// Package auth handles admin credential hashing, HTTP Basic Auth, API
// tokens, and CSRF protection for the web UI.
package auth

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

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

// Argon2id parameters. Tuned for a low-traffic single-admin login, not a
// high-throughput multi-tenant service.
const (
	argonTime    = 1
	argonMemory  = 64 * 1024 // KiB
	argonThreads = 4
	argonKeyLen  = 32
	saltLen      = 16
)

var ErrInvalidHash = errors.New("invalid password hash format")

// HashPassword returns an encoded argon2id hash in the standard
// $argon2id$v=19$m=...,t=...,p=...$salt$hash format.
func HashPassword(password string) (string, error) {
	salt := make([]byte, saltLen)
	if _, err := rand.Read(salt); err != nil {
		return "", err
	}
	hash := argon2.IDKey([]byte(password), salt, argonTime, argonMemory, argonThreads, argonKeyLen)
	b64Salt := base64.RawStdEncoding.EncodeToString(salt)
	b64Hash := base64.RawStdEncoding.EncodeToString(hash)
	return fmt.Sprintf("$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s", argon2.Version, argonMemory, argonTime, argonThreads, b64Salt, b64Hash), nil
}

// VerifyPassword checks password against an encoded hash produced by
// HashPassword, using constant-time comparison.
func VerifyPassword(password, encoded string) (bool, error) {
	parts := strings.Split(encoded, "$")
	if len(parts) != 6 || parts[1] != "argon2id" {
		return false, ErrInvalidHash
	}
	var version int
	if _, err := fmt.Sscanf(parts[2], "v=%d", &version); err != nil {
		return false, ErrInvalidHash
	}
	var memory uint32
	var time uint32
	var threads uint8
	if _, err := fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &memory, &time, &threads); err != nil {
		return false, ErrInvalidHash
	}
	salt, err := base64.RawStdEncoding.DecodeString(parts[4])
	if err != nil {
		return false, ErrInvalidHash
	}
	want, err := base64.RawStdEncoding.DecodeString(parts[5])
	if err != nil {
		return false, ErrInvalidHash
	}
	got := argon2.IDKey([]byte(password), salt, time, memory, threads, uint32(len(want)))
	return subtle.ConstantTimeCompare(got, want) == 1, nil
}

// GenerateRandomPassword returns a strong, human-typeable temporary
// password (24 base64url characters, no padding).
func GenerateRandomPassword() (string, error) {
	b := make([]byte, 18)
	if _, err := rand.Read(b); err != nil {
		return "", err
	}
	return base64.RawURLEncoding.EncodeToString(b), nil
}