package service

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

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

// argon2id parameters. ~64 MiB memory / 1 pass / 4 lanes is the interactive
// profile recommended by the argon2 authors and is comfortable on a
// self-hosted box. They are encoded into every stored hash, so raising them
// later leaves old hashes verifiable.
const (
	argonMemKiB  = 64 * 1024 // 64 MiB
	argonTime    = 1
	argonThreads = 4
	argonKeyLen  = 32
	argonSaltLen = 16
)

// errBadHash marks a stored hash string that could not be parsed — a data or
// programming error, not a wrong password. Callers treat it as an auth failure
// but should log it.
var errBadHash = errors.New("service: malformed password hash")

// b64 is the padding-free base64 used inside the PHC-style hash string.
var b64 = base64.RawStdEncoding

// hashPassword returns a self-describing argon2id hash in the PHC string format
// "$argon2id$v=19$m=...,t=...,p=...$salt$hash" (all base64, no padding).
func hashPassword(password string) (string, error) {
	salt := make([]byte, argonSaltLen)
	if _, err := rand.Read(salt); err != nil {
		return "", fmt.Errorf("service: generate salt: %w", err)
	}
	key := argon2.IDKey([]byte(password), salt, argonTime, argonMemKiB, argonThreads, argonKeyLen)
	return fmt.Sprintf("$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s",
		argon2.Version, argonMemKiB, argonTime, argonThreads,
		b64.EncodeToString(salt), b64.EncodeToString(key),
	), nil
}

// verifyPassword reports whether password matches the encoded argon2id hash. The
// comparison is constant-time. A malformed encoded value returns errBadHash.
func verifyPassword(encoded, password string) (bool, error) {
	mem, t, threads, salt, want, err := decodeHash(encoded)
	if err != nil {
		return false, err
	}
	got := argon2.IDKey([]byte(password), salt, t, mem, threads, uint32(len(want)))
	return subtle.ConstantTimeCompare(got, want) == 1, nil
}

// decodeHash parses a PHC-format argon2id string into its parameters, salt, and
// derived key.
func decodeHash(encoded string) (mem, t uint32, threads uint8, salt, key []byte, err error) {
	parts := strings.Split(encoded, "$")
	// ["", "argon2id", "v=19", "m=..,t=..,p=..", "<salt>", "<hash>"]
	if len(parts) != 6 || parts[1] != "argon2id" {
		return 0, 0, 0, nil, nil, errBadHash
	}

	var version int
	if _, err := fmt.Sscanf(parts[2], "v=%d", &version); err != nil || version != argon2.Version {
		return 0, 0, 0, nil, nil, errBadHash
	}
	if _, err := fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &mem, &t, &threads); err != nil {
		return 0, 0, 0, nil, nil, errBadHash
	}
	if salt, err = b64.DecodeString(parts[4]); err != nil {
		return 0, 0, 0, nil, nil, errBadHash
	}
	if key, err = b64.DecodeString(parts[5]); err != nil || len(key) == 0 {
		return 0, 0, 0, nil, nil, errBadHash
	}
	return mem, t, threads, salt, key, nil
}