package passwords

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

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

const (
	saltLength  = 16
	memoryCost  = 64 * 1024
	timeCost    = 3
	parallelism = 4
	keyLength   = 32
)

// Hash hashes a plaintext password using Argon2id.
func Hash(password string) (string, error) {
	salt := make([]byte, saltLength)
	if _, err := rand.Read(salt); err != nil {
		return "", fmt.Errorf("read argon2 salt: %w", err)
	}
	hash := argon2.IDKey([]byte(password), salt, timeCost, memoryCost, parallelism, keyLength)
	encodedSalt := base64.RawStdEncoding.EncodeToString(salt)
	encodedHash := base64.RawStdEncoding.EncodeToString(hash)
	return fmt.Sprintf("$argon2id$v=19$m=%d,t=%d,p=%d$%s$%s", memoryCost, timeCost, parallelism, encodedSalt, encodedHash), nil
}

// Verify verifies a plaintext password against its encoded Argon2id hash.
func Verify(password string, encoded string) (bool, error) {
	parts := strings.Split(encoded, "$")
	if len(parts) != 6 {
		return false, fmt.Errorf("invalid password hash format")
	}
	if parts[1] != "argon2id" || parts[2] != "v=19" {
		return false, fmt.Errorf("unsupported password hash format")
	}

	var memory uint32
	var iterations uint32
	var lanes uint8
	if _, err := fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &memory, &iterations, &lanes); err != nil {
		return false, fmt.Errorf("parse password hash params: %w", err)
	}
	salt, err := base64.RawStdEncoding.DecodeString(parts[4])
	if err != nil {
		return false, fmt.Errorf("decode password hash salt: %w", err)
	}
	expectedHash, err := base64.RawStdEncoding.DecodeString(parts[5])
	if err != nil {
		return false, fmt.Errorf("decode password hash body: %w", err)
	}
	actualHash := argon2.IDKey([]byte(password), salt, iterations, memory, lanes, uint32(len(expectedHash)))
	return subtle.ConstantTimeCompare(actualHash, expectedHash) == 1, nil
}