package service

import (
	"context"
	"crypto/sha1"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"math"
	"net/http"
	"regexp"
	"strconv"
	"strings"
	"time"
	"unicode"
	"unicode/utf8"

	"git.umbrella.haus/ae/qnote/internal/data"
	"github.com/rs/zerolog/log"
)

const (
	minPasswordLength  = 12 // Entropy checks prevent short passwords anyway
	maxPasswordLength  = 72 // Limitation of bcrypt
	minPasswordEntropy = 60.0

	minUsernameLength = 3
	maxUsernameLength = 20

	maxFutureExpirationYears = 10

	hibpAPI = "https://api.pwnedpasswords.com/range" // Doesn't require an API key
)

var (
	usernameRegex = regexp.MustCompile("^[a-z0-9_]+$")

	// Format: @exp:2025-06-15 or @expires:2025-06-15
	dateFormatRegex = regexp.MustCompile(`^@(?:exp|expires):(\d{4}-\d{2}-\d{2})`)

	// Format: @exp:+7d or @expires:+7d (7 days from now),
	// supports d (days), w (weeks), m (months), y (years)
	relativeFormatRegex = regexp.MustCompile(`^@(?:exp|expires):\+(\d+)([dwmy])`)

	ErrNoExpirationDateFound = errors.New("no expiration date found")
	ErrInvalidExpirationDate = errors.New("invalid expiration date format")
	ErrPastExpirationDate    = errors.New("expiration date cannot be in the past")
	ErrExpirationTooFar      = fmt.Errorf("expiration date too far in the future (max. %d years)", maxFutureExpirationYears)
)

func respondJSON(w http.ResponseWriter, status int, data any) {
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(status)
	json.NewEncoder(w).Encode(data)
}

func respondError(w http.ResponseWriter, status int, message string) {
	respondJSON(w, status, map[string]string{"error": message})
}

// Validate the given password using a hybrid approach: length (max. set due to bcrypt's input
// upper limit of 72 bytes), entropy, and HIBP API.
func validatePassword(password string) error {
	if len(password) < minPasswordLength {
		return fmt.Errorf("password must be at least %d characters", minPasswordLength)
	}
	if len(password) > maxPasswordLength {
		return fmt.Errorf("password cannot be longer than %d characters", maxPasswordLength)
	}

	// Simple entropy approximation
	if calcPasswordEntropy(password) < minPasswordEntropy {
		return fmt.Errorf("password is too weak, try adding more uppercase letters, digits, and symbols")
	}

	if compromised, _ := isPasswordCompromised(password); compromised {
		return errors.New("password was found in database leaks")
	}

	return nil
}

// Approximate given password's entropy. Notably the way the entropy is calculated is really
// conservative and punishes relatively harshly if the password contains a lot of repetition
// (small set of unique characters).
func calcPasswordEntropy(password string) float64 {
	hasLower, hasUpper, hasDigit, hasSymbol := false, false, false, false
	uniqueChars := make(map[rune]bool)

	for _, c := range password {
		uniqueChars[c] = true

		switch {
		case unicode.IsLower(c):
			hasLower = true
		case unicode.IsUpper(c):
			hasUpper = true
		case unicode.IsDigit(c):
			hasDigit = true
		default:
			if !unicode.IsLetter(c) && !unicode.IsDigit(c) {
				hasSymbol = true // Broader symbol collection than in the frontend
			}
		}
	}

	poolSize := 0
	if hasLower {
		poolSize += 26
	}
	if hasUpper {
		poolSize += 26
	}
	if hasDigit {
		poolSize += 10
	}
	if hasSymbol {
		poolSize += 40
	}

	if poolSize == 0 {
		return 0
	}

	basicEntropy := float64(len(password) * int(math.Log2(float64(poolSize))))
	diversityAdjustedEntropy := math.Log2(float64(poolSize)) + float64(len(password)-1)*math.Log2(float64(len(uniqueChars)))

	return math.Min(basicEntropy, diversityAdjustedEntropy)
}

// Send the first five bytes of the password's SHA-1 hash to HIBP API, then check if the rest of
// the hash is present in the APi's response data (k-Anonymity model).
func isPasswordCompromised(password string) (bool, error) {
	hash := sha1.Sum([]byte(password))
	hashStr := strings.ToUpper(hex.EncodeToString(hash[:]))
	prefix, suffix := hashStr[:5], hashStr[5:]

	resp, err := http.Get(fmt.Sprintf("%s/%s", hibpAPI, prefix))
	if err != nil {
		return false, err
	}
	defer resp.Body.Close()

	body, err := io.ReadAll(resp.Body)
	if err != nil {
		return false, err
	}

	return strings.Contains(strings.ToUpper(string(body)), suffix), nil
}

// Normalize the username by making it lowercase and trimming any leading or trailing whitespace.
func normalizeUsername(username string) string {
	return strings.ToLower(strings.TrimSpace(username))
}

// Validate the given username by making sure it only contains alphanumeric characters or
// underscores and adheres the hardcoded minimum and maximum length rules.
func validateUsername(username string) error {
	if utf8.RuneCountInString(username) < minUsernameLength {
		return fmt.Errorf("username must be at least %d characters", minUsernameLength)
	}
	if utf8.RuneCountInString(username) > maxUsernameLength {
		return fmt.Errorf("username cannot be longer than %d characters", maxUsernameLength)
	}

	if !usernameRegex.MatchString(username) {
		return errors.New("username can only contain numbers, letters, and underscores")
	}

	return nil
}

// Parse `limit` and `offset` 32-bit integer URL parameters from the given request. Defaults to
// limit of 50 and offset 0 if parameters are missing/invalid.
func getPaginationParams(r *http.Request) (limit int32, offset int32) {
	defaultLimit := 50
	defaultOffset := 0

	limitStr := r.URL.Query().Get("limit")
	if limitStr != "" {
		if l, err := strconv.Atoi(limitStr); err == nil && l > 0 {
			defaultLimit = l
		}
	}

	offsetStr := r.URL.Query().Get("offset")
	if offsetStr != "" {
		if o, err := strconv.Atoi(offsetStr); err == nil && o >= 0 {
			defaultOffset = o
		}
	}

	return int32(defaultLimit), int32(defaultOffset)
}

// Concatenate the title and content strings, calculate a SHA-1 hash of the resulting string, and
// return the resulting hash as a string.
func sha1ContentHash(title, content string) string {
	hashContent := title + content
	hash := sha1.Sum([]byte(hashContent))
	hashStr := strings.ToUpper(hex.EncodeToString(hash[:]))

	return hashStr
}

func parseTitleExpiration(title *string) (*time.Time, error) {
	// Absolute date format: '@exp:YYYY-MM-DD' (or '@expires:')
	if match := dateFormatRegex.FindStringSubmatch(*title); match != nil {
		dateStr := match[1]

		expiresAt, err := time.Parse("2006-01-02", dateStr)
		if err != nil {
			return nil, ErrInvalidExpirationDate
		}

		if err := validateExpirationDate(expiresAt); err != nil {
			return nil, err
		}

		// Set midnight at the end of the specified day (+0000 UTC)
		expiresAt = time.Date(expiresAt.Year(), expiresAt.Month(), expiresAt.Day(), 23, 59, 59, 0, time.UTC)

		return &expiresAt, nil
	}

	if match := relativeFormatRegex.FindStringSubmatch(*title); match != nil {
		amount := match[1]
		unit := match[2]

		var amountInt int
		_, err := fmt.Sscanf(amount, "%d", &amountInt)
		if err != nil || amountInt <= 0 {
			return nil, ErrInvalidExpirationDate
		}

		now := time.Now()
		var expiresAt time.Time

		switch unit {
		case "d":
			expiresAt = now.AddDate(0, 0, amountInt)
		case "w":
			expiresAt = now.AddDate(0, 0, amountInt*7)
		case "m":
			expiresAt = now.AddDate(0, amountInt, 0)
		case "y":
			expiresAt = now.AddDate(amountInt, 0, 0)
		default:
			return nil, ErrInvalidExpirationDate
		}

		if err := validateExpirationDate(expiresAt); err != nil {
			return nil, err
		}

		// Set midnight at the end of the specified day (+0000 UTC)
		expiresAt = time.Date(expiresAt.Year(), expiresAt.Month(), expiresAt.Day(), 23, 59, 59, 0, time.UTC)

		return &expiresAt, nil
	}

	return nil, ErrNoExpirationDateFound
}

func validateExpirationDate(date time.Time) error {
	now := time.Now()

	if date.Before(now) {
		return ErrPastExpirationDate
	}

	maxDate := now.AddDate(maxFutureExpirationYears, 0, 0)
	if date.After(maxDate) {
		return ErrExpirationTooFar
	}

	return nil
}

func cleanupNotes(ctx context.Context, q *data.Queries) {
	expiredNotes, err := q.ListExpiredNotes(ctx)
	if err != nil {
		log.Error().Err(err).Msg("Failed querying expired notes")
		return
	}

	if len(expiredNotes) == 0 {
		return
	}

	// Log what we're about to delete to be able to track potential bugs in the expiration implementation
	for _, note := range expiredNotes {
		log.Debug().Msgf("Deleting expired note: %s (ID: %s, UID: %s), expired at %s",
			note.Title, note.NoteID, note.OwnerID, note.ExpiresAt.Format(time.RFC3339))
	}

	if err = q.DeleteExpiredNotes(ctx); err != nil {
		log.Error().Err(err).Msg("Failed deleting expired notes")
		return
	}

	log.Info().Msgf("Successfully deleted %d expired notes during scheduled cleanup", len(expiredNotes))
}

func cleanupRefreshTokens(ctx context.Context, q *data.Queries) {
	rowsAffected, err := q.DeleteExpiredRefreshTokens(ctx)
	if err != nil {
		log.Error().Err(err).Msg("Failed cleaning up refresh tokens")
		return
	}

	if rowsAffected > 0 {
		log.Info().Msgf("Cleaned up %d expired/revoked refresh tokens during scheduled cleanup", rowsAffected)
	}
}