firmware/src/modules/Native/StoreForwardPlusPlus.cpp

// create second module for satellites?

// The central node will generate a 256 or 128 bit value as its seed. This is the value other nodes subscribe to, and serves as
// the root of the chain

//

// Basic design:
// This module watches a channel for text messages.
// each message gets sha256 summed, and then appended to a git-style blockchain. Probably need a counter, too
// then the message, metadata, hash, and git hash information are saved. sqlite?

// nodes/sub-controllers can subscribe to a database

// A node can DM the controller, querying if a single message is on the chain, or asking for the last message hash

// if the message is not on the chain, the node can resend the message

// if the node lacks messages, it can request them

// will need the concept of sub-controllers, that subscribe to the central controller, can help push updates

// catch-up messages only go out when the mesh is low use %

// Normal firmware will only attempt to sync the chain a few times, then just ask for the latest few messages. A phone app can try
// harder

// host will periodically advertise its presence

// at least initially, there can only be one authoritative host

// first draft is just to save channel 0 in a git-style database

// message objects get a hash value
// the message chain gets a commit hash
//

#include "StoreForwardPlusPlus.h"
#include "MeshService.h"
#include "RTC.h"
#include "SHA256.h"
#include "meshUtils.h"

StoreForwardPlusPlusModule::StoreForwardPlusPlusModule()
    : ProtobufModule("StoreForwardpp", meshtastic_PortNum_STORE_FORWARD_PLUSPLUS_APP, &meshtastic_StoreForwardPlusPlus_msg),
      concurrency::OSThread("StoreForwardpp")
{
    LOG_WARN("StoreForwardPlusPlusModule init");
    if (portduino_config.sfpp_stratum0)
        LOG_WARN("SF++ stratum0");
    int res = sqlite3_open("test.db", &ppDb);
    LOG_WARN("Result1 %u", res);
    char *err = nullptr;
    res = sqlite3_exec(ppDb, "         \
        CREATE TABLE channel_messages( \
        destination INT NOT NULL,      \
        sender INT NOT NULL,           \
        packet_id INT NOT NULL,        \
        want_ack BOOL NOT NULL,        \
        channel_hash INT NOT NULL,      \
        encrypted_bytes BLOB NOT NULL, \
        message_hash BLOB NOT NULL,    \
        rx_time INT NOT NULL,          \
        commit_hash BLOB NOT NULL,     \
        payload TEXT,                  \
        PRIMARY KEY (message_hash)     \
        );",
                       NULL, NULL, &err);
    LOG_WARN("Result2 %u", res);
    if (err != nullptr)
        ;
    LOG_ERROR("%s", err);
    sqlite3_free(err);

    // create table DMs
    res = sqlite3_exec(ppDb, "         \
        CREATE TABLE direct_messages( \
        destination INT NOT NULL,      \
        sender INT NOT NULL,           \
        packet_id INT NOT NULL,        \
        want_ack BOOL NOT NULL,        \
        channel_hash INT NOT NULL,      \
        commit_hash BLOB NOT NULL,     \
        encrypted_bytes BLOB NOT NULL, \
        message_hash BLOB NOT NULL,    \
        payload TEXT,                  \
        rx_time INT NOT NULL,          \
        PRIMARY KEY (message_hash)     \
        );",
                       NULL, NULL, &err);
    LOG_WARN("Result2 %u", res);
    if (err != nullptr)
        ;
    LOG_ERROR("%s", err);
    sqlite3_free(err);

    // create table mappings
    // create table DMs
    res = sqlite3_exec(ppDb, "         \
        CREATE TABLE mappings( \
        chain_type INT NOT NULL,      \
        identifier INT NOT NULL,           \
        root_hash BLOB NOT NULL,     \
        PRIMARY KEY (identifier)     \
        );",
                       NULL, NULL, &err);
    LOG_WARN("Result2 %u", res);
    if (err != nullptr)
        ;
    LOG_ERROR("%s", err);
    sqlite3_free(err);

    // store schema version somewhere

    std::string insert_statement = "INSERT INTO channel_messages (destination, sender, packet_id, want_ack, channel_hash, \
        encrypted_bytes, message_hash, rx_time, commit_hash, payload) VALUES(?, ?, ?, ?, ?, ?, ?, ?, ?, ?);";
    sqlite3_prepare_v2(ppDb, insert_statement.c_str(), insert_statement.length(), &stmt, NULL);

    encryptedOk = true;

    this->setInterval(15 * 1000);
}

int32_t StoreForwardPlusPlusModule::runOnce()
{
    LOG_WARN("StoreForward++ runONce");
    if (getRTCQuality() < RTCQualityNTP) {
        LOG_WARN("StoreForward++ deferred due to time quality %u", getRTCQuality());
        return 60 * 60 * 1000;
    }
    uint8_t root_hash_bytes[32] = {0};
    ChannelHash hash = channels.getHash(0);
    getOrAddRootFromChannelHash(hash, root_hash_bytes);

    // get tip of chain for this channel
    uint8_t last_message_chain_hash[32] = {0};
    uint8_t last_message_hash[32] = {0};
    LOG_WARN("here5");

    if (!getChainEnd(hash, last_message_chain_hash, last_message_hash)) {
        LOG_WARN("Store and Forward++ database lookup returned null");
        return 60 * 60 * 1000;
    }

    meshtastic_StoreForwardPlusPlus storeforward = meshtastic_StoreForwardPlusPlus_init_zero;
    storeforward.sfpp_message_type = meshtastic_StoreForwardPlusPlus_SFPP_message_type_CANON_ANNOUNCE;
    // set root hash

    // set message hash
    storeforward.message_hash.size = 32;
    memcpy(storeforward.message_hash.bytes, last_message_hash, 32);

    // set chain hash
    storeforward.chain_hash.size = 32;
    memcpy(storeforward.chain_hash.bytes, last_message_chain_hash, 32);

    // set root hash
    storeforward.root_hash.size = 32;
    memcpy(storeforward.root_hash.bytes, root_hash_bytes, 32);

    // storeforward.
    meshtastic_MeshPacket *p = allocDataProtobuf(storeforward);
    p->to = NODENUM_BROADCAST;
    p->decoded.want_response = false;
    p->priority = meshtastic_MeshPacket_Priority_BACKGROUND;
    p->channel = 0;
    LOG_INFO("Send packet to mesh");
    service->sendToMesh(p, RX_SRC_LOCAL, true);

    return 60 * 60 * 1000;
}

bool StoreForwardPlusPlusModule::handleReceivedProtobuf(const meshtastic_MeshPacket &mp, meshtastic_StoreForwardPlusPlus *t)
{
    LOG_WARN("in handleReceivedProtobuf");
    LOG_WARN("Sfp++ node %u is informing us of packet", mp.from);
    printBytes("chain_hash ", t->chain_hash.bytes, t->chain_hash.size);
    if (t->sfpp_message_type == meshtastic_StoreForwardPlusPlus_SFPP_message_type_CANON_ANNOUNCE) {
        // check chain_hash.size

        if (portduino_config.sfpp_stratum0) {
            LOG_WARN("Received a CANON_ANNOUNCE while stratum 0");
        } else {
            uint8_t tmp_root_hash_bytes[32] = {0};

            LOG_WARN("Received a CANON_ANNOUNCE");
            if (getRootFromChannelHash(router->p_encrypted->channel, tmp_root_hash_bytes)) {
                // we found the hash, check if it's the right one
                if (memcmp(tmp_root_hash_bytes, t->root_hash.bytes, 32) != 0) {
                    LOG_WARN("Found root hash, and it doesn't match!");
                    return true;
                }
            } else {
                // There's the possibility that
                addRootToMappings(router->p_encrypted->channel, t->root_hash.bytes);
                LOG_WARN("Adding root hash to mappings");
            }

            // get tip of chain for this channel
            uint8_t last_message_chain_hash[32] = {0};
            uint8_t last_message_hash[32] = {0};

            // get chain tip
            if (getChainEnd(router->p_encrypted->channel, last_message_chain_hash, last_message_hash)) {
                if (memcmp(last_message_chain_hash, t->chain_hash.bytes, 32) == 0) {
                    LOG_WARN("End of chain matches!");
                } else
                    ("End of chain does not match!");
            } else {
                LOG_WARN("No Messages on this chain, request!");
                requestNextMessage(t->root_hash.bytes, t->root_hash.bytes);
            }

            // compare to chain tip in incoming message

            // if not found, request the next message
        }
    } else if (t->sfpp_message_type == meshtastic_StoreForwardPlusPlus_SFPP_message_type_LINK_REQUEST) {
        uint8_t next_chain_hash[32] = {0};

        LOG_WARN("Received link request");
        getNextHash(t->root_hash.bytes, t->chain_hash.bytes, next_chain_hash);
        printBytes("next chain hash: ", next_chain_hash, 32);

        broadcastLink(next_chain_hash, t->root_hash.bytes);

        // if root and chain hashes are the same, grab the first message on the chain
        // if different, get the message directly after.
        // check if the root

    } else if (t->sfpp_message_type == meshtastic_StoreForwardPlusPlus_SFPP_message_type_LINK_PROVIDE) {
        LOG_WARN("Link Provide received!");
    }

    return true;
}

ProcessMessage StoreForwardPlusPlusModule::handleReceived(const meshtastic_MeshPacket &mp)
{
    // To avoid terrible time problems, require NTP or GPS time
    if (getRTCQuality() < RTCQualityNTP) {
        return ProcessMessage::CONTINUE; // Let others look at this message also if they want
    }

    // the sender+destination pair is an interesting unique id (though ordering) (smaller one goes first?)
    // so messages with a unique pair become a chain
    // These get a table

    // message to broadcast get a chain per channel hash
    // second table
    // for now, channel messages are limited to decryptable
    // limited to text messages

    // create a unique-from-nodenums() class that returns a 64-bit value

    SHA256 message_hash, chain_hash;
    uint8_t message_hash_bytes[32] = {0};
    uint8_t chain_hash_bytes[32] = {0};
    uint8_t root_hash_bytes[32] = {0};

    // For the moment, this is strictly LoRa
    if (mp.transport_mechanism != meshtastic_MeshPacket_TransportMechanism_TRANSPORT_LORA) {
        return ProcessMessage::CONTINUE; // Let others look at this message also if they want
    }

    // will eventually host DMs and other undecodable messages
    if (mp.which_payload_variant != meshtastic_MeshPacket_decoded_tag) {
        return ProcessMessage::CONTINUE; // Let others look at this message also if they want
    }
    // refuse without valid time?
    LOG_WARN("in handleReceived");
    if (mp.decoded.portnum == meshtastic_PortNum_TEXT_MESSAGE_APP && mp.to == NODENUM_BROADCAST) {

        // need to resolve the channel hash to the root hash
        getRootFromChannelHash(router->p_encrypted->channel, root_hash_bytes);

        std::string getEntry_string =
            "select commit_hash from channel_messages where channel_hash=? order by rowid desc LIMIT 1;";
        sqlite3_stmt *getEntry;
        int rc = sqlite3_prepare_v2(ppDb, getEntry_string.c_str(), getEntry_string.size(), &getEntry, NULL);
        sqlite3_bind_int(getEntry, 1, router->p_encrypted->channel);
        sqlite3_step(getEntry);

        // this is allocated by sqlite3 and will be deleted when finalize is called
        uint8_t *last_message_hash = (uint8_t *)sqlite3_column_blob(getEntry, 0);
        if (last_message_hash) {
            printBytes("last message: 0x", last_message_hash, 32);
        } else {
            printBytes("new chain root: 0x", root_hash_bytes, 32);
        }

        // do not include rxtime in the message hash. We want these to match when more then one node receives and compares notes.
        // feel free to include it in the commit hash

        message_hash.reset();
        message_hash.update(router->p_encrypted->encrypted.bytes, router->p_encrypted->encrypted.size);
        message_hash.update(&mp.to, sizeof(mp.to));
        message_hash.update(&mp.from, sizeof(mp.from));
        message_hash.update(&mp.id, sizeof(mp.id));
        message_hash.finalize(message_hash_bytes, 32);

        chain_hash.reset();
        if (last_message_hash) {
            chain_hash.update(last_message_hash, 32);
        } else {
            chain_hash.update(root_hash_bytes, 32);
        }
        chain_hash.update(message_hash_bytes, 32);
        // message_hash.update(&mp.rx_time, sizeof(mp.rx_time));
        chain_hash.finalize(chain_hash_bytes, 32);

        sqlite3_finalize(getEntry);

        // select HEX(commit_hash),HEX(channel_hash), payload, destination from channel_messages order by rowid desc;

        // push a message into the local chain DB
        // destination
        sqlite3_bind_int(stmt, 1, mp.to);
        // sender
        sqlite3_bind_int(stmt, 2, mp.from);
        // packet_id
        sqlite3_bind_int(stmt, 3, mp.id);
        // want_ack
        sqlite3_bind_int(stmt, 4, mp.want_ack);
        // channel_hash
        sqlite3_bind_int(stmt, 5, router->p_encrypted->channel);
        // encrypted_bytes
        sqlite3_bind_blob(stmt, 6, router->p_encrypted->encrypted.bytes, router->p_encrypted->encrypted.size, NULL);

        // message_hash
        sqlite3_bind_blob(stmt, 7, message_hash_bytes, 32, NULL);
        // rx_time
        sqlite3_bind_int(stmt, 8, mp.rx_time);

        // commit_hash
        sqlite3_bind_blob(stmt, 9, chain_hash_bytes, 32, NULL);
        // payload
        sqlite3_bind_text(stmt, 10, (char *)mp.decoded.payload.bytes, mp.decoded.payload.size, NULL);

        sqlite3_step(stmt);
        sqlite3_reset(stmt);

        return ProcessMessage::CONTINUE; // Let others look at this message also if they want

        // one of the command messages
    } else if (mp.decoded.portnum == meshtastic_PortNum_STORE_FORWARD_PLUSPLUS_APP) {
        LOG_WARN("Got a STORE_FORWARD++ packet");
        meshtastic_StoreForwardPlusPlus scratch;
        pb_decode_from_bytes(mp.decoded.payload.bytes, mp.decoded.payload.size, meshtastic_StoreForwardPlusPlus_fields, &scratch);
        handleReceivedProtobuf(mp, &scratch);
        // when we get an update this way, if the message isn't on the chain, this node hasn't seen it, and can rebroadcast.
        return ProcessMessage::CONTINUE;
    }
    return ProcessMessage::CONTINUE;
}

bool StoreForwardPlusPlusModule::getRootFromChannelHash(ChannelHash _ch_hash, uint8_t *_root_hash)
{
    bool found = false;
    sqlite3_stmt *getHash;
    int rc = sqlite3_prepare_v2(ppDb, "select root_hash from mappings where identifier=?;", -1, &getHash, NULL);
    sqlite3_bind_int(getHash, 1, _ch_hash);
    sqlite3_step(getHash);
    uint8_t *tmp_root_hash = (uint8_t *)sqlite3_column_blob(getHash, 0);
    if (tmp_root_hash) {
        LOG_WARN("Found root hash!");
        memcpy(_root_hash, tmp_root_hash, 32);
        found = true;
    }
    sqlite3_finalize(getHash);
    return found;
}

ChannelHash StoreForwardPlusPlusModule::getChannelHashFromRoot(uint8_t *_root_hash)
{
    sqlite3_stmt *getHash;
    int rc = sqlite3_prepare_v2(ppDb, "select identifier from mappings where root_hash=?;", -1, &getHash, NULL);
    sqlite3_bind_blob(getHash, 1, _root_hash, 32, NULL);
    sqlite3_step(getHash);
    ChannelHash tmp_hash = (ChannelHash)sqlite3_column_int(getHash, 0);
    sqlite3_finalize(getHash);
    return tmp_hash;
}

// return code indicates newly created chain
bool StoreForwardPlusPlusModule::getOrAddRootFromChannelHash(ChannelHash _ch_hash, uint8_t *_root_hash)
{
    LOG_WARN("getOrAddRootFromChannelHash()");
    bool isNew = !getRootFromChannelHash(_ch_hash, _root_hash);

    if (isNew) {
        if (portduino_config.sfpp_stratum0) {
            LOG_WARN("Generating Root hash!");
            // generate root hash
            SHA256 chain_hash;
            chain_hash.update(&_ch_hash, sizeof(_ch_hash));
            NodeNum ourNode = nodeDB->getNodeNum();
            chain_hash.update(&ourNode, sizeof(ourNode));
            uint32_t rtc_sec = getValidTime(RTCQuality::RTCQualityDevice, true);
            chain_hash.update(&rtc_sec, sizeof(rtc_sec));
            chain_hash.finalize(_root_hash, 32);

            addRootToMappings(_ch_hash, _root_hash);
            LOG_WARN("here4");
        }
    }
    return isNew;
}

bool StoreForwardPlusPlusModule::addRootToMappings(ChannelHash _ch_hash, uint8_t *_root_hash)
{
    LOG_WARN("addRootToMappings()");
    printBytes("_root_hash", _root_hash, 32);
    sqlite3_stmt *getHash;

    // write to the table
    int rc =
        sqlite3_prepare_v2(ppDb, "INSERT INTO mappings (chain_type, identifier, root_hash) VALUES(?, ?, ?);", -1, &getHash, NULL);
    LOG_WARN("%d", rc);
    int type = chain_types::channel_chain;
    // note, must be an int variable

    sqlite3_bind_int(getHash, 1, type);
    sqlite3_bind_int(getHash, 2, _ch_hash);
    sqlite3_bind_blob(getHash, 3, _root_hash, 32, NULL);
    LOG_WARN("here1");
    // sqlite3_bind_int(getHash, 4, nodeToAdd);
    rc = sqlite3_step(getHash);
    LOG_WARN("here2 %u, %s", rc, sqlite3_errmsg(ppDb));
    sqlite3_finalize(getHash);
    LOG_WARN("here3");
    return true;
}

bool StoreForwardPlusPlusModule::getChainEnd(ChannelHash _ch_hash, uint8_t *_chain_hash, uint8_t *_message_hash)
{
    LOG_WARN("getChainEnd");

    std::string getEntry_string =
        "select commit_hash, message_hash from channel_messages where channel_hash=? order by rowid desc LIMIT 1;";
    sqlite3_stmt *getEntry;
    int rc = sqlite3_prepare_v2(ppDb, getEntry_string.c_str(), getEntry_string.size(), &getEntry, NULL);
    sqlite3_bind_int(getEntry, 1, _ch_hash);
    sqlite3_step(getEntry);
    uint8_t *last_message_chain_hash = (uint8_t *)sqlite3_column_blob(getEntry, 0);
    uint8_t *last_message_hash = (uint8_t *)sqlite3_column_blob(getEntry, 1);

    if (last_message_chain_hash == nullptr || last_message_hash == nullptr) {
        LOG_WARN("Store and Forward++ database lookup returned null");
        sqlite3_finalize(getEntry);

        return false;
    }
    memcpy(_chain_hash, last_message_chain_hash, 32);
    memcpy(_message_hash, last_message_hash, 32);
    sqlite3_finalize(getEntry);
    return true;
}

void StoreForwardPlusPlusModule::requestNextMessage(uint8_t *_root_hash, uint8_t *_chain_hash)
{

    meshtastic_StoreForwardPlusPlus storeforward = meshtastic_StoreForwardPlusPlus_init_zero;
    storeforward.sfpp_message_type = meshtastic_StoreForwardPlusPlus_SFPP_message_type_LINK_REQUEST;
    // set root hash

    // set chain hash
    storeforward.chain_hash.size = 32;
    memcpy(storeforward.chain_hash.bytes, _chain_hash, 32);

    // set root hash
    storeforward.root_hash.size = 32;
    memcpy(storeforward.root_hash.bytes, _root_hash, 32);

    // storeforward.
    meshtastic_MeshPacket *p = allocDataProtobuf(storeforward);
    p->to = NODENUM_BROADCAST;
    p->decoded.want_response = false;
    p->priority = meshtastic_MeshPacket_Priority_BACKGROUND;
    p->channel = 0;
    LOG_INFO("Send packet to mesh");
    service->sendToMesh(p, RX_SRC_LOCAL, true);
}

bool StoreForwardPlusPlusModule::getNextHash(uint8_t *_root_hash, uint8_t *_chain_hash, uint8_t *next_chain_hash)
{
    LOG_WARN("getNextHash");

    ChannelHash _channel_hash = getChannelHashFromRoot(_root_hash);
    LOG_WARN("_channel_hash %u", _channel_hash);

    sqlite3_stmt *getHash;
    int rc = sqlite3_prepare_v2(ppDb, "select commit_hash from channel_messages where channel_hash=? order by rowid ASC;", -1,
                                &getHash, NULL);

    LOG_WARN("%d", rc);
    if (rc != SQLITE_OK) {
        LOG_WARN("here2 %u, %s", rc, sqlite3_errmsg(ppDb));
    }
    sqlite3_bind_int(getHash, 1, _channel_hash);
    sqlite3_step(getHash);
    bool found_hash = false;

    // asking for the first entry on the chain
    if (memcmp(_root_hash, _chain_hash, 32) == 0) {
        uint8_t *tmp_chain_hash = (uint8_t *)sqlite3_column_blob(getHash, 0);
        memcpy(next_chain_hash, tmp_chain_hash, 32);
        found_hash = true;
    } else {
        uint8_t *tmp_chain_hash;
        while (sqlite3_step(getHash) != SQLITE_DONE) {
            tmp_chain_hash = (uint8_t *)sqlite3_column_blob(getHash, 0);
            if (found_hash) {
                memcpy(next_chain_hash, tmp_chain_hash, 32);
                break;
            }
            if (memcmp(tmp_chain_hash, _chain_hash, 32) == 0)
                found_hash = true;
        }
    }

    sqlite3_finalize(getHash);
    return found_hash;
}

bool StoreForwardPlusPlusModule::broadcastLink(uint8_t *_chain_hash, uint8_t *_root_hash)
{
    sqlite3_stmt *getHash;
    int rc = sqlite3_prepare_v2(ppDb, "select destination, sender, packet_id, encrypted_bytes, message_hash, rx_time \
        from channel_messages where commit_hash=?;",
                                -1, &getHash, NULL);

    LOG_WARN("%d", rc);
    if (rc != SQLITE_OK) {
        LOG_WARN("here2 %u, %s", rc, sqlite3_errmsg(ppDb));
    }
    sqlite3_bind_blob(getHash, 1, _chain_hash, 32, NULL);
    sqlite3_step(getHash);

    meshtastic_StoreForwardPlusPlus storeforward = meshtastic_StoreForwardPlusPlus_init_zero;
    storeforward.sfpp_message_type = meshtastic_StoreForwardPlusPlus_SFPP_message_type_LINK_PROVIDE;

    storeforward.encapsulated_to = sqlite3_column_int(getHash, 0);
    storeforward.encapsulated_from = sqlite3_column_int(getHash, 1);
    storeforward.encapsulated_id = sqlite3_column_int(getHash, 2);

    uint8_t *_payload = (uint8_t *)sqlite3_column_blob(getHash, 3);
    storeforward.message.size = sqlite3_column_bytes(getHash, 3);
    memcpy(storeforward.message.bytes, _payload, storeforward.message.size);

    uint8_t *_message_hash = (uint8_t *)sqlite3_column_blob(getHash, 4);
    storeforward.message_hash.size = 32;
    memcpy(storeforward.message_hash.bytes, _message_hash, storeforward.message_hash.size);

    storeforward.encapsulated_rxtime = sqlite3_column_int(getHash, 5);

    storeforward.chain_hash.size = 32;
    memcpy(storeforward.chain_hash.bytes, _chain_hash, 32);

    storeforward.root_hash.size = 32;
    memcpy(storeforward.root_hash.bytes, _root_hash, 32);

    sqlite3_finalize(getHash);

    meshtastic_MeshPacket *p = allocDataProtobuf(storeforward);
    p->to = NODENUM_BROADCAST;
    p->decoded.want_response = false;
    p->priority = meshtastic_MeshPacket_Priority_BACKGROUND;
    p->channel = 0;
    LOG_INFO("Send link to mesh");
    service->sendToMesh(p, RX_SRC_LOCAL, true);
    return true;
}

// announce latest hash
// chain_end_announce

// check if hash is known
// hash_query

// request next message
// link_request

// send encapsulated message
// link_provide_whole
// link_provide_half1
// link_provide_half2

// onboard request message?

// get x from top?

// messages
// Given this chain root, do you have a packet that matches this message hash?
// responds with chain hash etc

// given this chain root, what is your last chain and message hash?
// given this chain root, what is your next message after this chain hash? (do we have an overhead problem here?) (blegh,
// fragmentation) (but also, trunking)

// broadcast on this chain root, here is my last chain hash

// consider third-order nodes

// I can't talk directly to strata, I can talk to a satellite. Inform sat of a message. Sat stores it as if had seen it locally,
// and pushes it to central

// message Eventually works out through chain

// sat can capture time of receipt

// terms:
// CANON
// stratum
// chain
// links on the chain