package passwords

import (
	"crypto/rand"
	"errors"
	"fmt"
	"math/big"
	"strings"
)

const (
	lowercaseChars = "abcdefghijkmnopqrstuvwxyz"
	uppercaseChars = "ABCDEFGHJKLMNPQRSTUVWXYZ"
	digitChars     = "23456789"
	symbolChars    = "!@#$%^&*()-_=+[]{}<>?/."
)

var ErrImpossibleProfile = errors.New("impossible password profile")

type Profile struct {
	Name           string
	Length         int
	Lowercase      bool
	Uppercase      bool
	Digits         bool
	Symbols        bool
	MinLowercase   int
	MinUppercase   int
	MinDigits      int
	MinSymbols     int
	ExcludeSimilar bool
}

func DefaultProfiles() map[string]Profile {
	return map[string]Profile{
		"strong": {
			Name:           "strong",
			Length:         24,
			Lowercase:      true,
			Uppercase:      true,
			Digits:         true,
			Symbols:        true,
			MinLowercase:   2,
			MinUppercase:   2,
			MinDigits:      2,
			MinSymbols:     2,
			ExcludeSimilar: true,
		},
		"memorable": {
			Name:           "memorable",
			Length:         20,
			Lowercase:      true,
			Uppercase:      true,
			Digits:         true,
			Symbols:        false,
			MinLowercase:   4,
			MinUppercase:   2,
			MinDigits:      2,
			ExcludeSimilar: true,
		},
	}
}

func Generate(profile Profile) (string, error) {
	if err := validateProfile(profile); err != nil {
		return "", err
	}

	var chars []byte
	var pool strings.Builder
	if profile.Lowercase {
		pool.WriteString(lowercaseChars)
		chars = append(chars, mustRandomChars(lowercaseChars, profile.MinLowercase)...)
	}
	if profile.Uppercase {
		pool.WriteString(uppercaseChars)
		chars = append(chars, mustRandomChars(uppercaseChars, profile.MinUppercase)...)
	}
	if profile.Digits {
		pool.WriteString(digitChars)
		chars = append(chars, mustRandomChars(digitChars, profile.MinDigits)...)
	}
	if profile.Symbols {
		pool.WriteString(symbolChars)
		chars = append(chars, mustRandomChars(symbolChars, profile.MinSymbols)...)
	}

	allChars := pool.String()
	for len(chars) < profile.Length {
		ch, err := randomChar(allChars)
		if err != nil {
			return "", err
		}
		chars = append(chars, ch)
	}

	if err := shuffle(chars); err != nil {
		return "", err
	}

	return string(chars), nil
}

func validateProfile(profile Profile) error {
	if profile.Length <= 0 {
		return fmt.Errorf("%w: length must be positive", ErrImpossibleProfile)
	}

	required := profile.MinLowercase + profile.MinUppercase + profile.MinDigits + profile.MinSymbols
	if required > profile.Length {
		return fmt.Errorf("%w: minimum character counts exceed length", ErrImpossibleProfile)
	}

	if profile.MinLowercase > 0 && !profile.Lowercase {
		return fmt.Errorf("%w: lowercase disabled with lowercase minimum", ErrImpossibleProfile)
	}
	if profile.MinUppercase > 0 && !profile.Uppercase {
		return fmt.Errorf("%w: uppercase disabled with uppercase minimum", ErrImpossibleProfile)
	}
	if profile.MinDigits > 0 && !profile.Digits {
		return fmt.Errorf("%w: digits disabled with digit minimum", ErrImpossibleProfile)
	}
	if profile.MinSymbols > 0 && !profile.Symbols {
		return fmt.Errorf("%w: symbols disabled with symbol minimum", ErrImpossibleProfile)
	}

	if !profile.Lowercase && !profile.Uppercase && !profile.Digits && !profile.Symbols {
		return fmt.Errorf("%w: no character sets enabled", ErrImpossibleProfile)
	}

	return nil
}

func mustRandomChars(chars string, count int) []byte {
	if count <= 0 {
		return nil
	}

	out := make([]byte, 0, count)
	for i := 0; i < count; i++ {
		ch, err := randomChar(chars)
		if err != nil {
			panic(err)
		}
		out = append(out, ch)
	}
	return out
}

func randomChar(chars string) (byte, error) {
	n, err := rand.Int(rand.Reader, big.NewInt(int64(len(chars))))
	if err != nil {
		return 0, fmt.Errorf("random index: %w", err)
	}
	return chars[n.Int64()], nil
}

func shuffle(chars []byte) error {
	for i := len(chars) - 1; i > 0; i-- {
		n, err := rand.Int(rand.Reader, big.NewInt(int64(i+1)))
		if err != nil {
			return fmt.Errorf("shuffle password: %w", err)
		}
		j := int(n.Int64())
		chars[i], chars[j] = chars[j], chars[i]
	}
	return nil
}