package classes

import (
	"context"
	"crypto/hmac"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"encoding/hex"
	"errors"
	"regexp"
	"strconv"
	"strings"
	"time"

	"git.golem15.com/golem15/summercms/modules/cooler"
)

// Signed one-time upload URLs for material files (D-10, D-12). The owner's
// agent asks for an upload URL over MCP and sends the file with curl, so
// the bytes never pass through MCP. Each plugin signs its own tickets with
// its own label: golem15.presentations has the same design, and the code is
// duplicated on purpose so jz.bm imports neither that plugin nor new
// golem15.user code. A ticket is
//
//	base64url(payload) + "." + base64url(HMAC-SHA256(key, uploadLabel + payload))
//
// with payload "<kind>|<target>|<user>|<expiry unix>|<nonce>", the nonce 16
// random bytes in hex. It is valid for UploadTTL and redeemable once:
// issuing stores the payload under the nonce in the app cache, redeeming
// pulls it atomically. A restart of a memory cache forgets every issued
// ticket, so they fail closed.

// uploadLabel separates the upload HMAC from any other use of the key.
const uploadLabel = "jz.bm.upload.v1:"

// uploadCachePrefix is the cache key prefix of issued tickets.
const uploadCachePrefix = "jz.bm.upload:"

// UploadTTL is how long an upload URL is valid.
const UploadTTL = 10 * time.Minute

// uploadTicketMaxLen bounds the ticket text read from a URL.
const uploadTicketMaxLen = 512

var (
	// ErrUploadTicket is every refused ticket: malformed, forged, signed
	// with another key or label, expired, unknown or already used.
	ErrUploadTicket = errors.New("bm: invalid or expired upload ticket")
	// ErrUploadKeyMissing means no key to sign upload tickets with (app.key
	// is empty): no upload URL is issued.
	ErrUploadKeyMissing = errors.New("bm: upload key is not configured")
)

var (
	uploadKind  = regexp.MustCompile(`^[a-z]{1,20}$`)
	uploadNonce = regexp.MustCompile(`^[0-9a-f]{32}$`)
)

// UploadTicket is what a signed upload URL grants: one upload of Kind to
// Target (a material id) by User, until Expires.
type UploadTicket struct {
	Kind    string
	Target  uint
	User    uint
	Expires time.Time
	Nonce   string
}

// payload is the signed text of t.
func (t UploadTicket) payload() string {
	return t.Kind + "|" + strconv.FormatUint(uint64(t.Target), 10) + "|" + strconv.FormatUint(uint64(t.User), 10) + "|" + strconv.FormatInt(t.Expires.Unix(), 10) + "|" + t.Nonce
}

// UploadKey is the signing key from the app key: trimmed, nil when empty.
func UploadKey(appKey string) []byte {
	if k := strings.TrimSpace(appKey); k != "" {
		return []byte(k)
	}
	return nil
}

func uploadMAC(key []byte, payload string) []byte {
	mac := hmac.New(sha256.New, key)
	mac.Write([]byte(uploadLabel + payload))
	return mac.Sum(nil)
}

// SignUpload signs t. It does not register the ticket; IssueUpload does.
func SignUpload(key []byte, t UploadTicket) (string, error) {
	if len(key) == 0 {
		return "", ErrUploadKeyMissing
	}
	payload := t.payload()
	return base64.RawURLEncoding.EncodeToString([]byte(payload)) + "." + base64.RawURLEncoding.EncodeToString(uploadMAC(key, payload)), nil
}

// VerifyUpload checks a ticket's signature and expiry at now. An expiry
// further away than UploadTTL (and a minute of clock skew) is refused too.
// Every failure is ErrUploadTicket.
func VerifyUpload(key []byte, ticket string, now time.Time) (UploadTicket, error) {
	if len(key) == 0 || ticket == "" || len(ticket) > uploadTicketMaxLen {
		return UploadTicket{}, ErrUploadTicket
	}
	parts := strings.Split(ticket, ".")
	if len(parts) != 2 {
		return UploadTicket{}, ErrUploadTicket
	}
	payload, err := base64.RawURLEncoding.DecodeString(parts[0])
	if err != nil {
		return UploadTicket{}, ErrUploadTicket
	}
	mac, err := base64.RawURLEncoding.DecodeString(parts[1])
	if err != nil {
		return UploadTicket{}, ErrUploadTicket
	}
	if !hmac.Equal(mac, uploadMAC(key, string(payload))) {
		return UploadTicket{}, ErrUploadTicket
	}
	t, ok := parseUploadPayload(string(payload))
	if !ok || !now.Before(t.Expires) || t.Expires.Sub(now) > UploadTTL+time.Minute {
		return UploadTicket{}, ErrUploadTicket
	}
	return t, nil
}

func parseUploadPayload(payload string) (UploadTicket, bool) {
	fields := strings.Split(payload, "|")
	if len(fields) != 5 || !uploadKind.MatchString(fields[0]) || !uploadNonce.MatchString(fields[4]) {
		return UploadTicket{}, false
	}
	target, err := strconv.ParseUint(fields[1], 10, 32)
	if err != nil || target == 0 {
		return UploadTicket{}, false
	}
	user, err := strconv.ParseUint(fields[2], 10, 32)
	if err != nil || user == 0 {
		return UploadTicket{}, false
	}
	expiry, err := strconv.ParseInt(fields[3], 10, 64)
	if err != nil {
		return UploadTicket{}, false
	}
	return UploadTicket{Kind: fields[0], Target: uint(target), User: uint(user), Expires: time.Unix(expiry, 0).UTC(), Nonce: fields[4]}, true
}

// IssueUpload signs a ticket for one upload of kind to target by user,
// valid for UploadTTL from now, and stores its payload in cache under the
// nonce for RedeemUpload. It answers the ticket and its expiry.
func IssueUpload(ctx context.Context, cache cooler.Cache, key []byte, kind string, target, user uint, now time.Time) (string, time.Time, error) {
	if len(key) == 0 {
		return "", time.Time{}, ErrUploadKeyMissing
	}
	if cache == nil {
		return "", time.Time{}, errors.New("bm: upload cache is not available")
	}
	if !uploadKind.MatchString(kind) || target == 0 || user == 0 {
		return "", time.Time{}, errors.New("bm: invalid upload target")
	}
	var nonce [16]byte
	if _, err := rand.Read(nonce[:]); err != nil {
		return "", time.Time{}, err
	}
	t := UploadTicket{Kind: kind, Target: target, User: user, Expires: now.Add(UploadTTL).Truncate(time.Second).UTC(), Nonce: hex.EncodeToString(nonce[:])}
	signed, err := SignUpload(key, t)
	if err != nil {
		return "", time.Time{}, err
	}
	if err := cache.Set(ctx, uploadCachePrefix+t.Nonce, []byte(t.payload()), UploadTTL); err != nil {
		return "", time.Time{}, err
	}
	return signed, t.Expires, nil
}

// RedeemUpload verifies ticket and takes it out of cache in one atomic Pull,
// so it works once. A ticket whose cache entry is missing (used, expired or
// forgotten by a restart) or differs from its payload is refused. Every
// failure is ErrUploadTicket.
func RedeemUpload(ctx context.Context, cache cooler.Cache, key []byte, ticket string, now time.Time) (UploadTicket, error) {
	t, err := VerifyUpload(key, ticket, now)
	if err != nil {
		return UploadTicket{}, err
	}
	if cache == nil {
		return UploadTicket{}, ErrUploadTicket
	}
	raw, found, err := cache.Pull(ctx, uploadCachePrefix+t.Nonce)
	if err != nil || !found || string(raw) != t.payload() {
		return UploadTicket{}, ErrUploadTicket
	}
	return t, nil
}