// Package auth provides cryptographic core for authentication.
package auth

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

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

// argon2Memory, argon2Time, argon2Threads, argon2KeyLen, and saltLen are parameters
// recommended by OWASP for Argon2id (memory=65536 KiB, time=3 iterations, parallelism=2).
const (
	argon2Memory  = 65536
	argon2Time    = 3
	argon2Threads = 2
	argon2KeyLen  = 32
	saltLen       = 16
)

// HashPassword generates an Argon2id hash of the provided password and returns it
// in PHC string format: $argon2id$v=19$m=...,t=...,p=...$<salt>$<hash>.
func HashPassword(password string) (string, error) {
	salt := make([]byte, saltLen)
	if _, err := rand.Read(salt); err != nil {
		return "", fmt.Errorf("auth: generate salt: %w", err)
	}
	hash := argon2.IDKey([]byte(password), salt, argon2Time, argon2Memory, argon2Threads, argon2KeyLen)
	b64Salt := base64.RawStdEncoding.EncodeToString(salt)
	b64Hash := base64.RawStdEncoding.EncodeToString(hash)
	phc := fmt.Sprintf("$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s", argon2.Version, argon2Memory, argon2Time, argon2Threads, b64Salt, b64Hash)
	return phc, nil
}

// VerifyPassword checks whether the provided password matches the given Argon2id PHC hash.
// It returns (true, nil) on match, (false, nil) on mismatch, or (false, error) on invalid format.
func VerifyPassword(phc, password string) (bool, error) {
	parts := strings.Split(phc, "$")
	if len(parts) != 6 || parts[1] != "argon2id" {
		return false, errors.New("auth: invalid hash format")
	}
	// Version
	vPart := parts[2]
	if !strings.HasPrefix(vPart, "v=") {
		return false, errors.New("auth: invalid version format")
	}
	v, err := strconv.Atoi(strings.TrimPrefix(vPart, "v="))
	if err != nil {
		return false, fmt.Errorf("auth: parse version: %w", err)
	}
	if v != argon2.Version {
		return false, errors.New("auth: incompatible version")
	}
	// Parameters
	mem, t, p := uint32(0), uint32(0), uint32(0)
	for _, kv := range strings.Split(parts[3], ",") {
		pair := strings.SplitN(kv, "=", 2)
		if len(pair) != 2 {
			return false, errors.New("auth: invalid parameter format")
		}
		key, val := pair[0], pair[1]
		switch key {
		case "m":
			x, err := strconv.ParseUint(val, 10, 32)
			if err != nil {
				return false, fmt.Errorf("auth: parse memory: %w", err)
			}
			mem = uint32(x)
		case "t":
			x, err := strconv.ParseUint(val, 10, 32)
			if err != nil {
				return false, fmt.Errorf("auth: parse time: %w", err)
			}
			t = uint32(x)
		case "p":
			x, err := strconv.ParseUint(val, 10, 32)
			if err != nil {
				return false, fmt.Errorf("auth: parse threads: %w", err)
			}
			p = uint32(x)
		default:
			return false, fmt.Errorf("auth: unknown parameter %q", key)
		}
	}
	salt, err := base64.RawStdEncoding.DecodeString(parts[4])
	if err != nil {
		return false, fmt.Errorf("auth: decode salt: %w", err)
	}
	expected, err := base64.RawStdEncoding.DecodeString(parts[5])
	if err != nil {
		return false, fmt.Errorf("auth: decode hash: %w", err)
	}
	computed := argon2.IDKey([]byte(password), salt, t, mem, uint8(p), uint32(len(expected)))
	if subtle.ConstantTimeCompare(computed, expected) == 1 {
		return true, nil
	}
	return false, nil
}