// Package config loads application configuration from defaults, a YAML/JSON
// document, environment variables (optionally via a .env file) and CLI flags.
//
// Precedence, highest first:
//  1. CLI flags (--conf, --log-level, --listen-addr, --listen-addr6, --port)
//  2. Environment variables
//  3. .env file located next to the executable or in the working directory
//  4. Configuration file supplied with --conf
//  5. Built-in defaults
package config

import (
	"errors"
	"flag"
	"fmt"
	"os"
	"path/filepath"
	"strconv"
	"strings"

	"gopkg.in/yaml.v3"
)

// Config holds every runtime setting supported by kereru-forms.
type Config struct {
	// LogLevel is the minimum log level (trace, debug, info, warn, error).
	LogLevel string `yaml:"logLevel" json:"logLevel"`
	// LogFormat is "json" or "console".
	LogFormat string `yaml:"logFormat" json:"logFormat"`
	// Environment is a free-form deployment label (development, staging, production).
	Environment string `yaml:"environment" json:"environment"`

	// HTTPAddr is the IPv4 address to listen on. Empty disables that listener.
	HTTPAddr string `yaml:"listenAddr" json:"listenAddr"`
	// HTTPAddr6 is the IPv6 address to listen on. Empty disables that listener.
	HTTPAddr6 string `yaml:"listenAddr6" json:"listenAddr6"`
	// Port is the TCP port shared by both listeners.
	Port string `yaml:"port" json:"port"`

	// DBDriver selects the storage backend. Currently "sqlite" is implemented.
	DBDriver string `yaml:"dbDriver" json:"dbDriver"`
	// DBDSN is the driver-specific connection string.
	DBDSN string `yaml:"dbDsn" json:"dbDsn"`

	// TelemetryEnabled turns on OpenTelemetry tracing/metrics export.
	TelemetryEnabled bool `yaml:"telemetryEnabled" json:"telemetryEnabled"`
	// OTLPEndpoint is the OTLP/HTTP collector endpoint (host:port or URL).
	OTLPEndpoint string `yaml:"otlpEndpoint" json:"otlpEndpoint"`

	// CORSOrigins is the list of allowed browser origins ("*" allows all).
	CORSOrigins []string `yaml:"corsOrigins" json:"corsOrigins"`

	// StaticDir optionally serves a built frontend from this directory.
	StaticDir string `yaml:"staticDir" json:"staticDir"`
}

// Default returns the built-in configuration.
func Default() *Config {
	return &Config{
		LogLevel:         "info",
		LogFormat:        "console",
		Environment:      "development",
		HTTPAddr:         "0.0.0.0",
		HTTPAddr6:        "",
		Port:             "8080",
		DBDriver:         "sqlite",
		DBDSN:            "file:kereru.db?_pragma=busy_timeout(5000)&_pragma=foreign_keys(1)",
		TelemetryEnabled: false,
		OTLPEndpoint:     "localhost:4318",
		CORSOrigins:      []string{"http://localhost:5173"},
		StaticDir:        "",
	}
}

// Load builds the configuration from args (usually os.Args[1:]).
func Load(args []string) (*Config, error) {
	cfg := Default()

	dotEnvPath := LoadDotEnv()
	_ = dotEnvPath

	confPath := scanConf(args)
	if confPath != "" {
		if err := loadFile(cfg, confPath); err != nil {
			return nil, err
		}
	}

	applyEnv(cfg)

	fs := flag.NewFlagSet("kereru-forms", flag.ContinueOnError)
	fs.SetOutput(os.Stderr)

	var (
		flagConf   = fs.String("conf", confPath, "path to a YAML or JSON configuration file")
		flagLevel  = fs.String("log-level", cfg.LogLevel, "log level: trace, debug, info, warn, error")
		flagFormat = fs.String("log-format", cfg.LogFormat, "log format: console or json")
		flagAddr   = fs.String("listen-addr", cfg.HTTPAddr, "IPv4 listen address (empty to disable)")
		flagAddr6  = fs.String("listen-addr6", cfg.HTTPAddr6, "IPv6 listen address (empty to disable)")
		flagPort   = fs.String("port", cfg.Port, "TCP listen port")
		flagDB     = fs.String("db-dsn", cfg.DBDSN, "database connection string")
		flagDriver = fs.String("db-driver", cfg.DBDriver, "database driver (sqlite)")
		flagStatic = fs.String("static-dir", cfg.StaticDir, "directory containing a built frontend (optional)")
	)
	fs.Usage = func() {
		fmt.Fprintf(fs.Output(), "Usage: kereru-forms [flags]\n\nFlags:\n")
		fs.PrintDefaults()
	}

	if err := fs.Parse(args); err != nil {
		return nil, err
	}
	_ = flagConf

	cfg.LogLevel = *flagLevel
	cfg.LogFormat = *flagFormat
	cfg.HTTPAddr = *flagAddr
	cfg.HTTPAddr6 = *flagAddr6
	cfg.Port = *flagPort
	cfg.DBDSN = *flagDB
	cfg.DBDriver = *flagDriver
	cfg.StaticDir = *flagStatic

	if err := cfg.Validate(); err != nil {
		return nil, err
	}
	return cfg, nil
}

// Validate checks that the configuration is usable.
func (c *Config) Validate() error {
	if strings.TrimSpace(c.Port) == "" {
		return errors.New("port must not be empty")
	}
	if _, err := strconv.Atoi(c.Port); err != nil {
		return fmt.Errorf("port %q is not a number", c.Port)
	}
	if c.HTTPAddr == "" && c.HTTPAddr6 == "" {
		return errors.New("at least one of listenAddr or listenAddr6 must be set")
	}
	if strings.TrimSpace(c.DBDriver) == "" {
		return errors.New("dbDriver must not be empty")
	}
	if strings.TrimSpace(c.DBDSN) == "" {
		return errors.New("dbDsn must not be empty")
	}
	return nil
}

// Addresses returns the listen addresses (host:port) for enabled listeners.
func (c *Config) Addresses() []string {
	var addrs []string
	if c.HTTPAddr != "" {
		addrs = append(addrs, joinHostPort(c.HTTPAddr, c.Port))
	}
	if c.HTTPAddr6 != "" {
		addrs = append(addrs, joinHostPort(c.HTTPAddr6, c.Port))
	}
	return addrs
}

func joinHostPort(host, port string) string {
	if strings.Contains(host, ":") && !strings.HasPrefix(host, "[") {
		return "[" + host + "]:" + port
	}
	return host + ":" + port
}

func scanConf(args []string) string {
	for i := 0; i < len(args); i++ {
		arg := args[i]
		if arg == "--conf" || arg == "-conf" {
			if i+1 < len(args) {
				return args[i+1]
			}
		}
		if strings.HasPrefix(arg, "--conf=") {
			return strings.TrimPrefix(arg, "--conf=")
		}
		if strings.HasPrefix(arg, "-conf=") {
			return strings.TrimPrefix(arg, "-conf=")
		}
	}
	return ""
}

func loadFile(cfg *Config, path string) error {
	data, err := os.ReadFile(path)
	if err != nil {
		return fmt.Errorf("read config %s: %w", path, err)
	}
	if strings.EqualFold(filepath.Ext(path), ".json") {
		if err := yaml.Unmarshal(data, cfg); err != nil {
			return fmt.Errorf("parse JSON config %s: %w", path, err)
		}
		return nil
	}
	if err := yaml.Unmarshal(data, cfg); err != nil {
		return fmt.Errorf("parse YAML config %s: %w", path, err)
	}
	return nil
}

func applyEnv(cfg *Config) {
	setString(&cfg.LogLevel, "KERERU_LOG_LEVEL")
	setString(&cfg.LogFormat, "KERERU_LOG_FORMAT")
	setString(&cfg.Environment, "KERERU_ENVIRONMENT")
	setString(&cfg.HTTPAddr, "KERERU_LISTEN_ADDR")
	setString(&cfg.HTTPAddr6, "KERERU_LISTEN_ADDR6")
	setString(&cfg.Port, "KERERU_PORT")
	setString(&cfg.DBDriver, "KERERU_DB_DRIVER")
	setString(&cfg.DBDSN, "KERERU_DB_DSN")
	setString(&cfg.StaticDir, "KERERU_STATIC_DIR")
	if v, ok := os.LookupEnv("KERERU_TELEMETRY_ENABLED"); ok {
		if b, err := strconv.ParseBool(v); err == nil {
			cfg.TelemetryEnabled = b
		}
	}
	setString(&cfg.OTLPEndpoint, "OTEL_EXPORTER_OTLP_ENDPOINT")
	if v, ok := os.LookupEnv("KERERU_CORS_ORIGINS"); ok {
		var origins []string
		for _, part := range strings.Split(v, ",") {
			if p := strings.TrimSpace(part); p != "" {
				origins = append(origins, p)
			}
		}
		if len(origins) > 0 {
			cfg.CORSOrigins = origins
		}
	}
}

func setString(dst *string, key string) {
	if v, ok := os.LookupEnv(key); ok {
		*dst = v
	}
}

// LoadDotEnv reads a .env file from the directory containing the executable
// or, failing that, the working directory. Existing environment variables are never
// overwritten. It returns the path that was loaded, if any.
func LoadDotEnv() string {
	candidateDirs := []string{}
	if exe, err := os.Executable(); err == nil {
		candidateDirs = append(candidateDirs, filepath.Dir(exe))
	}
	if wd, err := os.Getwd(); err == nil {
		candidateDirs = append(candidateDirs, wd)
	}

	for _, dir := range candidateDirs {
		path := filepath.Join(dir, ".env")
		data, err := os.ReadFile(path)
		if err != nil {
			continue
		}
		for _, line := range strings.Split(string(data), "\n") {
			line = strings.TrimSpace(line)
			if line == "" || strings.HasPrefix(line, "#") {
				continue
			}
			line = strings.TrimPrefix(line, "export ")
			key, value, ok := strings.Cut(line, "=")
			if !ok {
				continue
			}
			key = strings.TrimSpace(key)
			value = strings.Trim(strings.TrimSpace(value), `"'`)
			if key == "" {
				continue
			}
			if _, exists := os.LookupEnv(key); !exists {
				_ = os.Setenv(key, value)
			}
		}
		return path
	}
	return ""
}