, or schema-version error
//   - (cfg, nil)  on success
//
// Unknown top-level keys invalidate the entire file with a descriptive
// error; ctask never partially applies a file with typos. Future schema
// versions are rejected with an upgrade message.
func LoadConfigFile(path string) (*ConfigFile, error) {
	data, err := os.ReadFile(path)
	if err != nil {
		if os.IsNotExist(err) {
			return nil, nil
		}
		return nil, err
	}

	// First pass: parse into a generic map of YAML nodes so we can
	// inspect raw key names without trusting the typed-struct unmarshal
	// (yaml.v3 silently drops unknown keys by default).
	var raw map[string]yaml.Node
	if err := yaml.Unmarshal(data, &raw); err != nil {
		return nil, fmt.Errorf("parsing config file: %w", err)
	}

	known := configFileKnownKeys()
	for key := range raw {
		if _, ok := known[key]; !ok {
			return nil, fmt.Errorf("unknown key: %q", key)
		}
	}

	// Second pass: typed unmarshal.
	var cfg ConfigFile
	if err := yaml.Unmarshal(data, &cfg); err != nil {
		return nil, fmt.Errorf("parsing config file: %w", err)
	}

	if cfg.SchemaVersion > CurrentConfigSchemaVersion {
		return nil, fmt.Errorf(
			"config file requires schema version %d, but this binary supports up to version %d. Please upgrade ctask.",
			cfg.SchemaVersion, CurrentConfigSchemaVersion)
	}

	return &cfg, nil
}

// ConfigFilePath returns the platform-appropriate global config path.
//
// Linux / macOS / WSL:
//   - $XDG_CONFIG_HOME/ctask/config.yaml when XDG_CONFIG_HOME is set
//   - ~/.config/ctask/config.yaml otherwise
//
// Native Windows:
//   - %APPDATA%\ctask\config.yaml (fallback: %USERPROFILE%\AppData\Roaming\ctask\config.yaml)
//
// The path is returned regardless of whether the file exists; callers
// stat the path themselves.
func ConfigFilePath() string {
	if runtime.GOOS == "windows" {
		appData := os.Getenv("APPDATA")
		if appData == "" {
			home, _ := os.UserHomeDir()
			appData = filepath.Join(home, "AppData", "Roaming")
		}
		return filepath.Join(appData, "ctask", "config.yaml")
	}
	if xdg := os.Getenv("XDG_CONFIG_HOME"); xdg != "" {
		return filepath.Join(xdg, "ctask", "config.yaml")
	}
	home, _ := os.UserHomeDir()
	return filepath.Join(home, ".config", "ctask", "config.yaml")
}