package config

import (
	"fmt"
	"io"
	"os"
	"reflect"
	"sort"
	"strings"
	"time"

	"github.com/mleku/lol/chk"
	"go-simpler.org/env"
)

type C struct {
	LogLevel string        `env:"REVERSE_LOG_LEVEL" default:"info" usage:"Log level: fatal error warn info debug trace"`
	Listen   string        `env:"REVERSE_LISTEN" usage:"Listen address for reverse proxy" default:":443"`
	Mapping  string        `env:"REVERSE_MAPPING" usage:"file containing domain/target mappings for reverse proxy" default:"~/.config/reverse/mapping.conf"`
	Cache    string        `env:"REVERSE_CERTCACHE" usage:"directory where certificates from letsencrypt are stored" default:"~/.cache/reverse"`
	HSTS     bool          `env:"REVERSE_HSTS" usage:"add Strict-TransportSecurity header" default:"false"`
	Email    string        `env:"REVERSE_EMAIL" usage:"email address presented to letsencrypt CA"`
	HTTP     string        `env:"REVERSE_HTTP" usage:"http address for HTTP->HTTPS upgrades" default:":80"`
	Idle     time.Duration `env:"REVERSE_IDLE" usage:"how long idle connection is kept before closing" default:"1m"`
}

func New() (cfg *C, err error) {
	cfg = new(C)
	if err = env.Load(cfg, &env.Options{SliceSep: ","}); chk.T(err) {
		return
	}
	if GetEnv() {
		PrintEnv(cfg, os.Stdout)
		os.Exit(0)
	}
	if HelpRequested() {
		PrintHelp(cfg, os.Stderr)
		os.Exit(0)
	}
	return
}

// HelpRequested determines if the command line arguments indicate a request for help
//
// # Return Values
//
//   - help: A boolean value indicating true if a help flag was detected in the
//     command line arguments, false otherwise
//
// # Expected Behaviour
//
// The function checks the first command line argument for common help flags and
// returns true if any of them are present. Returns false if no help flag is found
func HelpRequested() (help bool) {
	if len(os.Args) > 1 {
		switch strings.ToLower(os.Args[1]) {
		case "help", "-h", "--h", "-help", "--help", "?":
			help = true
		}
	}
	return
}

// GetEnv checks if the first command line argument is "env" and returns
// whether the environment configuration should be printed.
//
// # Return Values
//
//   - requested: A boolean indicating true if the 'env' argument was
//     provided, false otherwise.
//
// # Expected Behaviour
//
// The function returns true when the first command line argument is "env"
// (case-insensitive), signalling that the environment configuration should be
// printed. Otherwise, it returns false.
func GetEnv() (requested bool) {
	if len(os.Args) > 1 {
		switch strings.ToLower(os.Args[1]) {
		case "env":
			requested = true
		}
	}
	return
}

// KV is a key/value pair.
type KV struct{ Key, Value string }

// KVSlice is a sortable slice of key/value pairs, designed for managing
// configuration data and enabling operations like merging and sorting based on
// keys.
type KVSlice []KV

func (kv KVSlice) Len() int           { return len(kv) }
func (kv KVSlice) Less(i, j int) bool { return kv[i].Key < kv[j].Key }
func (kv KVSlice) Swap(i, j int)      { kv[i], kv[j] = kv[j], kv[i] }

// Compose merges two KVSlice instances into a new slice where key-value pairs
// from the second slice override any duplicate keys from the first slice.
//
// # Parameters
//
//   - kv2: The second KVSlice whose entries will be merged with the receiver.
//
// # Return Values
//
//   - out: A new KVSlice containing all entries from both slices, with keys
//     from kv2 taking precedence over keys from the receiver.
//
// # Expected Behaviour
//
// The method returns a new KVSlice that combines the contents of the receiver
// and kv2. If any key exists in both slices, the value from kv2 is used. The
// resulting slice remains sorted by keys as per the KVSlice implementation.
func (kv KVSlice) Compose(kv2 KVSlice) (out KVSlice) {
	// duplicate the initial KVSlice
	for _, p := range kv {
		out = append(out, p)
	}
out:
	for i, p := range kv2 {
		for j, q := range out {
			// if the key is repeated, replace the value
			if p.Key == q.Key {
				out[j].Value = kv2[i].Value
				continue out
			}
		}
		out = append(out, p)
	}
	return
}

// EnvKV generates key/value pairs from a configuration object's struct tags
//
// # Parameters
//
//   - cfg: A configuration object whose struct fields are processed for env tags
//
// # Return Values
//
//   - m: A KVSlice containing key/value pairs derived from the config's env tags
//
// # Expected Behaviour
//
// Processes each field of the config object, extracting values tagged with
// "env" and converting them to strings. Skips fields without an "env" tag.
// Handles various value types including strings, integers, booleans, durations,
// and string slices by joining elements with commas.
func EnvKV(cfg any) (m KVSlice) {
	t := reflect.TypeOf(cfg)
	for i := 0; i < t.NumField(); i++ {
		k := t.Field(i).Tag.Get("env")
		v := reflect.ValueOf(cfg).Field(i).Interface()
		var val string
		switch v.(type) {
		case string:
			val = v.(string)
		case int, bool, time.Duration:
			val = fmt.Sprint(v)
		case []string:
			arr := v.([]string)
			if len(arr) > 0 {
				val = strings.Join(arr, ",")
			}
		}
		// this can happen with embedded structs
		if k == "" {
			continue
		}
		m = append(m, KV{k, val})
	}
	return
}

// PrintEnv outputs sorted environment key/value pairs from a configuration object
// to the provided writer
//
// # Parameters
//
//   - cfg: Pointer to the configuration object containing env tags
//
//   - printer: Destination for the output, typically an io.Writer implementation
//
// # Expected Behaviour
//
// Outputs each environment variable derived from the config's struct tags in
// sorted order, formatted as "key=value\n" to the specified writer
func PrintEnv(cfg *C, printer io.Writer) {
	kvs := EnvKV(*cfg)
	sort.Sort(kvs)
	for _, v := range kvs {
		_, _ = fmt.Fprintf(printer, "%s=%s\n", v.Key, v.Value)
	}
}

// PrintHelp prints help information including application version, environment
// variable configuration, and details about .env file handling to the provided
// writer
//
// # Parameters
//
//   - cfg: Configuration object containing app name and config directory path
//
//   - printer: Output destination for the help text
//
// # Expected Behaviour
//
// Prints application name and version followed by environment variable
// configuration details, explains .env file behaviour including automatic
// loading and custom path options, and displays current configuration values
// using PrintEnv. Outputs all information to the specified writer
func PrintHelp(cfg *C, printer io.Writer) {
	_, _ = fmt.Fprintf(printer, "reverse usage:\n\n")
	env.Usage(cfg, printer, &env.Options{SliceSep: ","})
	_, _ = fmt.Fprintf(
		printer,
		"\nCLI parameter 'help' also prints this information\n"+
			"use the parameter 'env' to print out the current configuration to the terminal\n\n",
	)
	_, _ = fmt.Fprintf(printer, "current configuration:\n\n")
	PrintEnv(cfg, printer)
	return
}