package net.rustnotmc.protocol;

import java.util.LinkedHashMap;
import java.util.Map;

/**
 * The packets that start a connection, and the table that maps them to ids.
 *
 * The ids are written down here rather than generated, because they end up in
 * every capture and every log line forever. Renumbering one is a protocol
 * change and should look like a deliberate act, not a side effect of moving a
 * class around.
 *
 * The order of a connection is fixed and short:
 *
 * <pre>
 *   client -> Hello        protocol version, what the client is
 *   server -> HelloAck     accepted version, server name, motd, online count
 *   client -> Auth         account and proof
 *   server -> AuthAck      session token, or a reason it was refused
 *   client -> Login        the session token
 *   server -> LoginAck     player id and entity id, or a reason
 *   either -> Disconnect   a human readable reason
 * </pre>
 */
public final class Packets {

    private Packets() {
    }

    /** Longest a name, motd or reason may be, in bytes. */
    public static final int MAX_TEXT = 512;

    /** Longest a handshake proof may be, in bytes. */
    public static final int MAX_PROOF = 256;

    /** A salt and a nonce, in the sizes the crypto actually produces. */
    public static final int SALT_BYTES = 16;
    public static final int CHALLENGE_BYTES = 32;

    /** Rounds of PBKDF2 that both ends must use. */
    public static final int KEY_ITERATIONS = 210000;

    /** Derived key length in bytes, matching PBKDF2WithHmacSHA256. */
    public static final int KEY_BYTES = 32;

    /** Longest the handshake body may be, in bytes. */
    public static final int MAX_HANDSHAKE_BODY = 8 * 1024;

    public static final int HELLO = 0x01;
    public static final int HELLO_ACK = 0x02;
    public static final int AUTH = 0x03;
    public static final int AUTH_ACK = 0x04;
    public static final int LOGIN = 0x05;
    public static final int LOGIN_ACK = 0x06;
    public static final int DISCONNECT = 0x7F;

    /** The client introducing itself, and naming the account it wants. */
    public static final class Hello {
        public final int protocolVersion;
        public final String account;
        public final String clientName;
        public final int clientVersion;

        public Hello(int protocolVersion, String account, String clientName, int clientVersion) {
            this.protocolVersion = protocolVersion;
            this.account = account;
            this.clientName = clientName;
            this.clientVersion = clientVersion;
        }

        public Buf write() {
            Buf buf = new Buf();
            buf.writeVarInt(this.protocolVersion);
            buf.writeString(this.account);
            buf.writeString(this.clientName);
            buf.writeVarInt(this.clientVersion);
            return buf;
        }

        public static Hello read(Buf in) {
            return new Hello(in.readVarInt(), in.readString(MAX_TEXT),
                    in.readString(MAX_TEXT), in.readVarInt());
        }
    }

    /** The server accepting, and describing itself. */
    public static final class HelloAck {
        public final int protocolVersion;
        public final String serverName;
        public final String motd;
        public final int onlinePlayers;
        public final int maxPlayers;

        /**
         * A value fresh for this connection.
         *
         * The proof is computed over it, so a recording of one login cannot be
         * replayed against the next one.
         */
        public final byte[] nonce;

        /**
         * The salt of the account that was named, when it exists.
         *
         * Sent before the client has proved anything, which does reveal whether
         * an account exists. That is a deliberate trade for a design where the
         * password never crosses the wire and the server stores no reversible
         * form of it.
         */
        public final byte[] salt;

        public HelloAck(int protocolVersion, String serverName, String motd,
                        int onlinePlayers, int maxPlayers, byte[] nonce, byte[] salt) {
            this.protocolVersion = protocolVersion;
            this.serverName = serverName;
            this.motd = motd;
            this.onlinePlayers = onlinePlayers;
            this.maxPlayers = maxPlayers;
            this.nonce = nonce;
            this.salt = salt;
        }

        public Buf write() {
            Buf buf = new Buf();
            buf.writeVarInt(this.protocolVersion);
            buf.writeString(this.serverName);
            buf.writeString(this.motd);
            buf.writeVarInt(this.onlinePlayers);
            buf.writeVarInt(this.maxPlayers);
            buf.writeBytes(this.nonce);
            buf.writeBytes(this.salt);
            return buf;
        }

        public static HelloAck read(Buf in) {
            return new HelloAck(in.readVarInt(), in.readString(MAX_TEXT),
                    in.readString(MAX_TEXT), in.readVarInt(), in.readVarInt(),
                    in.readBytes(CHALLENGE_BYTES), in.readBytes(SALT_BYTES));
        }
    }

    /**
     * The client claiming an account, with proof rather than with the password.
     *
     * The proof is HMAC-SHA256 over the derived key, the nonce and the account
     * name. The password itself is never sent, the server stores no reversible
     * form of it, and a proof captured from one connection is useless against
     * the next because the nonce differs.
     */
    public static final class Auth {
        public final String account;
        public final byte[] proof;

        public Auth(String account, byte[] proof) {
            this.account = account;
            this.proof = proof;
        }

        public Buf write() {
            Buf buf = new Buf();
            buf.writeString(this.account);
            buf.writeBytes(this.proof);
            return buf;
        }

        public static Auth read(Buf in) {
            return new Auth(in.readString(MAX_TEXT), in.readBytes(KEY_BYTES));
        }

        /** Derives the key from a password and salt the way the server does. */
        public static byte[] deriveKey(char[] password, byte[] salt) {
            try {
                javax.crypto.SecretKeyFactory factory =
                        javax.crypto.SecretKeyFactory.getInstance("PBKDF2WithHmacSHA256");
                javax.crypto.spec.PBEKeySpec spec = new javax.crypto.spec.PBEKeySpec(
                        password, salt, KEY_ITERATIONS, KEY_BYTES * 8);
                try {
                    return factory.generateSecret(spec).getEncoded();
                } finally {
                    // PBEKeySpec keeps its own copy of the password, and only
                    // clearPassword reaches it
                    spec.clearPassword();
                }
            } catch (java.security.NoSuchAlgorithmException
                    | java.security.spec.InvalidKeySpecException e) {
                throw new IllegalStateException("PBKDF2WithHmacSHA256 is unavailable", e);
            }
        }

        /** Builds the proof for a password, a salt and a nonce. */
        public static byte[] proofFor(String account, char[] password, byte[] salt, byte[] nonce) {
            byte[] key = deriveKey(password, salt);
            try {
                javax.crypto.Mac mac = javax.crypto.Mac.getInstance("HmacSHA256");
                mac.init(new javax.crypto.spec.SecretKeySpec(key, "HmacSHA256"));
                mac.update(nonce);
                mac.update(account.getBytes(java.nio.charset.StandardCharsets.UTF_8));
                return mac.doFinal();
            } catch (java.security.NoSuchAlgorithmException | java.security.InvalidKeyException e) {
                throw new IllegalStateException("HmacSHA256 is unavailable", e);
            } finally {
                java.util.Arrays.fill(key, (byte) 0);
            }
        }

        /** Compares two proofs without leaking how much matched. */
        public static boolean proofsEqual(byte[] a, byte[] b) {
            if (a == null || b == null || a.length != b.length) {
                return false;
            }
            int diff = 0;
            for (int i = 0; i < a.length; i++) {
                diff |= a[i] ^ b[i];
            }
            return diff == 0;
        }
    }

    /** Whether the account was accepted, and on what terms. */
    public static final class AuthAck {
        public final boolean accepted;
        public final String reason;
        public final String token;
        public final long expiresAt;

        private AuthAck(boolean accepted, String reason, String token, long expiresAt) {
            this.accepted = accepted;
            this.reason = reason;
            this.token = token;
            this.expiresAt = expiresAt;
        }

        public static AuthAck ok(String token, long expiresAt) {
            return new AuthAck(true, "", token, expiresAt);
        }

        public static AuthAck refused(String reason) {
            return new AuthAck(false, reason, "", 0L);
        }

        public Buf write() {
            Buf buf = new Buf();
            buf.writeBoolean(this.accepted);
            buf.writeString(this.reason);
            if (this.accepted) {
                buf.writeString(this.token);
                buf.writeLong(this.expiresAt);
            }
            return buf;
        }

        public static AuthAck read(Buf in) {
            boolean accepted = in.readBoolean();
            // the reason is written either way, so it has to be read either way:
            // skipping it here shifts everything after it by one field
            String reason = in.readString(MAX_TEXT);
            if (!accepted) {
                return refused(reason);
            }
            return ok(in.readString(MAX_TEXT), in.readLong());
        }
    }

    /** The client redeeming the token it was just handed. */
    public static final class Login {
        public final String account;
        public final String token;

        public Login(String account, String token) {
            this.account = account;
            this.token = token;
        }

        public Buf write() {
            Buf buf = new Buf();
            buf.writeString(this.account);
            buf.writeString(this.token);
            return buf;
        }

        public static Login read(Buf in) {
            return new Login(in.readString(MAX_TEXT), in.readString(MAX_TEXT));
        }
    }

    /** Whether the player is in. */
    public static final class LoginAck {
        public final boolean accepted;
        public final String reason;
        public final int playerId;
        public final int entityId;

        private LoginAck(boolean accepted, String reason, int playerId, int entityId) {
            this.accepted = accepted;
            this.reason = reason;
            this.playerId = playerId;
            this.entityId = entityId;
        }

        public static LoginAck ok(int playerId, int entityId) {
            return new LoginAck(true, "", playerId, entityId);
        }

        public static LoginAck refused(String reason) {
            return new LoginAck(false, reason, -1, -1);
        }

        public Buf write() {
            Buf buf = new Buf();
            buf.writeBoolean(this.accepted);
            buf.writeString(this.reason);
            if (this.accepted) {
                buf.writeVarInt(this.playerId);
                buf.writeVarInt(this.entityId);
            }
            return buf;
        }

        public static LoginAck read(Buf in) {
            boolean accepted = in.readBoolean();
            String reason = in.readString(MAX_TEXT);
            if (!accepted) {
                return refused(reason);
            }
            return ok(in.readVarInt(), in.readVarInt());
        }
    }

    /** A reason, said out loud, before the connection goes away. */
    public static final class Disconnect {
        public final String reason;

        public Disconnect(String reason) {
            this.reason = reason;
        }

        public Buf write() {
            return new Buf().writeString(this.reason);
        }

        public static Disconnect read(Buf in) {
            return new Disconnect(in.readString(MAX_TEXT));
        }
    }

    /**
     * Every packet id this build knows, and what it decodes into.
     *
     * Written as plain data so that adding a packet is one line and an unknown
     * id is a loud error instead of a silent skip.
     */
    public static Map<Integer, String> names() {
        Map<Integer, String> map = new LinkedHashMap<Integer, String>();
        map.put(Integer.valueOf(HELLO), "Hello");
        map.put(Integer.valueOf(HELLO_ACK), "HelloAck");
        map.put(Integer.valueOf(AUTH), "Auth");
        map.put(Integer.valueOf(AUTH_ACK), "AuthAck");
        map.put(Integer.valueOf(LOGIN), "Login");
        map.put(Integer.valueOf(LOGIN_ACK), "LoginAck");
        map.put(Integer.valueOf(DISCONNECT), "Disconnect");
        return map;
    }
}