// Package authn implements password hashing and high-entropy credential generation.
package authn

import (
	"crypto/rand"
	"crypto/subtle"
	"encoding/base64"
	"errors"
	"fmt"
	"strconv"
	"strings"

	"golang.org/x/crypto/argon2"
)

var ErrInvalidPasswordHash = errors.New("invalid password hash")

type Argon2Parameters struct {
	MemoryKiB uint32
	Time      uint32
	Threads   uint8
	SaltBytes uint32
	KeyBytes  uint32
}

type PasswordHasher struct {
	parameters Argon2Parameters
}

func NewPasswordHasher() PasswordHasher {
	return PasswordHasher{parameters: Argon2Parameters{
		MemoryKiB: 64 * 1024,
		Time:      3,
		Threads:   4,
		SaltBytes: 16,
		KeyBytes:  32,
	}}
}

func newPasswordHasher(parameters Argon2Parameters) PasswordHasher {
	return PasswordHasher{parameters: parameters}
}

func (h PasswordHasher) Hash(password string) (string, error) {
	salt := make([]byte, h.parameters.SaltBytes)
	if _, err := rand.Read(salt); err != nil {
		return "", fmt.Errorf("generate password salt: %w", err)
	}
	derived := argon2.IDKey([]byte(password), salt, h.parameters.Time, h.parameters.MemoryKiB, h.parameters.Threads, h.parameters.KeyBytes)
	return fmt.Sprintf(
		"$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s",
		argon2.Version,
		h.parameters.MemoryKiB,
		h.parameters.Time,
		h.parameters.Threads,
		base64.RawStdEncoding.EncodeToString(salt),
		base64.RawStdEncoding.EncodeToString(derived),
	), nil
}

func (h PasswordHasher) Verify(password, encoded string) (bool, error) {
	parameters, salt, expected, err := parsePasswordHash(encoded)
	if err != nil {
		return false, err
	}
	actual := argon2.IDKey([]byte(password), salt, parameters.Time, parameters.MemoryKiB, parameters.Threads, uint32(len(expected)))
	return subtle.ConstantTimeCompare(actual, expected) == 1, nil
}

func parsePasswordHash(encoded string) (Argon2Parameters, []byte, []byte, error) {
	parts := strings.Split(encoded, "$")
	if len(parts) != 6 || parts[0] != "" || parts[1] != "argon2id" {
		return Argon2Parameters{}, nil, nil, ErrInvalidPasswordHash
	}

	version, err := parseNamedUint(parts[2], "v", 8)
	if err != nil || version != argon2.Version {
		return Argon2Parameters{}, nil, nil, ErrInvalidPasswordHash
	}
	parameters, err := parseArgon2Parameters(parts[3])
	if err != nil {
		return Argon2Parameters{}, nil, nil, err
	}
	salt, err := base64.RawStdEncoding.Strict().DecodeString(parts[4])
	if err != nil || len(salt) < 16 || len(salt) > 64 {
		return Argon2Parameters{}, nil, nil, ErrInvalidPasswordHash
	}
	derived, err := base64.RawStdEncoding.Strict().DecodeString(parts[5])
	if err != nil || len(derived) < 16 || len(derived) > 64 {
		return Argon2Parameters{}, nil, nil, ErrInvalidPasswordHash
	}
	parameters.SaltBytes = uint32(len(salt))
	parameters.KeyBytes = uint32(len(derived))
	return parameters, salt, derived, nil
}

func parseArgon2Parameters(value string) (Argon2Parameters, error) {
	parts := strings.Split(value, ",")
	if len(parts) != 3 {
		return Argon2Parameters{}, ErrInvalidPasswordHash
	}
	memory, err := parseNamedUint(parts[0], "m", 32)
	if err != nil || memory < 8*1024 || memory > 1024*1024 {
		return Argon2Parameters{}, ErrInvalidPasswordHash
	}
	timeCost, err := parseNamedUint(parts[1], "t", 32)
	if err != nil || timeCost == 0 || timeCost > 10 {
		return Argon2Parameters{}, ErrInvalidPasswordHash
	}
	threads, err := parseNamedUint(parts[2], "p", 8)
	if err != nil || threads == 0 || threads > 16 {
		return Argon2Parameters{}, ErrInvalidPasswordHash
	}
	return Argon2Parameters{MemoryKiB: uint32(memory), Time: uint32(timeCost), Threads: uint8(threads)}, nil
}

func parseNamedUint(value, name string, bitSize int) (uint64, error) {
	prefix := name + "="
	if !strings.HasPrefix(value, prefix) {
		return 0, ErrInvalidPasswordHash
	}
	parsed, err := strconv.ParseUint(strings.TrimPrefix(value, prefix), 10, bitSize)
	if err != nil {
		return 0, ErrInvalidPasswordHash
	}
	return parsed, nil
}