package domain

import (
	"bytes"
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"sort"
	"time"
)

// SignedJoinAuthorisation is the transport envelope. The signing primitive is
// supplied by the backend signer so this policy stays independent of key
// storage and cryptographic algorithm choice.
type SignedJoinAuthorisation struct {
	Authorisation JoinAuthorisation
	Signature     []byte
}

// JoinAuthorisationBytes is the canonical claim encoding. KeyID is appended
// last and is covered by the signature, so an attacker cannot redirect an
// authorisation at a different key than the one that signed it. Game/scripts/
// match_net.gd builds the identical byte sequence; the two must change
// together.
func JoinAuthorisationBytes(auth JoinAuthorisation) []byte {
	return []byte(fmt.Sprintf("%s\x00%s\x00%s\x00%s\x00%d\x00%d\x00%s\x00%d\x00%s\x00%s",
		auth.MatchID, auth.ServerID, auth.PlayerID, auth.SteamID, auth.Slot, auth.Team, auth.Protocol, auth.Generation, auth.ExpiresAt.UTC().Format(time.RFC3339Nano), auth.KeyID))
}

func SignJoinAuthorisation(auth JoinAuthorisation, sign func([]byte) ([]byte, error)) (SignedJoinAuthorisation, error) {
	if sign == nil {
		return SignedJoinAuthorisation{}, ErrJoinAuthorisation
	}
	signature, err := sign(JoinAuthorisationBytes(auth))
	if err != nil || len(signature) == 0 {
		return SignedJoinAuthorisation{}, ErrJoinAuthorisation
	}
	return SignedJoinAuthorisation{Authorisation: auth, Signature: append([]byte(nil), signature...)}, nil
}

// SignJoinAuthorisationHMAC is the interoperable production profile used by
// the Godot allocated server. The key is mounted out-of-band; the signed
// bytes remain the same canonical claim bytes used by the generic signer.
// The caller must have set auth.KeyID to the ID of this key, so the verifier
// can pick the right one out of its key set.
func SignJoinAuthorisationHMAC(auth JoinAuthorisation, key []byte) (SignedJoinAuthorisation, error) {
	if len(key) == 0 {
		return SignedJoinAuthorisation{}, ErrJoinAuthorisation
	}
	mac := hmac.New(sha256.New, key)
	_, _ = mac.Write(JoinAuthorisationBytes(auth))
	return SignedJoinAuthorisation{Authorisation: auth, Signature: mac.Sum(nil)}, nil
}

func (r *RankedConnections) AdmitSigned(signed SignedJoinAuthorisation, verify func([]byte, []byte) bool, now time.Time) (uint64, error) {
	if len(signed.Signature) == 0 || verify == nil || !verify(JoinAuthorisationBytes(signed.Authorisation), signed.Signature) {
		return 0, ErrJoinAuthorisation
	}
	return r.Admit(signed.Authorisation, now)
}

// AssignmentRosterDigest binds a manifest to the exact roster it was issued
// with. Signing each authorisation individually proves each claim, but the
// manifest also has to commit to the set, so a server cannot be handed a
// truncated roster whose entries are each individually valid.
//
// Entries are hashed in slot order so the digest is independent of the order
// the caller happened to build them in.
func AssignmentRosterDigest(roster []SignedJoinAuthorisation) (string, error) {
	if len(roster) == 0 {
		return "", ErrJoinAuthorisation
	}
	ordered := make([]SignedJoinAuthorisation, len(roster))
	copy(ordered, roster)
	sort.Slice(ordered, func(i, j int) bool {
		return ordered[i].Authorisation.Slot < ordered[j].Authorisation.Slot
	})
	digest := sha256.New()
	for _, signed := range ordered {
		if signed.Authorisation.PlayerID == "" {
			return "", ErrJoinAuthorisation
		}
		digest.Write(JoinAuthorisationBytes(signed.Authorisation))
		digest.Write([]byte{0})
	}
	return hex.EncodeToString(digest.Sum(nil)), nil
}

// VerifyJoinAuthorisationHMAC builds the verifier the persistence boundary
// re-checks each signature with, selecting the key named by the claim. Keys is
// key ID to raw key; an unknown ID verifies as false rather than falling back
// to any other key.
func VerifyJoinAuthorisationHMAC(keys map[string][]byte) func([]byte, []byte) bool {
	return func(claims, signature []byte) bool {
		if len(keys) == 0 || len(claims) == 0 || len(signature) == 0 {
			return false
		}
		// The key ID is the last NUL-separated field of the canonical bytes.
		separator := bytes.LastIndexByte(claims, 0)
		if separator < 0 {
			return false
		}
		key, known := keys[string(claims[separator+1:])]
		if !known || len(key) == 0 {
			return false
		}
		mac := hmac.New(sha256.New, key)
		mac.Write(claims)
		return hmac.Equal(mac.Sum(nil), signature)
	}
}