package auth

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

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

const (
	argonTime    = 1
	argonMemory  = 64 * 1024
	argonThreads = 4
	argonKeyLen  = 32
	argonSaltLen = 16

	// Upper bounds guard against a corrupted/attacker-controlled
	// password_hash forcing an oversized argon2 computation (DoS).
	argonMaxMemoryKiB = 1 << 21 // 2 GiB in KiB
	argonMaxTime      = 10
	argonMaxThreads   = 16
)

// paramsRe strictly matches the "m=<n>,t=<n>,p=<n>" parameter segment,
// requiring the whole segment to be consumed (no trailing garbage).
var paramsRe = regexp.MustCompile(`^m=([0-9]+),t=([0-9]+),p=([0-9]+)$`)

func HashPassword(password string) (string, error) {
	salt := make([]byte, argonSaltLen)
	if _, err := rand.Read(salt); err != nil {
		return "", err
	}
	key := argon2.IDKey([]byte(password), salt, argonTime, argonMemory, argonThreads, argonKeyLen)
	b64 := base64.RawStdEncoding.EncodeToString
	return fmt.Sprintf("$argon2id$v=19$m=%d,t=%d,p=%d$%s$%s",
		argonMemory, argonTime, argonThreads, b64(salt), b64(key)), nil
}

func VerifyPassword(encoded, password string) (bool, error) {
	parts := strings.Split(encoded, "$")
	if len(parts) != 6 || parts[1] != "argon2id" {
		return false, fmt.Errorf("auth: bad hash format")
	}
	if parts[2] != "v=19" {
		return false, fmt.Errorf("auth: unsupported version")
	}

	matches := paramsRe.FindStringSubmatch(parts[3])
	if matches == nil {
		return false, fmt.Errorf("auth: bad hash format")
	}
	m64, err := strconv.ParseUint(matches[1], 10, 32)
	if err != nil {
		return false, fmt.Errorf("auth: bad hash format")
	}
	t64, err := strconv.ParseUint(matches[2], 10, 32)
	if err != nil {
		return false, fmt.Errorf("auth: bad hash format")
	}
	p64, err := strconv.ParseUint(matches[3], 10, 8)
	if err != nil {
		return false, fmt.Errorf("auth: bad hash format")
	}
	m, t, p := uint32(m64), uint32(t64), uint8(p64)

	// argon2.IDKey panics if time<1 ("number of rounds too small") or
	// threads<1 ("parallelism degree too low"). Reject before calling it.
	// Minimum memory per argon2 spec is 8*parallelism (in KiB).
	if t < 1 || p < 1 || m < 8*uint32(p) {
		return false, fmt.Errorf("auth: bad hash params")
	}
	// Upper bounds guard against DoS via inflated parameters in a
	// corrupted or attacker-controlled stored hash.
	if m > argonMaxMemoryKiB || t > argonMaxTime || p > argonMaxThreads {
		return false, fmt.Errorf("auth: bad hash params")
	}

	salt, err := base64.RawStdEncoding.DecodeString(parts[4])
	if err != nil {
		return false, err
	}
	want, err := base64.RawStdEncoding.DecodeString(parts[5])
	if err != nil {
		return false, err
	}
	got := argon2.IDKey([]byte(password), salt, t, m, p, uint32(len(want)))
	return subtle.ConstantTimeCompare(got, want) == 1, nil
}