core/internal/config/config.go

package config

import (
	"encoding/json"
	"fmt"
	"os"
	"path/filepath"
	"strconv"
	"time"
)

// Config holds process configuration loaded from environment variables.
type Config struct {
	HTTPAddr      string
	ReadTimeout   time.Duration
	WriteTimeout  time.Duration
	ShutdownGrace time.Duration
	DatabaseDSN   string
	MigrationsDir string
}

func Load() (Config, error) {
	readTimeout, err := envDuration("READ_TIMEOUT", 10*time.Second)
	if err != nil {
		return Config{}, err
	}

	writeTimeout, err := envDuration("WRITE_TIMEOUT", 15*time.Second)
	if err != nil {
		return Config{}, err
	}

	shutdownGrace, err := envDuration("SHUTDOWN_GRACE", 10*time.Second)
	if err != nil {
		return Config{}, err
	}

	cfg := Config{
		HTTPAddr:      envOrDefault("HTTP_ADDR", ":9999"),
		ReadTimeout:   readTimeout,
		WriteTimeout:  writeTimeout,
		ShutdownGrace: shutdownGrace,
		DatabaseDSN:   os.Getenv("DATABASE_DSN"),
		MigrationsDir: envOrDefault("MIGRATIONS_DIR", "migrations"),
	}

	fileCfg, err := loadFileConfig()
	if err != nil {
		return Config{}, err
	}
	applyFileConfig(&cfg, fileCfg)

	return cfg, nil
}

func envOrDefault(key, fallback string) string {
	if value := os.Getenv(key); value != "" {
		return value
	}
	return fallback
}

func envDuration(key string, fallback time.Duration) (time.Duration, error) {
	value := os.Getenv(key)
	if value == "" {
		return fallback, nil
	}

	seconds, err := strconv.Atoi(value)
	if err != nil {
		return 0, fmt.Errorf("%s must be an integer number of seconds: %w", key, err)
	}
	if seconds <= 0 {
		return 0, fmt.Errorf("%s must be > 0", key)
	}

	return time.Duration(seconds) * time.Second, nil
}

type fileConfig struct {
	HTTPAddr             *string `json:"http_addr"`
	ReadTimeoutSeconds   *int    `json:"read_timeout_seconds"`
	WriteTimeoutSeconds  *int    `json:"write_timeout_seconds"`
	ShutdownGraceSeconds *int    `json:"shutdown_grace_seconds"`
	DatabaseDSN          *string `json:"database_dsn"`
	Database             *dbFileConfig `json:"database"`
	MigrationsDir        *string `json:"migrations_dir"`
}

type dbFileConfig struct {
	User     *string `json:"user"`
	Password *string `json:"password"`
	Host     *string `json:"host"`
	Port     *int    `json:"port"`
	Name     *string `json:"name"`
	Params   *string `json:"params"`
}

func loadFileConfig() (fileConfig, error) {
	path := os.Getenv("CONFIG_FILE")
	if path == "" {
		path = "config.json"
		if _, err := os.Stat(path); err != nil {
			if os.IsNotExist(err) {
				return fileConfig{}, nil
			}
			return fileConfig{}, err
		}
	} else {
		if _, err := os.Stat(path); err != nil {
			return fileConfig{}, fmt.Errorf("config file %s: %w", path, err)
		}
	}

	raw, err := os.ReadFile(filepath.Clean(path))
	if err != nil {
		return fileConfig{}, err
	}

	var cfg fileConfig
	if err := json.Unmarshal(raw, &cfg); err != nil {
		return fileConfig{}, fmt.Errorf("parse config file %s: %w", path, err)
	}

	return cfg, nil
}

func applyFileConfig(cfg *Config, fileCfg fileConfig) {
	if fileCfg.HTTPAddr != nil && os.Getenv("HTTP_ADDR") == "" {
		cfg.HTTPAddr = *fileCfg.HTTPAddr
	}
	if fileCfg.ReadTimeoutSeconds != nil && os.Getenv("READ_TIMEOUT") == "" {
		if *fileCfg.ReadTimeoutSeconds > 0 {
			cfg.ReadTimeout = time.Duration(*fileCfg.ReadTimeoutSeconds) * time.Second
		}
	}
	if fileCfg.WriteTimeoutSeconds != nil && os.Getenv("WRITE_TIMEOUT") == "" {
		if *fileCfg.WriteTimeoutSeconds > 0 {
			cfg.WriteTimeout = time.Duration(*fileCfg.WriteTimeoutSeconds) * time.Second
		}
	}
	if fileCfg.ShutdownGraceSeconds != nil && os.Getenv("SHUTDOWN_GRACE") == "" {
		if *fileCfg.ShutdownGraceSeconds > 0 {
			cfg.ShutdownGrace = time.Duration(*fileCfg.ShutdownGraceSeconds) * time.Second
		}
	}
	if fileCfg.DatabaseDSN != nil && os.Getenv("DATABASE_DSN") == "" {
		cfg.DatabaseDSN = *fileCfg.DatabaseDSN
	}
	if cfg.DatabaseDSN == "" && os.Getenv("DATABASE_DSN") == "" {
		if dsn := buildDSN(fileCfg.Database); dsn != "" {
			cfg.DatabaseDSN = dsn
		}
	}
	if fileCfg.MigrationsDir != nil && os.Getenv("MIGRATIONS_DIR") == "" {
		cfg.MigrationsDir = *fileCfg.MigrationsDir
	}
}

func buildDSN(db *dbFileConfig) string {
	if db == nil || db.Name == nil {
		return ""
	}

	user := valueOrDefault(db.User, "reanimator")
	password := valueOrDefault(db.Password, "reanimator")
	host := valueOrDefault(db.Host, "127.0.0.1")
	port := intValueOrDefault(db.Port, 3306)
	params := valueOrDefault(db.Params, "parseTime=true")

	return fmt.Sprintf("%s:%s@tcp(%s:%d)/%s?%s", user, password, host, port, *db.Name, params)
}

func valueOrDefault(value *string, fallback string) string {
	if value == nil || *value == "" {
		return fallback
	}
	return *value
}

func intValueOrDefault(value *int, fallback int) int {
	if value == nil || *value <= 0 {
		return fallback
	}
	return *value
}