/*

 * Copyright (c) 2021 Fastly, Kazuho Oku

 *

 * Permission is hereby granted, free of charge, to any person obtaining a copy

 * of this software and associated documentation files (the "Software"), to

 * deal in the Software without restriction, including without limitation the

 * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or

 * sell copies of the Software, and to permit persons to whom the Software is

 * furnished to do so, subject to the following conditions:

 *

 * The above copyright notice and this permission notice shall be included in

 * all copies or substantial portions of the Software.

 *

 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR

 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,

 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE

 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER

 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING

 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS

 * IN THE SOFTWARE.

 */

#include <math.h>

#include "quicly/cc.h"

#include "quicly.h"

/**

 * Calculates the increase ratio to be used in congestion avoidance phase.

 */

static uint32_t calc_bytes_per_mtu_increase(uint32_t cwnd, uint32_t rtt, uint32_t mtu)

{

    /* Reno: CWND size after reduction */

    uint32_t reno = cwnd * QUICLY_RENO_BETA;

    /* Cubic: Cubic reaches original CWND (i.e., Wmax) in K seconds, therefore:

     *   amount_to_increase = 0.3 * Wmax

     *   amount_to_be_acked = K * Wmax / RTT_at_Wmax

     * where

     *   K = (0.3 / 0.4 * Wmax / MTU)^(1/3)

     *

     * Hence:

     *   bytes_per_mtu_increase = amount_to_be_acked / amount_to_increase * MTU

     *     = (K * Wmax / RTT_at_Wmax) / (0.3 * Wmax) * MTU

     *     = K * MTU / (0.3 * RTT_at_Wmax)

     *

     * In addition, we have to adjust the value to take fast convergence into account. On a path with stable capacity, 50% of

     * congestion events adjust Wmax to 0.85x of before calculating K. If that happens, the modified K (K') is:

     *

     *   K' = (0.3 / 0.4 * 0.85 * Wmax / MTU)^(1/3) = 0.85^(1/3) * K

     *

     * where K' represents the time to reach 0.85 * Wmax. As the cubic curve is point symmetric at the point where this curve

     * reaches 0.85 * Wmax, it would take 2 * K' seconds to reach Wmax.

     *

     * Therefore, by amortizing the two modes, the congestion period of Cubic with fast convergence is calculated as:

     *

     *   bytes_per_mtu_increase = ((1 + 0.85^(1/3) * 2) / 2) * K * MTU / (0.3 * RTT_at_Wmax)

     */

    uint32_t cubic = 1.447 / 0.3 * 1000 * cbrt(0.3 / 0.4 * cwnd / mtu) / rtt * mtu;

    return reno < cubic ? reno : cubic;

}

/* TODO: Avoid increase if sender was application limited. */

static void pico_on_acked(quicly_cc_t *cc, const quicly_loss_t *loss, uint32_t bytes, uint64_t largest_acked, uint32_t inflight,

                          uint64_t next_pn, int64_t now, uint32_t max_udp_payload_size)

{

    assert(inflight >= bytes);

    /* Do not increase congestion window while in recovery. */

    if (largest_acked < cc->recovery_end)

        return;

    cc->state.pico.stash += bytes;

    /* Calculate the amount of bytes required to be acked for incrementing CWND by one MTU. */

    uint32_t bytes_per_mtu_increase;

    if (cc->cwnd < cc->ssthresh) {

        bytes_per_mtu_increase = max_udp_payload_size;

    } else {

        bytes_per_mtu_increase = cc->state.pico.bytes_per_mtu_increase;

    }

    /* Bail out if we do not yet have enough bytes being acked. */

    if (cc->state.pico.stash < bytes_per_mtu_increase)

        return;

    /* Update CWND, reducing stash relative to the amount we've adjusted the CWND */

    uint32_t count = cc->state.pico.stash / bytes_per_mtu_increase;

    cc->cwnd += count * max_udp_payload_size;

    cc->state.pico.stash -= count * bytes_per_mtu_increase;

    if (cc->cwnd_maximum < cc->cwnd)

        cc->cwnd_maximum = cc->cwnd;

}

static void pico_on_lost(quicly_cc_t *cc, const quicly_loss_t *loss, uint32_t bytes, uint64_t lost_pn, uint64_t next_pn,

                         int64_t now, uint32_t max_udp_payload_size)

{

    /* Nothing to do if loss is in recovery window. */

    if (lost_pn < cc->recovery_end)

        return;

    cc->recovery_end = next_pn;

    ++cc->num_loss_episodes;

    if (cc->cwnd_exiting_slow_start == 0)

        cc->cwnd_exiting_slow_start = cc->cwnd;

    /* Calculate increase rate. */

    cc->state.pico.bytes_per_mtu_increase = calc_bytes_per_mtu_increase(cc->cwnd, loss->rtt.smoothed, max_udp_payload_size);

    /* Reduce congestion window. */

    cc->cwnd *= QUICLY_RENO_BETA;

    if (cc->cwnd < QUICLY_MIN_CWND * max_udp_payload_size)

        cc->cwnd = QUICLY_MIN_CWND * max_udp_payload_size;

    cc->ssthresh = cc->cwnd;

    if (cc->cwnd_minimum > cc->cwnd)

        cc->cwnd_minimum = cc->cwnd;

}

static void pico_on_persistent_congestion(quicly_cc_t *cc, const quicly_loss_t *loss, int64_t now)

{

    /* TODO */

}

static void pico_on_sent(quicly_cc_t *cc, const quicly_loss_t *loss, uint32_t bytes, int64_t now)

{

    /* Unused */

}

static void pico_init_pico_state(quicly_cc_t *cc, uint32_t stash)

{

    cc->state.pico.stash = stash;

    cc->state.pico.bytes_per_mtu_increase = cc->cwnd * QUICLY_RENO_BETA; /* use Reno, for simplicity */

}

static void pico_reset(quicly_cc_t *cc, uint32_t initcwnd)

{

    *cc = (quicly_cc_t){

        .type = &quicly_cc_type_pico,

        .cwnd = initcwnd,

        .cwnd_initial = initcwnd,

        .cwnd_maximum = initcwnd,

        .cwnd_minimum = UINT32_MAX,

        .ssthresh = UINT32_MAX,

    };

    pico_init_pico_state(cc, 0);

}

static int pico_on_switch(quicly_cc_t *cc)

{

    if (cc->type == &quicly_cc_type_pico) {

        return 1; /* nothing to do */

    } else if (cc->type == &quicly_cc_type_reno) {

        cc->type = &quicly_cc_type_pico;

        pico_init_pico_state(cc, cc->state.reno.stash);

        return 1;

    } else if (cc->type == &quicly_cc_type_cubic) {

        /* When in slow start, state can be reused as-is; otherwise, restart. */

        if (cc->cwnd_exiting_slow_start == 0) {

            cc->type = &quicly_cc_type_pico;

            pico_init_pico_state(cc, 0);

        } else {

            pico_reset(cc, cc->cwnd_initial);

        }

        return 1;

    }

    return 0;

}

static void pico_init(quicly_init_cc_t *self, quicly_cc_t *cc, uint32_t initcwnd, int64_t now)

{

    pico_reset(cc, initcwnd);

}

quicly_cc_type_t quicly_cc_type_pico = {

    "pico", &quicly_cc_pico_init, pico_on_acked, pico_on_lost, pico_on_persistent_congestion, pico_on_sent, pico_on_switch};

quicly_init_cc_t quicly_cc_pico_init = {pico_init};