config/manager.go

package config

import (
	"fmt"
	"path/filepath"
	"reflect"
	"strings"
	"sync"

	"github.com/go-playground/validator/v10"
	"github.com/spf13/viper"
)

// Schema marks configuration struct types. Use a struct type for T in [NewManager].
type Schema any

// ManagerOpts defines options for the configuration manager.
type ManagerOpts[T Schema] struct {
	// PreLoad is called before reading the configuration file. The schema pointer
	// may be newly allocated and empty.
	PreLoad func(*T) error

	// PostLoad is called after the file is unmarshalled and validated. Use it to
	// populate derived fields; validation runs again after PostLoad returns.
	PostLoad func(*T) error

	// EnvPrefix enables environment variable overrides with this prefix.
	EnvPrefix string

	// Defaults is a map of default values (dot-notation keys).
	Defaults map[string]any
}

// Manager loads and validates a typed configuration schema via viper.
// Prefer [Manager.Load] during startup and [Manager.C] for quick access after load.
type Manager[T Schema] struct {
	preLoad  func(*T) error
	postLoad func(*T) error

	config *T
	loaded bool
	mu     sync.RWMutex

	viper *viper.Viper
}

// NewManager creates a configuration manager for schema T. T must be a struct type.
func NewManager[T Schema](configName, configType, configPath string) *Manager[T] {
	var zero T
	if reflect.TypeOf(zero).Kind() != reflect.Struct {
		panic(fmt.Sprintf("config: schema type %T must be a struct", zero))
	}

	m := &Manager[T]{
		viper: viper.New(),
	}

	m.viper.SetConfigName(configName)
	m.viper.SetConfigType(configType)
	m.viper.AddConfigPath(configPath)

	return m
}

// WithOpts sets additional options for the Manager.
func (m *Manager[T]) WithOpts(opts *ManagerOpts[T]) *Manager[T] {
	if opts == nil {
		return m
	}

	if opts.PreLoad != nil {
		m.preLoad = opts.PreLoad
	}
	if opts.PostLoad != nil {
		m.postLoad = opts.PostLoad
	}
	if opts.Defaults != nil {
		for key, value := range opts.Defaults {
			m.viper.SetDefault(key, value)
		}
	}
	if opts.EnvPrefix != "" {
		m.WithEnvOverride(opts.EnvPrefix)
	}

	return m
}

// WithEnvOverride sets the environment variable prefix and enables AutomaticEnv.
func (m *Manager[T]) WithEnvOverride(prefix string) *Manager[T] {
	m.viper.SetEnvPrefix(prefix)
	m.viper.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
	m.viper.AutomaticEnv()
	return m
}

// C returns the loaded configuration, loading the file first if needed.
// Panics if loading or validation fails.
func (m *Manager[T]) C() *T {
	m.mu.RLock()
	if m.loaded {
		cfg := m.config
		m.mu.RUnlock()
		return cfg
	}
	m.mu.RUnlock()

	m.mu.Lock()
	defer m.mu.Unlock()

	if m.loaded {
		return m.config
	}

	if err := m.loadUnsafe(); err != nil {
		panic(fmt.Errorf("failed to load configuration: %w", err))
	}

	return m.config
}

// Config is an alias for [Manager.C].
func (m *Manager[T]) Config() *T {
	return m.C()
}

// ConfigPath returns the absolute path to the configuration file that was used.
func (m *Manager[T]) ConfigPath() string {
	if m.viper.ConfigFileUsed() == "" {
		return ""
	}

	absPath, err := filepath.Abs(m.viper.ConfigFileUsed())
	if err != nil {
		return m.viper.ConfigFileUsed()
	}

	return absPath
}

// Viper returns the underlying viper instance. Prefer [Manager.Load] and [Manager.C]
// so validation and loaded state stay consistent.
func (m *Manager[T]) Viper() *viper.Viper {
	return m.viper
}

// Load reads and validates the configuration file.
func (m *Manager[T]) Load() error {
	m.mu.Lock()
	defer m.mu.Unlock()

	if m.loaded {
		return nil
	}

	return m.loadUnsafe()
}

func (m *Manager[T]) loadUnsafe() error {
	m.config = new(T)

	if m.preLoad != nil {
		if err := m.preLoad(m.config); err != nil {
			return fmt.Errorf("pre-load function failed: %w", err)
		}
	}

	if err := m.viper.ReadInConfig(); err != nil {
		return fmt.Errorf("viper failed to read config file: %w", err)
	}
	if err := m.viper.Unmarshal(m.config); err != nil {
		return fmt.Errorf("viper failed to unmarshal config file: %w", err)
	}

	validate := validator.New(validator.WithRequiredStructEnabled())
	if err := validate.Struct(m.config); err != nil {
		return fmt.Errorf("validator failed to validate config file: %w", err)
	}

	if m.postLoad != nil {
		if err := m.postLoad(m.config); err != nil {
			return fmt.Errorf("post-load function failed: %w", err)
		}
		if err := validate.Struct(m.config); err != nil {
			return fmt.Errorf("validator failed after post-load: %w", err)
		}
	}

	m.loaded = true
	return nil
}