// Package steam handles "Sign in through Steam" (OpenID 2.0) and SteamID64 validation.
package steam

import (
	"context"
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"regexp"
	"strconv"
	"strings"
	"sync"
	"time"
)

const (
	opEndpoint     = "https://steamcommunity.com/openid/login"
	openIDNS       = "http://specs.openid.net/auth/2.0"
	identifierSel  = "http://specs.openid.net/auth/2.0/identifier_select"
	claimedPrefix  = "https://steamcommunity.com/openid/id/"
	nonceMaxAge    = 5 * time.Minute
	steamID64Base  = 76561197960265728
	steamID64Limit = steamID64Base + 1<<32
)

var steamID64Re = regexp.MustCompile(`^7656119[0-9]{10}$`)

// ValidID reports whether s is an individual account's SteamID64.
func ValidID(s string) bool {
	if !steamID64Re.MatchString(s) {
		return false
	}
	n, err := strconv.ParseUint(s, 10, 64)
	return err == nil && n > steamID64Base && n < steamID64Limit
}

// LoginURL is where to send the browser to sign in. returnTo must be under realm.
func LoginURL(realm, returnTo string) string {
	q := url.Values{
		"openid.ns":         {openIDNS},
		"openid.mode":       {"checkid_setup"},
		"openid.return_to":  {returnTo},
		"openid.realm":      {realm},
		"openid.identity":   {identifierSel},
		"openid.claimed_id": {identifierSel},
	}
	return opEndpoint + "?" + q.Encode()
}

// Verify checks a callback from Steam with Steam itself and returns the signed-in SteamID64.
func Verify(ctx context.Context, client *http.Client, returnTo string, q url.Values) (string, error) {
	if q.Get("openid.mode") != "id_res" {
		return "", errors.New("steam: sign-in was cancelled")
	}
	if q.Get("openid.op_endpoint") != opEndpoint {
		return "", errors.New("steam: wrong OpenID provider")
	}
	if q.Get("openid.return_to") != returnTo {
		return "", errors.New("steam: return address doesn't match")
	}
	claimed := q.Get("openid.claimed_id")
	if claimed != q.Get("openid.identity") || !strings.HasPrefix(claimed, claimedPrefix) {
		return "", errors.New("steam: unexpected identity")
	}
	id := strings.TrimPrefix(claimed, claimedPrefix)
	if !ValidID(id) {
		return "", errors.New("steam: identity isn't a SteamID64")
	}
	if err := checkNonce(q.Get("openid.response_nonce")); err != nil {
		return "", err
	}
	// A callback URL is a bearer credential until the nonce expires, so accept each one once.
	if !seen.claim(q.Get("openid.response_nonce")) {
		return "", errors.New("steam: this sign-in link was already used")
	}

	check := url.Values{}
	for k, v := range q {
		if strings.HasPrefix(k, "openid.") {
			check[k] = v
		}
	}
	check.Set("openid.mode", "check_authentication")

	req, err := http.NewRequestWithContext(ctx, http.MethodPost, opEndpoint, strings.NewReader(check.Encode()))
	if err != nil {
		return "", err
	}
	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
	resp, err := client.Do(req)
	if err != nil {
		return "", fmt.Errorf("steam: verify: %w", err)
	}
	defer resp.Body.Close()
	body, err := io.ReadAll(io.LimitReader(resp.Body, 4096))
	if err != nil {
		return "", fmt.Errorf("steam: verify: %w", err)
	}
	for line := range strings.SplitSeq(string(body), "\n") {
		if strings.TrimSpace(line) == "is_valid:true" {
			return id, nil
		}
	}
	return "", errors.New("steam: Steam didn't confirm the sign-in")
}

// The nonce starts with its UTC creation time, e.g. 2026-09-25T20:00:00Z0a1b2c.
func checkNonce(nonce string) error {
	if len(nonce) < 20 {
		return errors.New("steam: missing nonce")
	}
	t, err := time.Parse(time.RFC3339, nonce[:20])
	if err != nil {
		return errors.New("steam: bad nonce")
	}
	if age := time.Since(t); age > nonceMaxAge || age < -nonceMaxAge {
		return errors.New("steam: sign-in link expired, try again")
	}
	return nil
}

type nonceSet struct {
	mu   sync.Mutex
	used map[string]time.Time
}

var seen = &nonceSet{used: map[string]time.Time{}}

func (n *nonceSet) claim(nonce string) bool {
	n.mu.Lock()
	defer n.mu.Unlock()
	now := time.Now()
	for k, t := range n.used {
		if now.Sub(t) > 2*nonceMaxAge {
			delete(n.used, k)
		}
	}
	if _, ok := n.used[nonce]; ok {
		return false
	}
	n.used[nonce] = now
	return true
}