/*
 * SpanDSP - a series of DSP components for telephony
 *
 * t38_gateway.c - An implementation of a T.38 gateway, less the packet exchange part
 *
 * Written by Steve Underwood <steveu@coppice.org>
 *
 * Copyright (C) 2005, 2006 Steve Underwood
 *
 * All rights reserved.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2, as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 *
 * $Id: t38_gateway.c,v 1.62 2006/12/08 12:47:29 steveu Exp $
 */

/*! \file */

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include <inttypes.h>
#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <time.h>
#include <string.h>
#if defined(HAVE_TGMATH_H)
#include <tgmath.h>
#endif
#if defined(HAVE_MATH_H)
#include <math.h>
#endif
#include <assert.h>
#include <tiffio.h>

#include "spandsp/telephony.h"
#include "spandsp/logging.h"
#include "spandsp/queue.h"
#include "spandsp/bit_operations.h"
#include "spandsp/power_meter.h"
#include "spandsp/complex.h"
#include "spandsp/tone_generate.h"
#include "spandsp/async.h"
#include "spandsp/hdlc.h"
#include "spandsp/silence_gen.h"
#include "spandsp/fsk.h"
#include "spandsp/v29rx.h"
#include "spandsp/v29tx.h"
#include "spandsp/v27ter_rx.h"
#include "spandsp/v27ter_tx.h"
#if defined(ENABLE_V17)
#include "spandsp/v17rx.h"
#include "spandsp/v17tx.h"
#endif
#include "spandsp/t4.h"

#include "spandsp/t30_fcf.h"
#include "spandsp/t35.h"
#include "spandsp/t30.h"

#include "spandsp/t38_core.h"
#include "spandsp/t38_gateway.h"

#define MS_PER_TX_CHUNK             30

#define INDICATOR_TX_COUNT          3

#define DISBIT1     0x01
#define DISBIT2     0x02
#define DISBIT3     0x04
#define DISBIT4     0x08
#define DISBIT5     0x10
#define DISBIT6     0x20
#define DISBIT7     0x40
#define DISBIT8     0x80

enum
{
    T38_NONE,
    T38_V27TER_RX,
    T38_V29_RX,
    T38_V17_RX
};

enum
{
    HDLC_FLAG_FINISHED = 0x01,
    HDLC_FLAG_CORRUPT_CRC = 0x02,
    HDLC_FLAG_PROCEED_WITH_OUTPUT = 0x04,
    HDLC_FLAG_MISSING_DATA = 0x08
};

static int restart_rx_modem(t38_gateway_state_t *s);
static void add_to_non_ecm_tx_buffer(t38_gateway_state_t *s, const uint8_t *buf, int len);
static int non_ecm_get_bit(void *user_data);
static int process_rx_indicator(t38_core_state_t *t, void *user_data, int indicator);

static void hdlc_underflow_handler(void *user_data)
{
    t38_gateway_state_t *s;
    int old_data_type;
    
    s = (t38_gateway_state_t *) user_data;
    span_log(&s->logging, SPAN_LOG_FLOW, "HDLC underflow at %d\n", s->hdlc_out);
    /* If the current HDLC buffer is not at the HDLC_FLAG_PROCEED_WITH_OUTPUT stage, this
       underflow must be an end of preamble condition. */
    if ((s->hdlc_flags[s->hdlc_out] & HDLC_FLAG_PROCEED_WITH_OUTPUT))
    {
        old_data_type = s->hdlc_contents[s->hdlc_out];
        s->hdlc_len[s->hdlc_out] = 0;
        s->hdlc_flags[s->hdlc_out] = 0;
        s->hdlc_contents[s->hdlc_out] = 0;
        if (++s->hdlc_out >= T38_TX_HDLC_BUFS)
            s->hdlc_out = 0;
        span_log(&s->logging, SPAN_LOG_FLOW, "HDLC next is 0x%X\n", s->hdlc_contents[s->hdlc_out]);
        if ((s->hdlc_contents[s->hdlc_out] & 0x100))
        {
            /* The next thing in the queue is an indicator, so we need to stop this modem. */
            span_log(&s->logging, SPAN_LOG_FLOW, "HDLC shutdown\n");
            hdlc_tx_frame(&s->hdlctx, NULL, 0);
        }
        else if ((s->hdlc_contents[s->hdlc_out] & 0x200))
        {
            /* Check if we should start sending the next frame */
            if ((s->hdlc_flags[s->hdlc_out] & HDLC_FLAG_PROCEED_WITH_OUTPUT))
            {
                /* This frame is ready to go, and uses the same modem we are running now. So, send
                   whatever we have. This might or might not be an entire frame. */
                span_log(&s->logging, SPAN_LOG_FLOW, "HDLC start next frame\n");
                hdlc_tx_frame(&s->hdlctx, s->hdlc_buf[s->hdlc_out], s->hdlc_len[s->hdlc_out]);
                if ((s->hdlc_flags[s->hdlc_out] & HDLC_FLAG_CORRUPT_CRC))
                    hdlc_tx_corrupt_frame(&s->hdlctx);
            }
        }
    }
}
/*- End of function --------------------------------------------------------*/

static int set_next_tx_type(t38_gateway_state_t *s)
{
    tone_gen_descriptor_t tone_desc;
    get_bit_func_t get_bit_func;
    void *get_bit_user_data;
    int indicator;

    if (s->next_tx_handler)
    {
        /* There is a handler queued, so that is the next one */
        s->tx_handler = s->next_tx_handler;
        s->tx_user_data = s->next_tx_user_data;
        s->next_tx_handler = NULL;
        return TRUE;
    }
    if (s->hdlc_in == s->hdlc_out)
        return FALSE;
    if ((s->hdlc_contents[s->hdlc_out] & 0x100) == 0)
        return FALSE;
    indicator = (s->hdlc_contents[s->hdlc_out] & 0xFF);
    s->hdlc_len[s->hdlc_out] = 0;
    s->hdlc_flags[s->hdlc_out] = 0;
    s->hdlc_contents[s->hdlc_out] = 0;
    if (++s->hdlc_out >= T38_TX_HDLC_BUFS)
        s->hdlc_out = 0;
    span_log(&s->logging, SPAN_LOG_FLOW, "Changing to %s\n", t38_indicator(indicator));
    if (s->short_train  &&  s->ecm_mode)
    {
        span_log(&s->logging, SPAN_LOG_FLOW, "HDLC mode\n");
        hdlc_tx_init(&s->hdlctx, FALSE, 2, TRUE, hdlc_underflow_handler, s);
        get_bit_func = (get_bit_func_t) hdlc_tx_get_bit;
        get_bit_user_data = (void *) &s->hdlctx;
    }
    else
    {
        span_log(&s->logging, SPAN_LOG_FLOW, "non-ECM mode\n");
        get_bit_func = non_ecm_get_bit;
        get_bit_user_data = (void *) s;
    }
    switch (indicator)
    {
    case T38_IND_NO_SIGNAL:
        /* Impose 75ms minimum on transmitted silence */
        silence_gen_set(&s->silence_gen, ms_to_samples(75));
        s->tx_handler = (span_tx_handler_t *) &(silence_gen);
        s->tx_user_data = &(s->silence_gen);
        s->next_tx_handler = NULL;
        break;
    case T38_IND_CNG:
        /* 0.5s of 1100Hz + 3.0s of silence repeating */
        make_tone_gen_descriptor(&tone_desc,
                                 1100,
                                 -11,
                                 0,
                                 0,
                                 500,
                                 3000,
                                 0,
                                 0,
                                 TRUE);
        tone_gen_init(&(s->tone_gen), &tone_desc);
        s->tx_handler = (span_tx_handler_t *) &(tone_gen);
        s->tx_user_data = &(s->tone_gen);
        silence_gen_set(&s->silence_gen, 0);
        s->next_tx_handler = (span_tx_handler_t *) &(silence_gen);
        s->next_tx_user_data = &(s->silence_gen);
        break;
    case T38_IND_CED:
        /* 0.2s of silence, then 2.6s to 4s of 2100Hz tone, then 75ms of silence. */
        silence_gen_alter(&s->silence_gen, ms_to_samples(200));
        make_tone_gen_descriptor(&tone_desc,
                                 2100,
                                 -11,
                                 0,
                                 0,
                                 2600,
                                 75,
                                 0,
                                 0,
                                 FALSE);
        tone_gen_init(&(s->tone_gen), &tone_desc);
        s->tx_handler = (span_tx_handler_t *) &(silence_gen);
        s->tx_user_data = &(s->silence_gen);
        s->next_tx_handler = (span_tx_handler_t *) &(tone_gen);
        s->next_tx_user_data = &(s->tone_gen);
        break;
    case T38_IND_V21_PREAMBLE:
        hdlc_tx_init(&s->hdlctx, FALSE, 2, TRUE, hdlc_underflow_handler, s);
        hdlc_tx_preamble(&s->hdlctx, 32);
        s->hdlc_len[s->hdlc_in] = 0;
        fsk_tx_init(&(s->v21tx), &preset_fsk_specs[FSK_V21CH2], (get_bit_func_t) hdlc_tx_get_bit, &s->hdlctx);
        /* Impose a minimum silence */
        silence_gen_alter(&s->silence_gen, ms_to_samples(75));
        s->tx_handler = (span_tx_handler_t *) &(silence_gen);
        s->tx_user_data = &(s->silence_gen);
        s->next_tx_handler = (span_tx_handler_t *) &(fsk_tx);
        s->next_tx_user_data = &(s->v21tx);
        break;
    case T38_IND_V27TER_2400_TRAINING:
        silence_gen_alter(&s->silence_gen, ms_to_samples(75));
        hdlc_tx_preamble(&s->hdlctx, 60);
        v27ter_tx_restart(&(s->v27ter_tx), 2400, s->use_tep);
        v27ter_tx_set_get_bit(&(s->v27ter_tx), get_bit_func, get_bit_user_data);
        s->tx_handler = (span_tx_handler_t *) &(silence_gen);
        s->tx_user_data = &(s->silence_gen);
        s->next_tx_handler = (span_tx_handler_t *) &(v27ter_tx);
        s->next_tx_user_data = &(s->v27ter_tx);
        break;
    case T38_IND_V27TER_4800_TRAINING:
        hdlc_tx_preamble(&s->hdlctx, 120);
        silence_gen_alter(&s->silence_gen, ms_to_samples(75));
        v27ter_tx_restart(&(s->v27ter_tx), 4800, s->use_tep);
        v27ter_tx_set_get_bit(&(s->v27ter_tx), get_bit_func, get_bit_user_data);
        s->tx_handler = (span_tx_handler_t *) &(silence_gen);
        s->tx_user_data = &(s->silence_gen);
        s->next_tx_handler = (span_tx_handler_t *) &(v27ter_tx);
        s->next_tx_user_data = &(s->v27ter_tx);
        break;
    case T38_IND_V29_7200_TRAINING:
        hdlc_tx_preamble(&s->hdlctx, 180);
        silence_gen_alter(&s->silence_gen, ms_to_samples(75));
        v29_tx_restart(&(s->v29tx), 7200, s->use_tep);
        v29_tx_set_get_bit(&(s->v29tx), get_bit_func, get_bit_user_data);
        s->tx_handler = (span_tx_handler_t *) &(silence_gen);
        s->tx_user_data = &(s->silence_gen);
        s->next_tx_handler = (span_tx_handler_t *) &(v29_tx);
        s->next_tx_user_data = &(s->v29tx);
        break;
    case T38_IND_V29_9600_TRAINING:
        hdlc_tx_preamble(&s->hdlctx, 240);
        silence_gen_alter(&s->silence_gen, ms_to_samples(75));
        v29_tx_restart(&(s->v29tx), 9600, s->use_tep);
        v29_tx_set_get_bit(&(s->v29tx), get_bit_func, get_bit_user_data);
        s->tx_handler = (span_tx_handler_t *) &(silence_gen);
        s->tx_user_data = &(s->silence_gen);
        s->next_tx_handler = (span_tx_handler_t *) &(v29_tx);
        s->next_tx_user_data = &(s->v29tx);
        break;
#if defined(ENABLE_V17)
    case T38_IND_V17_7200_SHORT_TRAINING:
        hdlc_tx_preamble(&s->hdlctx, 180);
        silence_gen_alter(&s->silence_gen, ms_to_samples(75));
        v17_tx_restart(&(s->v17tx), 7200, s->use_tep, s->short_train);
        v17_tx_set_get_bit(&(s->v17tx), get_bit_func, get_bit_user_data);
        s->tx_handler = (span_tx_handler_t *) &(silence_gen);
        s->tx_user_data = &(s->silence_gen);
        s->next_tx_handler = (span_tx_handler_t *) &(v17_tx);
        s->next_tx_user_data = &(s->v17tx);
        break;
    case T38_IND_V17_7200_LONG_TRAINING:
        hdlc_tx_preamble(&s->hdlctx, 180);
        silence_gen_alter(&s->silence_gen, ms_to_samples(75));
        v17_tx_restart(&(s->v17tx), 7200, s->use_tep, s->short_train);
        v17_tx_set_get_bit(&(s->v17tx), get_bit_func, get_bit_user_data);
        s->tx_handler = (span_tx_handler_t *) &(silence_gen);
        s->tx_user_data = &(s->silence_gen);
        s->next_tx_handler = (span_tx_handler_t *) &(v17_tx);
        s->next_tx_user_data = &(s->v17tx);
        break;
    case T38_IND_V17_9600_SHORT_TRAINING:
        hdlc_tx_preamble(&s->hdlctx, 240);
        silence_gen_alter(&s->silence_gen, ms_to_samples(75));
        v17_tx_restart(&(s->v17tx), 9600, s->use_tep, s->short_train);
        v17_tx_set_get_bit(&(s->v17tx), get_bit_func, get_bit_user_data);
        s->tx_handler = (span_tx_handler_t *) &(silence_gen);
        s->tx_user_data = &(s->silence_gen);
        s->next_tx_handler = (span_tx_handler_t *) &(v17_tx);
        s->next_tx_user_data = &(s->v17tx);
        break;
    case T38_IND_V17_9600_LONG_TRAINING:
        hdlc_tx_preamble(&s->hdlctx, 240);
        silence_gen_alter(&s->silence_gen, ms_to_samples(75));
        v17_tx_restart(&(s->v17tx), 9600, s->use_tep, s->short_train);
        v17_tx_set_get_bit(&(s->v17tx), get_bit_func, get_bit_user_data);
        s->tx_handler = (span_tx_handler_t *) &(silence_gen);
        s->tx_user_data = &(s->silence_gen);
        s->next_tx_handler = (span_tx_handler_t *) &(v17_tx);
        s->next_tx_user_data = &(s->v17tx);
        break;
    case T38_IND_V17_12000_SHORT_TRAINING:
        hdlc_tx_preamble(&s->hdlctx, 300);
        silence_gen_alter(&s->silence_gen, ms_to_samples(75));
        v17_tx_restart(&(s->v17tx), 12000, s->use_tep, s->short_train);
        v17_tx_set_get_bit(&(s->v17tx), get_bit_func, get_bit_user_data);
        s->tx_handler = (span_tx_handler_t *) &(silence_gen);
        s->tx_user_data = &(s->silence_gen);
        s->next_tx_handler = (span_tx_handler_t *) &(v17_tx);
        s->next_tx_user_data = &(s->v17tx);
        break;
    case T38_IND_V17_12000_LONG_TRAINING:
        hdlc_tx_preamble(&s->hdlctx, 300);
        silence_gen_alter(&s->silence_gen, ms_to_samples(75));
        v17_tx_restart(&(s->v17tx), 12000, s->use_tep, s->short_train);
        v17_tx_set_get_bit(&(s->v17tx), get_bit_func, get_bit_user_data);
        s->tx_handler = (span_tx_handler_t *) &(silence_gen);
        s->tx_user_data = &(s->silence_gen);
        s->next_tx_handler = (span_tx_handler_t *) &(v17_tx);
        s->next_tx_user_data = &(s->v17tx);
        break;
    case T38_IND_V17_14400_SHORT_TRAINING:
        hdlc_tx_preamble(&s->hdlctx, 360);
        silence_gen_alter(&s->silence_gen, ms_to_samples(75));
        v17_tx_restart(&(s->v17tx), 14400, s->use_tep, s->short_train);
        v17_tx_set_get_bit(&(s->v17tx), get_bit_func, get_bit_user_data);
        s->tx_handler = (span_tx_handler_t *) &(silence_gen);
        s->tx_user_data = &(s->silence_gen);
        s->next_tx_handler = (span_tx_handler_t *) &(v17_tx);
        s->next_tx_user_data = &(s->v17tx);
        break;
    case T38_IND_V17_14400_LONG_TRAINING:
        hdlc_tx_preamble(&s->hdlctx, 360);
        silence_gen_alter(&s->silence_gen, ms_to_samples(75));
        v17_tx_restart(&(s->v17tx), 14400, s->use_tep, s->short_train);
        v17_tx_set_get_bit(&(s->v17tx), get_bit_func, get_bit_user_data);
        s->tx_handler = (span_tx_handler_t *) &(silence_gen);
        s->tx_user_data = &(s->silence_gen);
        s->next_tx_handler = (span_tx_handler_t *) &(v17_tx);
        s->next_tx_user_data = &(s->v17tx);
        break;
#endif
    case T38_IND_V8_ANSAM:
        break;
    case T38_IND_V8_SIGNAL:
        break;
    case T38_IND_V34_CNTL_CHANNEL_1200:
        break;
    case T38_IND_V34_PRI_CHANNEL:
        break;
    case T38_IND_V34_CC_RETRAIN:
        break;
    case T38_IND_V33_12000_TRAINING:
        break;
    case T38_IND_V33_14400_TRAINING:
        break;
    default:
        break;
    }
    s->non_ecm_bit_no = 0;
    s->current_non_ecm_octet = 0xFF;
    s->non_ecm_flow_control_fill_octet = 0xFF;
    s->non_ecm_at_initial_all_ones = TRUE;
    s->bit_stream = 0;
    if (s->non_ecm_flow_control_fill_octets)
    {
        span_log(&s->logging, SPAN_LOG_WARNING, "Flow control generated %d octets\n", s->non_ecm_flow_control_fill_octets);
        s->non_ecm_flow_control_fill_octets = 0;
    }
    s->in_progress_rx_indicator = indicator;
    return TRUE;
}
/*- End of function --------------------------------------------------------*/

static void pump_out_final_hdlc(t38_gateway_state_t *s, int good_fcs)
{
    if (!good_fcs)
        s->hdlc_flags[s->hdlc_in] |= HDLC_FLAG_CORRUPT_CRC;
    if (s->hdlc_in == s->hdlc_out)
    {
        /* This is the frame in progress at the output. */
        if ((s->hdlc_flags[s->hdlc_out] & HDLC_FLAG_PROCEED_WITH_OUTPUT) == 0)
        {
            /* Output of this frame has not yet begun. Throw it all out now. */
            hdlc_tx_frame(&s->hdlctx, s->hdlc_buf[s->hdlc_out], s->hdlc_len[s->hdlc_out]);
        }
        if ((s->hdlc_flags[s->hdlc_out] & HDLC_FLAG_CORRUPT_CRC))
            hdlc_tx_corrupt_frame(&s->hdlctx);
    }
    s->hdlc_flags[s->hdlc_in] |= (HDLC_FLAG_PROCEED_WITH_OUTPUT | HDLC_FLAG_FINISHED);
    if (++s->hdlc_in >= T38_TX_HDLC_BUFS)
        s->hdlc_in = 0;
}
/*- End of function --------------------------------------------------------*/

static void constrain_fast_modem(t38_gateway_state_t *s, uint8_t *buf, int len)
{
    /* We may need to adjust the capabilities, so they do not exceed our own */
    if (len < 5)
        return;
    /* TODO: fiddle the contents */
    switch (buf[4] & (DISBIT6 | DISBIT5 | DISBIT4 | DISBIT3))
    {
    case 0:
    case DISBIT4:
        /* V.27ter only */
        break;
    case DISBIT3:
    case (DISBIT4 | DISBIT3):
        /* V.27ter and V.29 */
        break;
    case (DISBIT6 | DISBIT4 | DISBIT3):
        /* V.27ter, V.29 and V.17 */
#if !defined(ENABLE_V17)
        buf[4] &= ~DISBIT6;
#endif
        break;
    case (DISBIT5 | DISBIT4):
    case (DISBIT6 | DISBIT4):
    case (DISBIT6 | DISBIT5 | DISBIT4):
    case (DISBIT6 | DISBIT5 | DISBIT4 | DISBIT3):
        /* Reserved */
        buf[4] &= ~(DISBIT6 | DISBIT5);
        buf[4] |= (DISBIT4 | DISBIT3);
        break;
    default:
        /* Not used */
        buf[4] &= ~(DISBIT6 | DISBIT5);
        buf[4] |= (DISBIT4 | DISBIT3);
        break;
    }
}
/*- End of function --------------------------------------------------------*/

static void monitor_control_messages(t38_gateway_state_t *s, uint8_t *buf, int len, int from_modem)
{
    span_log(&s->logging, SPAN_LOG_FLOW, "monitor 0x%x\n", buf[2]);
    if (len < 3)
        return;
    /* Monitor the control messages, so we can see what is happening to things like
       training success/failure. */
    switch (buf[2])
    {
    case T30_CFR:
        /* We are changing from TCF exchange to image exchange */
        /* Successful training means we should change to short training */
        s->short_train = TRUE;
        span_log(&s->logging, SPAN_LOG_FLOW, "CFR - short = %d, ECM = %d\n", s->short_train, s->ecm_mode);
        if (!from_modem)
            restart_rx_modem(s);
        break;
    case T30_RTP:
        /* We are going back to the exchange of fresh TCF */
        s->short_train = FALSE;
        break;
    case T30_DTC:
    case T30_DCS:
    case T30_DCS + 1:
        /* We need to check which modem type is about to be used. */
        switch (buf[4] & (DISBIT6 | DISBIT5 | DISBIT4 | DISBIT3))
        {
        case 0:
            s->fast_bit_rate = 2400;
            s->fast_modem = T38_V27TER_RX;
            s->short_train = FALSE;
            break;
        case DISBIT4:
            s->fast_bit_rate = 4800;
            s->fast_modem = T38_V27TER_RX;
            s->short_train = FALSE;
            break;
        case DISBIT3:
            s->fast_bit_rate = 9600;
            s->fast_modem = T38_V29_RX;
            s->short_train = FALSE;
            break;
        case (DISBIT4 | DISBIT3):
            s->fast_bit_rate = 7200;
            s->fast_modem = T38_V29_RX;
            s->short_train = FALSE;
            break;
#if defined(ENABLE_V17)
        case DISBIT6:
            s->fast_bit_rate = 14400;
            s->fast_modem = T38_V17_RX;
            s->short_train = FALSE;
            break;
        case (DISBIT6 | DISBIT4):
            s->fast_bit_rate = 12000;
            s->fast_modem = T38_V17_RX;
            s->short_train = FALSE;
            break;
        case (DISBIT6 | DISBIT3):
            s->fast_bit_rate = 9600;
            s->fast_modem = T38_V17_RX;
            s->short_train = FALSE;
            break;
        case (DISBIT6 | DISBIT4 | DISBIT3):
            s->fast_bit_rate = 7200;
            s->fast_modem = T38_V17_RX;
            s->short_train = FALSE;
            break;
#endif
        case (DISBIT5 | DISBIT3):
        case (DISBIT5 | DISBIT4 | DISBIT3):
        case (DISBIT6 | DISBIT5):
        case (DISBIT6 | DISBIT5 | DISBIT3):
        case (DISBIT6 | DISBIT5 | DISBIT4):
        case (DISBIT6 | DISBIT5 | DISBIT4 | DISBIT3):
            /* Reserved */
            s->fast_bit_rate = 0;
            s->fast_modem = T38_NONE;
            break;
        default:
            /* Not used */
            s->fast_bit_rate = 0;
            s->fast_modem = T38_NONE;
            break;
        }
        s->ecm_mode = (buf[6] & DISBIT3);
        break;
    default:
        break;
    }
}
/*- End of function --------------------------------------------------------*/

static void queue_missing_indicator(t38_gateway_state_t *s, int data_type)
{
    t38_core_state_t *t;
    
    t = &s->t38;
    /* Missing packets might have lost us the indicator that should have put us in
       the required mode of operation. It might be a bit late to fill in such a gap
       now, but we should try. We may also want to force indicators into the queue,
       such as when the data says 'end of signal'. */
    switch (data_type)
    {
    case -1:
        if (s->t38.current_rx_indicator != T38_IND_NO_SIGNAL)
            process_rx_indicator(t, (void *) s, T38_IND_NO_SIGNAL);
        break;
    case T38_DATA_V21:
        if (s->t38.current_rx_indicator != T38_IND_V21_PREAMBLE)
            process_rx_indicator(t, (void *) s, T38_IND_V21_PREAMBLE);
        break;
    case T38_DATA_V27TER_2400:
        if (s->t38.current_rx_indicator != T38_IND_V27TER_2400_TRAINING)
            process_rx_indicator(t, (void *) s, T38_IND_V27TER_2400_TRAINING);
        break;
    case T38_DATA_V27TER_4800:
        if (s->t38.current_rx_indicator != T38_IND_V27TER_4800_TRAINING)
            process_rx_indicator(t, (void *) s, T38_IND_V27TER_4800_TRAINING);
        break;
    case T38_DATA_V29_7200:
        if (s->t38.current_rx_indicator != T38_IND_V29_7200_TRAINING)
            process_rx_indicator(t, (void *) s, T38_IND_V29_7200_TRAINING);
        break;
    case T38_DATA_V29_9600:
        if (s->t38.current_rx_indicator != T38_IND_V29_9600_TRAINING)
            process_rx_indicator(t, (void *) s, T38_IND_V29_9600_TRAINING);
        break;
    case T38_DATA_V17_7200:
        if (s->t38.current_rx_indicator != T38_IND_V17_7200_SHORT_TRAINING  &&  s->t38.current_rx_indicator != T38_IND_V17_7200_LONG_TRAINING)
            process_rx_indicator(t, (void *) s, T38_IND_V17_7200_LONG_TRAINING);
        break;
    case T38_DATA_V17_9600:
        if (s->t38.current_rx_indicator != T38_IND_V17_9600_SHORT_TRAINING  &&  s->t38.current_rx_indicator != T38_IND_V17_9600_LONG_TRAINING)
            process_rx_indicator(t, (void *) s, T38_IND_V17_9600_LONG_TRAINING);
        break;
    case T38_DATA_V17_12000:
        if (s->t38.current_rx_indicator != T38_IND_V17_12000_SHORT_TRAINING  &&  s->t38.current_rx_indicator != T38_IND_V17_12000_LONG_TRAINING)
            process_rx_indicator(t, (void *) s, T38_IND_V17_12000_LONG_TRAINING);
        break;
    case T38_DATA_V17_14400:
        if (s->t38.current_rx_indicator != T38_IND_V17_14400_SHORT_TRAINING  &&  s->t38.current_rx_indicator != T38_IND_V17_14400_LONG_TRAINING)
            process_rx_indicator(t, (void *) s, T38_IND_V17_14400_LONG_TRAINING);
        break;
    case T38_DATA_V8:
        break;
    case T38_DATA_V34_PRI_RATE:
        break;
    case T38_DATA_V34_CC_1200:
        break;
    case T38_DATA_V34_PRI_CH:
        break;
    case T38_DATA_V33_12000:
        break;
    case T38_DATA_V33_14400:
        break;
    }
}
/*- End of function --------------------------------------------------------*/

static int process_rx_missing(t38_core_state_t *t, void *user_data, int rx_seq_no, int expected_seq_no)
{
    t38_gateway_state_t *s;
    
    s = (t38_gateway_state_t *) user_data;
    s->hdlc_flags[s->hdlc_in] |= HDLC_FLAG_MISSING_DATA;
    return 0;
}
/*- End of function --------------------------------------------------------*/

static int process_rx_indicator(t38_core_state_t *t, void *user_data, int indicator)
{
    t38_gateway_state_t *s;
    
    s = (t38_gateway_state_t *) user_data;

    if (s->t38.current_rx_indicator != indicator)
    {
        if (s->hdlc_contents[s->hdlc_in])
        {
            if (++s->hdlc_in >= T38_TX_HDLC_BUFS)
                s->hdlc_in = 0;
        }
        s->hdlc_contents[s->hdlc_in] = (indicator | 0x100);
        if (++s->hdlc_in >= T38_TX_HDLC_BUFS)
            s->hdlc_in = 0;
        span_log(&s->logging,
                 SPAN_LOG_FLOW,
                 "Queued change - (%d) %s -> %s\n",
                 silence_gen_remainder(&(s->silence_gen)),
                 t38_indicator(s->t38.current_rx_indicator),
                 t38_indicator(indicator));
        s->current_rx_field_class = T38_FIELD_CLASS_NONE;
    }
    return 0;
}
/*- End of function --------------------------------------------------------*/

static int process_rx_data(t38_core_state_t *t, void *user_data, int data_type, int field_type, const uint8_t *buf, int len)
{
    int i;
    int previous;
    t38_gateway_state_t *s;
    
    s = (t38_gateway_state_t *) user_data;
    switch (field_type)
    {
    case T38_FIELD_HDLC_DATA:
        s->current_rx_field_class = T38_FIELD_CLASS_HDLC;
        if (s->hdlc_contents[s->hdlc_in] != (data_type | 0x200))
            queue_missing_indicator(s, data_type);
        previous = s->hdlc_len[s->hdlc_in];
        /* Check if this data would overflow the buffer. */
        if (s->hdlc_len[s->hdlc_in] + len > T38_MAX_HDLC_LEN)
            break;
        s->hdlc_contents[s->hdlc_in] = (data_type | 0x200);
        if (data_type == T38_DATA_V21)
        {
            for (i = 0;  i < len;  i++)
            {
                s->hdlc_buf[s->hdlc_in][s->hdlc_len[s->hdlc_in]++] = bit_reverse8(buf[i]);
                /* Edit the message, if we need to control the communication between the end points. */
                switch (s->hdlc_len[s->hdlc_in])
                {
                case 4:
                    /* Check if we need to corrupt this message */
                    if (s->hdlc_buf[s->hdlc_in][2] == T30_NSF
                        ||
                        s->hdlc_buf[s->hdlc_in][2] == T30_NSC
                        ||
                        s->hdlc_buf[s->hdlc_in][2] == T30_NSS)
                    {
                        /* Corrupt the message, so it will be ignored */
                        span_log(&s->logging, SPAN_LOG_FLOW, "Corrupting non-specific procedures message\n");
                        s->hdlc_buf[s->hdlc_in][3] = 0;
                    }
                    break;
                case 6:
                    switch (s->hdlc_buf[s->hdlc_in][2])
                    {
                    case T30_DIS:
                        /* We may need to adjust the capabilities, so they do not exceed our own */
                        span_log(&s->logging, SPAN_LOG_FLOW, "Constraining the fast modem\n");
                        constrain_fast_modem(s, s->hdlc_buf[s->hdlc_in], s->hdlc_len[s->hdlc_in]);
                        break;
                    default:
                        break;
                    }
                    break;
                case 7:
                    if (!s->ecm_allowed)
                    {
                        switch (s->hdlc_buf[s->hdlc_in][2])
                        {
                        case T30_DIS:
                            /* Do not allow ECM or T.6 coding */
                            span_log(&s->logging, SPAN_LOG_FLOW, "Inhibiting ECM\n");
                            s->hdlc_buf[s->hdlc_in][6] &= ~(DISBIT3 | DISBIT7);
                            break;
                        }
                    }
                    break;
                }
            }
            /* We need to send out the control messages as they are arriving. They are
               too slow to capture a whole frame, and then pass it on. */
            /* Don't start pumping data into the actual output stream until there is
               enough backlog to create some elasticity for jitter tolerance. */
            if (s->hdlc_len[s->hdlc_in] >= 8)
            {
                if (s->hdlc_in == s->hdlc_out)
                {
                    if ((s->hdlc_flags[s->hdlc_in] & HDLC_FLAG_PROCEED_WITH_OUTPUT) == 0)
                        previous = 0;
                    hdlc_tx_frame(&s->hdlctx, s->hdlc_buf[s->hdlc_out] + previous, s->hdlc_len[s->hdlc_out] - previous);
                }
                s->hdlc_flags[s->hdlc_in] |= HDLC_FLAG_PROCEED_WITH_OUTPUT;
            }
        }
        else
        {
            /* For the faster frames, take in the whole frame before sending it out. Also, there
               is no need to monitor, or modify, the contents of the faster frames. */
            for (i = 0;  i < len;  i++)
                s->hdlc_buf[s->hdlc_in][s->hdlc_len[s->hdlc_in] + i] = bit_reverse8(buf[i]);
            s->hdlc_len[s->hdlc_in] += len;
        }
        break;
    case T38_FIELD_HDLC_FCS_OK:
        s->current_rx_field_class = T38_FIELD_CLASS_HDLC;
        if (len > 0)
        {
            span_log(&s->logging, SPAN_LOG_WARNING, "There is data in a T38_FIELD_HDLC_FCS_OK!\n");
            /* The sender has incorrectly included data in this message. It is unclear what we should do
               with it, to maximise tolerance of buggy implementations. */
        }
        span_log(&s->logging, SPAN_LOG_FLOW, "Type %s - CRC good\n", t30_frametype(s->hdlc_buf[s->hdlc_in][2]));
        if (s->hdlc_contents[s->hdlc_in] != (data_type | 0x200))
            queue_missing_indicator(s, data_type);
        /* Don't deal with zero length frames. Some T.38 implementations send multiple T38_FIELD_HDLC_FCS_OK
           packets, when they have sent no data for the body of the frame. */
        if (s->hdlc_len[s->hdlc_in] > 0)
        {
            s->hdlc_contents[s->hdlc_in] = (data_type | 0x200);
            if (data_type == T38_DATA_V21  &&  (s->hdlc_flags[s->hdlc_in] & HDLC_FLAG_MISSING_DATA) == 0)
                monitor_control_messages(s, s->hdlc_buf[s->hdlc_in], s->hdlc_len[s->hdlc_in], FALSE);
            pump_out_final_hdlc(s, (s->hdlc_flags[s->hdlc_in] & HDLC_FLAG_MISSING_DATA) == 0);
        }
        s->hdlc_len[s->hdlc_in] = 0;
        s->hdlc_flags[s->hdlc_in] = 0;
        break;
    case T38_FIELD_HDLC_FCS_BAD:
        s->current_rx_field_class = T38_FIELD_CLASS_HDLC;
        if (len > 0)
        {
            span_log(&s->logging, SPAN_LOG_WARNING, "There is data in a T38_FIELD_HDLC_FCS_BAD!\n");
            /* The sender has incorrectly included data in this message. We can safely ignore it, as the
               bad FCS means we will throw away the whole message, anyway. */
        }
        span_log(&s->logging, SPAN_LOG_FLOW, "Type %s - CRC bad\n", t30_frametype(s->hdlc_buf[s->hdlc_in][2]));
        if (s->hdlc_contents[s->hdlc_in] != (data_type | 0x200))
            queue_missing_indicator(s, data_type);
        if (s->hdlc_len[s->hdlc_in] > 0)
        {
            s->hdlc_contents[s->hdlc_in] = (data_type | 0x200);
            pump_out_final_hdlc(s, FALSE);
        }
        s->hdlc_len[s->hdlc_in] = 0;
        s->hdlc_flags[s->hdlc_in] = 0;
        break;
    case T38_FIELD_HDLC_FCS_OK_SIG_END:
        s->current_rx_field_class = T38_FIELD_CLASS_HDLC;
        if (len > 0)
        {
            span_log(&s->logging, SPAN_LOG_WARNING, "There is data in a T38_FIELD_HDLC_FCS_OK_SIG_END!\n");
            /* The sender has incorrectly included data in this message. It is unclear what we should do
               with it, to maximise tolerance of buggy implementations. */
        }
        span_log(&s->logging, SPAN_LOG_FLOW, "Type %s - CRC OK, sig end\n", t30_frametype(s->hdlc_buf[s->hdlc_in][2]));
        if (s->hdlc_contents[s->hdlc_in] != (data_type | 0x200))
            queue_missing_indicator(s, data_type);
        /* Don't deal with zero length frames. Some T.38 implementations send multiple T38_FIELD_HDLC_FCS_OK
           packets, when they have sent no data for the body of the frame. */
        if (s->hdlc_len[s->hdlc_in] > 0)
        {
            s->hdlc_contents[s->hdlc_in] = (data_type | 0x200);
            if (data_type == T38_DATA_V21  &&  (s->hdlc_flags[s->hdlc_in] & HDLC_FLAG_MISSING_DATA) == 0)
                monitor_control_messages(s, s->hdlc_buf[s->hdlc_in], s->hdlc_len[s->hdlc_in], FALSE);
            pump_out_final_hdlc(s, (s->hdlc_flags[s->hdlc_in] & HDLC_FLAG_MISSING_DATA) == 0);
        }
        s->hdlc_len[s->hdlc_in] = 0;
        s->hdlc_flags[s->hdlc_in] = 0;
        s->hdlc_contents[s->hdlc_in] = 0;
        queue_missing_indicator(s, -1);
        break;
    case T38_FIELD_HDLC_FCS_BAD_SIG_END:
        s->current_rx_field_class = T38_FIELD_CLASS_HDLC;
        if (len > 0)
        {
            span_log(&s->logging, SPAN_LOG_WARNING, "There is data in a T38_FIELD_HDLC_FCS_BAD_SIG_END!\n");
            /* The sender has incorrectly included data in this message. We can safely ignore it, as the
               bad FCS means we will throw away the whole message, anyway. */
        }
        span_log(&s->logging, SPAN_LOG_FLOW, "Type %s - CRC bad, sig end\n", t30_frametype(s->hdlc_buf[s->hdlc_in][2]));
        if (s->hdlc_contents[s->hdlc_in] != (data_type | 0x200))
            queue_missing_indicator(s, data_type);
        if (s->hdlc_len[s->hdlc_in] > 0)
        {
            s->hdlc_contents[s->hdlc_in] = (data_type | 0x200);
            pump_out_final_hdlc(s, FALSE);
        }
        s->hdlc_len[s->hdlc_in] = 0;
        s->hdlc_flags[s->hdlc_in] = 0;
        s->hdlc_contents[s->hdlc_in] = 0;
        queue_missing_indicator(s, -1);
        break;
    case T38_FIELD_HDLC_SIG_END:
        if (len > 0)
        {
            span_log(&s->logging, SPAN_LOG_WARNING, "There is data in a T38_FIELD_HDLC_SIG_END!\n");
            /* The sender has incorrectly included data in this message, but there seems nothing meaningful
               it could be. There could not be an FCS good/bad report beyond this. */
        }
        if (s->hdlc_contents[s->hdlc_in] != (data_type | 0x200))
            queue_missing_indicator(s, data_type);
        /* WORKAROUND: At least some Mediatrix boxes have a bug, where they can send this message at the
                       end of non-ECM data. We need to tolerate this. */
        if (s->current_rx_field_class != T38_FIELD_CLASS_NON_ECM)
        {
            /* This message is expected under 2 circumstances. One is as an alternative to T38_FIELD_HDLC_FCS_OK_SIG_END - 
               i.e. they send T38_FIELD_HDLC_FCS_OK, and then T38_FIELD_HDLC_SIG_END when the carrier actually drops.
               The other is because the HDLC signal drops unexpectedly - i.e. not just after a final frame. */
            s->hdlc_len[s->hdlc_in] = 0;
            s->hdlc_flags[s->hdlc_in] = 0;
            s->hdlc_contents[s->hdlc_in] = 0;
        }
        else
        {
            span_log(&s->logging, SPAN_LOG_WARNING, "T38_FIELD_HDLC_SIG_END received at the end of non-ECM data!\n");
            /* Don't flow control the data any more. Just pump out the remainder as fast as we can. */
            s->non_ecm_tx_latest_eol_ptr = s->non_ecm_tx_in_ptr;
            s->non_ecm_data_finished = TRUE;
        }
        queue_missing_indicator(s, -1);
        s->current_rx_field_class = T38_FIELD_CLASS_NONE;
        break;
    case T38_FIELD_T4_NON_ECM_DATA:
        s->current_rx_field_class = T38_FIELD_CLASS_NON_ECM;
        if (s->hdlc_contents[s->hdlc_in] != (data_type | 0x200))
            queue_missing_indicator(s, data_type);
        add_to_non_ecm_tx_buffer(s, buf, len);
        break;
    case T38_FIELD_T4_NON_ECM_SIG_END:
        if (len > 0)
        {
            if (s->hdlc_contents[s->hdlc_in] != (data_type | 0x200))
                queue_missing_indicator(s, data_type);
            add_to_non_ecm_tx_buffer(s, buf, len);
        }
        if (s->hdlc_contents[s->hdlc_in] != (data_type | 0x200))
            queue_missing_indicator(s, data_type);
        /* WORKAROUND: At least some Mediatrix boxes have a bug, where they can send HDLC signal end where
                       they should send non-ECM signal end. It is possible they also do the opposite.
                       We need to tolerate this. */
        if (s->current_rx_field_class != T38_FIELD_CLASS_HDLC)
        {
            /* Don't flow control the data any more. Just pump out the remainder as fast as we can. */
            s->non_ecm_tx_latest_eol_ptr = s->non_ecm_tx_in_ptr;
            s->non_ecm_data_finished = TRUE;
        }
        else
        {
            span_log(&s->logging, SPAN_LOG_WARNING, "T38_FIELD_NON_ECM_SIG_END received at the end of HDLC data!\n");
            s->hdlc_len[s->hdlc_in] = 0;
            s->hdlc_flags[s->hdlc_in] = 0;
            s->hdlc_contents[s->hdlc_in] = 0;
        }
        queue_missing_indicator(s, -1);
        s->current_rx_field_class = T38_FIELD_CLASS_NONE;
        break;
    case T38_FIELD_CM_MESSAGE:
    case T38_FIELD_JM_MESSAGE:
    case T38_FIELD_CI_MESSAGE:
    case T38_FIELD_V34RATE:
    default:
        break;
    }
    s->current_rx_field_type = field_type;
    s->current_rx_data_type = data_type;

#if 0
    if (span_log_test(&s->logging, SPAN_LOG_FLOW))
    {
        int i;

        if (len > 0)
        {
            span_log(&s->logging, SPAN_LOG_FLOW, "Data: ");
            for (i = 0;  i < len;  i++)
                span_log(&s->logging, SPAN_LOG_FLOW | SPAN_LOG_SUPPRESS_LABELLING, " %02X", buf[i]);
        }
    }
    span_log(&s->logging, SPAN_LOG_FLOW | SPAN_LOG_SUPPRESS_LABELLING, "\n");
#endif
    return 0;
}
/*- End of function --------------------------------------------------------*/

static void announce_training(t38_gateway_state_t *s)
{
    int ind;

    ind = T38_IND_NO_SIGNAL;
    switch (s->fast_rx_active)
    {
#if defined(ENABLE_V17)
    case T38_V17_RX:
        switch (s->fast_bit_rate)
        {
        case 7200:
            ind = (s->short_train)  ?  T38_IND_V17_7200_SHORT_TRAINING  :  T38_IND_V17_7200_LONG_TRAINING;
            s->current_tx_data_type = T38_DATA_V17_7200;
            s->octets_per_data_packet = MS_PER_TX_CHUNK*7200/(8*1000);
            break;
        case 9600:
            ind = (s->short_train)  ?  T38_IND_V17_9600_SHORT_TRAINING  :  T38_IND_V17_9600_LONG_TRAINING;
            s->current_tx_data_type = T38_DATA_V17_9600;
            s->octets_per_data_packet = MS_PER_TX_CHUNK*9600/(8*1000);
            break;
        case 12000:
            ind = (s->short_train)  ?  T38_IND_V17_12000_SHORT_TRAINING  :  T38_IND_V17_12000_LONG_TRAINING;
            s->current_tx_data_type = T38_DATA_V17_12000;
            s->octets_per_data_packet = MS_PER_TX_CHUNK*12000/(8*1000);
            break;
        default:
        case 14400:
            ind = (s->short_train)  ?  T38_IND_V17_14400_SHORT_TRAINING  :  T38_IND_V17_14400_LONG_TRAINING;
            s->current_tx_data_type = T38_DATA_V17_14400;
            s->octets_per_data_packet = MS_PER_TX_CHUNK*14400/(8*1000);
            break;
        }
        break;
#endif
    case T38_V27TER_RX:
        switch (s->fast_bit_rate)
        {
        case 2400:
            ind = T38_IND_V27TER_2400_TRAINING;
            s->current_tx_data_type = T38_DATA_V27TER_2400;
            s->octets_per_data_packet = MS_PER_TX_CHUNK*2400/(8*1000);
            break;
        default:
        case 4800:
            ind = T38_IND_V27TER_4800_TRAINING;
            s->current_tx_data_type = T38_DATA_V27TER_4800;
            s->octets_per_data_packet = MS_PER_TX_CHUNK*4800/(8*1000);
            break;
        }
        break;
    case T38_V29_RX:
        switch (s->fast_bit_rate)
        {
        case 7200:
            ind = T38_IND_V29_7200_TRAINING;
            s->current_tx_data_type = T38_DATA_V29_7200;
            s->octets_per_data_packet = MS_PER_TX_CHUNK*7200/(8*1000);
            break;
        default:
        case 9600:
            ind = T38_IND_V29_9600_TRAINING;
            s->current_tx_data_type = T38_DATA_V29_9600;
            s->octets_per_data_packet = MS_PER_TX_CHUNK*9600/(8*1000);
            break;
        }
        break;
    }
    t38_core_send_indicator(&s->t38, ind, INDICATOR_TX_COUNT);
}
/*- End of function --------------------------------------------------------*/

static void non_ecm_put_bit(void *user_data, int bit)
{
    t38_gateway_state_t *s;

    s = (t38_gateway_state_t *) user_data;
    if (bit < 0)
    {
        /* Special conditions */
        switch (bit)
        {
        case PUTBIT_TRAINING_FAILED:
            span_log(&s->logging, SPAN_LOG_FLOW, "Non-ECM carrier training failed\n");
            break;
        case PUTBIT_TRAINING_SUCCEEDED:
            /* The modem is now trained */
            span_log(&s->logging, SPAN_LOG_FLOW, "Non-ECM carrier trained\n");
            s->rx_signal_present = TRUE;
            s->v21_rx_active = FALSE;
            announce_training(s);
            s->samples_since_last_tx_packet = 0;
            s->rx_data_ptr = 0;
            break;
        case PUTBIT_CARRIER_UP:
            span_log(&s->logging, SPAN_LOG_FLOW, "Non-ECM carrier up\n");
            break;
        case PUTBIT_CARRIER_DOWN:
            span_log(&s->logging, SPAN_LOG_FLOW, "Non-ECM carrier down\n");
            switch (s->current_tx_data_type)
            {
            case T38_DATA_V17_7200:
            case T38_DATA_V17_9600:
            case T38_DATA_V17_12000:
            case T38_DATA_V17_14400:
            case T38_DATA_V27TER_2400:
            case T38_DATA_V27TER_4800:
            case T38_DATA_V29_7200:
            case T38_DATA_V29_9600:
                t38_core_send_data(&s->t38, s->current_tx_data_type, T38_FIELD_T4_NON_ECM_SIG_END, NULL, 0);
                t38_core_send_indicator(&s->t38, T38_IND_NO_SIGNAL, INDICATOR_TX_COUNT);
                s->rx_signal_present = FALSE;
                restart_rx_modem(s);
                break;
            }
            break;
        default:
            span_log(&s->logging, SPAN_LOG_WARNING, "Unexpected non-ECM special bit - %d!\n", bit);
            break;
        }
        return;
    }
    s->current_non_ecm_octet = (s->current_non_ecm_octet << 1) | (bit & 1);
    if (++s->non_ecm_bit_no >= 8)
    {
        s->rx_data[s->rx_data_ptr++] = (uint8_t) s->current_non_ecm_octet;
        if (++s->octets_since_last_tx_packet >= s->octets_per_data_packet)
        {
            t38_core_send_data(&s->t38, s->current_tx_data_type, T38_FIELD_T4_NON_ECM_DATA, s->rx_data, s->rx_data_ptr);
            /* Since we delay transmission by 2 octets, we should now have sent the last of the data octets when
                we have just received the last of the CRC octets. */
            s->rx_data_ptr = 0;
            s->samples_since_last_tx_packet = 0;
            s->octets_since_last_tx_packet = 0;
        }
        s->non_ecm_bit_no = 0;
        s->current_non_ecm_octet = 0;
    }
}
/*- End of function --------------------------------------------------------*/

static int non_ecm_get_bit(void *user_data)
{
    t38_gateway_state_t *s;
    int bit;

    /* A rate adapting data stuffer for non-ECM image data */
    s = (t38_gateway_state_t *) user_data;
    if (s->non_ecm_bit_no <= 0)
    {
        /* We need another byte */
        if (s->non_ecm_tx_out_ptr != s->non_ecm_tx_latest_eol_ptr)
        {
            s->current_non_ecm_octet = s->non_ecm_tx_data[s->non_ecm_tx_out_ptr];
            s->non_ecm_tx_out_ptr = (s->non_ecm_tx_out_ptr + 1) & (T38_TX_BUF_LEN - 1);
        }
        else
        {
            if (s->non_ecm_data_finished)
            {
                /* The queue is empty, and we have received the end of data signal. This must
                   really be the end to transmission. */
                s->non_ecm_data_finished = FALSE;
                /* Reset the data pointers for next time. */
                s->non_ecm_tx_out_ptr = 0;
                s->non_ecm_tx_in_ptr = 0;
                s->non_ecm_tx_latest_eol_ptr = 0;
                return PUTBIT_END_OF_DATA;
            }
            /* The queue is empty, but this does not appear to be the end of the data. Idle with
               fill octets, which should be safe at this point. */
            s->current_non_ecm_octet = s->non_ecm_flow_control_fill_octet;
            s->non_ecm_flow_control_fill_octets++;
        }
        s->non_ecm_bit_no = 8;
    }
    s->non_ecm_bit_no--;
    bit = (s->current_non_ecm_octet >> 7) & 1;
    s->current_non_ecm_octet <<= 1;
    return bit;
}
/*- End of function --------------------------------------------------------*/

static void add_to_non_ecm_tx_buffer(t38_gateway_state_t *s, const uint8_t *buf, int len)
{
    int i;
    int upper;
    int lower;

    /* A rate adapting data stuffer for non-ECM image data */
    i = 0;
    if (s->non_ecm_at_initial_all_ones)
    {
        /* Dump initial 0xFF bytes. We will add enough of our own to makes things flow
           smoothly. If we don't strip these off we might end up delaying the start of
           forwarding by a large amount, as we could end up with a large block of 0xFF
           bytes before the real data begins. This is especially true with PC FAX
           systems. */
        for (  ;  i < len;  i++)
        {
            if (buf[i] != 0xFF)
            {
                s->non_ecm_at_initial_all_ones = FALSE;
                break;
            }
        }
    }
    if (s->short_train)
    {
        for (  ;  i < len;  i++)
        {
            /* Check for EOLs, because at an EOL we can pause and pump out zeros while
               waiting for more incoming data. */
            if (buf[i])
            {
                /* There might be an EOL here. Look for at least 11 zeros, followed by a one. */
                upper = bottom_bit((s->bit_stream | 0x800) & 0xFFF);
                lower = top_bit(buf[i] & 0xFF);
                if (lower > 0  &&  upper - lower >= 3)
                {
                    s->non_ecm_tx_latest_eol_ptr = s->non_ecm_tx_in_ptr;
                    s->non_ecm_flow_control_fill_octet = 0x00;
                }
            }
            s->bit_stream = (s->bit_stream << 8) | buf[i];
            s->non_ecm_tx_data[s->non_ecm_tx_in_ptr] = buf[i];
            /* TODO: We can't buffer overflow, since we wrap around. However, the tail could overwrite
                     itself if things fall badly behind. */
            s->non_ecm_tx_in_ptr = (s->non_ecm_tx_in_ptr + 1) & (T38_TX_BUF_LEN - 1);
        }
    }
    else
    {
        for (  ;  i < len;  i++)
        {
            /* Check for zero bytes, as we can pause and pump out zeros while waiting
               for more incoming data. */
            if (buf[i] == 0)
            {
                s->non_ecm_tx_latest_eol_ptr = s->non_ecm_tx_in_ptr;
                s->non_ecm_flow_control_fill_octet = 0x00;
            }
            s->non_ecm_tx_data[s->non_ecm_tx_in_ptr] = buf[i];
            /* TODO: We can't buffer overflow, since we wrap around. However, the tail could overwrite
                     itself if things fall badly behind. */
            s->non_ecm_tx_in_ptr = (s->non_ecm_tx_in_ptr + 1) & (T38_TX_BUF_LEN - 1);
        }
    }
}
/*- End of function --------------------------------------------------------*/

static void t38_hdlc_rx_put_bit(hdlc_rx_state_t *t, int new_bit)
{
    t38_gateway_state_t *s;
    int final;

    s = (t38_gateway_state_t *) t->user_data;
    if (new_bit < 0)
    {
        /* Special conditions */
        switch (new_bit)
        {
        case PUTBIT_TRAINING_FAILED:
            span_log(&s->logging, SPAN_LOG_FLOW, "HDLC carrier training failed\n");
            break;
        case PUTBIT_TRAINING_SUCCEEDED:
            /* The modem is now trained */
            span_log(&s->logging, SPAN_LOG_FLOW, "HDLC carrier trained\n");
            s->rx_signal_present = TRUE;
            s->v21_rx_active = FALSE;
            announce_training(s);
            /* Behave like HDLC preamble has been announced */
            t->framing_ok_announced = TRUE;
            s->samples_since_last_tx_packet = 0;
            s->rx_data_ptr = 0;
            break;
        case PUTBIT_CARRIER_UP:
            span_log(&s->logging, SPAN_LOG_FLOW, "HDLC carrier up\n");
            /* Reset the HDLC receiver. */
            t->len = 0;
            t->num_bits = 0;
            t->flags_seen = 0;
            t->framing_ok_announced = FALSE;
            s->rx_signal_present = TRUE;
            break;
        case PUTBIT_CARRIER_DOWN:
            span_log(&s->logging, SPAN_LOG_FLOW, "HDLC carrier down\n");
            if (t->framing_ok_announced)
            {
                t38_core_send_data(&s->t38, s->current_tx_data_type, T38_FIELD_HDLC_SIG_END, NULL, 0);
                t38_core_send_indicator(&s->t38, T38_IND_NO_SIGNAL, INDICATOR_TX_COUNT);
                t->framing_ok_announced = FALSE;
            }
            s->rx_signal_present = FALSE;
            restart_rx_modem(s);
            break;
        default:
            span_log(&s->logging, SPAN_LOG_WARNING, "Unexpected HDLC special bit - %d!\n", new_bit);
            break;
        }
        return;
    }
    t->raw_bit_stream |= (new_bit & 1);
    if ((t->raw_bit_stream & 0x3F) == 0x3E)
    {
        if ((t->raw_bit_stream & 0x40) == 0)
        {
            /* Stuffing */
        }
        else if ((t->raw_bit_stream & 0x80))
        {
            /* Hit HDLC abort */
            t->rx_aborts++;
            t->len = 0;
            t->num_bits = 0;
            s->corrupt_the_frame = FALSE;
            s->octets_since_last_tx_packet = 0;
        }
        else
        {
            /* Hit HDLC flag */
            if (t->flags_seen >= t->framing_ok_threshold)
            {
                if (t->len)
                {
                    if (t->len >= 2)
                    {
                        if ((s->crc & 0xFFFF) == 0xF0B8)
                        {
                            t->rx_frames++;
                            t->rx_bytes += t->len - 2;
                            final = (t->len - 2 >= 2  &&  t->buffer[0] == 0xFF  &&  t->buffer[1] == 0x13);
                            span_log(&s->logging, SPAN_LOG_FLOW, "E Type %s\n", t30_frametype(t->buffer[2]));
                            if (s->current_tx_data_type == T38_DATA_V21)
                                monitor_control_messages(s, t->buffer, t->len, TRUE);
                            if (s->rx_data_ptr)
                            {
                                t38_core_send_data(&s->t38, s->current_tx_data_type, T38_FIELD_HDLC_DATA, s->rx_data, s->rx_data_ptr);
                                s->rx_data_ptr = 0;
                            }
                            /* It seems some boxes may not like us sending a _SIG_END here, and then another
                               when the carrier actually drops. Lets just send T38_FIELD_HDLC_FCS_OK here. */
                            t38_core_send_data(&s->t38, s->current_tx_data_type, T38_FIELD_HDLC_FCS_OK, NULL, 0);
                        }
                        else
                        {
                            t->rx_crc_errors++;
                            final = (t->len - 2 >= 2  &&  t->buffer[0] == 0xFF  &&  t->buffer[1] == 0x13);
                            span_log(&s->logging, SPAN_LOG_FLOW, "F Type %s\n", t30_frametype(t->buffer[2]));
                            if (s->rx_data_ptr)
                            {
                                t38_core_send_data(&s->t38, s->current_tx_data_type, T38_FIELD_HDLC_DATA, s->rx_data, s->rx_data_ptr);
                                s->rx_data_ptr = 0;
                            }
                            /* It seems some boxes may not like us sending a _SIG_END here, and then another
                               when the carrier actually drops. Lets just send T38_FIELD_HDLC_FCS_OK here. */
                            t38_core_send_data(&s->t38, s->current_tx_data_type, T38_FIELD_HDLC_FCS_BAD, NULL, 0);
                        }
                    }
                    else
                    {
                        /* Frame too short */
                        t->rx_length_errors++;
                    }
                }
            }
            else
            {
                if (++t->flags_seen >= t->framing_ok_threshold  &&  !t->framing_ok_announced)
                {
                    if (s->current_tx_data_type == T38_DATA_V21)
                        t38_core_send_indicator(&s->t38, T38_IND_V21_PREAMBLE, INDICATOR_TX_COUNT);
                    if (s->in_progress_rx_indicator == T38_IND_CNG)
                        set_next_tx_type(s);
                    t->framing_ok_announced = TRUE;
                }
            }
            s->crc = 0xFFFF;
            t->len = 0;
            t->num_bits = 0;
            s->corrupt_the_frame = FALSE;
            s->octets_since_last_tx_packet = 0;
        }
    }
    else
    {
        if (t->framing_ok_announced)
        {
            t->byte_in_progress = (t->byte_in_progress >> 1) | ((t->raw_bit_stream & 0x01) << 7);
            if (++t->num_bits == 8)
            {
                if (t->len >= (int) sizeof(t->buffer))
                {
                    /* Frame too long */
                    t->rx_length_errors++;
                    t->flags_seen = t->framing_ok_threshold - 1;
                    t->len = 0;
                }
                else
                {
                    t->buffer[t->len] = (uint8_t) t->byte_in_progress;
                    /* Calculate the CRC progressively, before we start altering the frame */
                    s->crc = crc_itu16_calc(&t->buffer[t->len], 1, s->crc);
                    if (t->len >= 2)
                    {
                        /* Make the transmission lag by two bytes, so we do not send the CRC, and
                           do not report the CRC result too late. */
                        if (s->current_tx_data_type == T38_DATA_V21)
                        {
                            /* Edit the message, if we need to control the communication between the
                               end points. */
                            switch (t->len)
                            {
                            case 4:
                                switch (t->buffer[2])
                                {
                                case T30_NSF:
                                case T30_NSC:
                                case T30_NSS:
                                    /* We need to corrupt the rest of this message */
                                    span_log(&s->logging, SPAN_LOG_FLOW, "Corrupting non-specific procedures message\n");
                                    s->corrupt_the_frame = TRUE;
                                    break;
                                default:
                                    break;
                                }
                                break;
                            case 6:
                                switch (t->buffer[2])
                                {
                                case T30_DIS:
                                    /* We may need to adjust the capabilities, so they do not exceed our own */
                                    span_log(&s->logging, SPAN_LOG_FLOW, "Constraining the fast modem\n");
                                    constrain_fast_modem(s, t->buffer, t->len);
                                    break;
                                default:
                                    break;
                                }
                                break;
                            case 7:
                                if (!s->ecm_allowed)
                                {
                                    switch (t->buffer[2])
                                    {
                                    case T30_DIS:
                                        /* Do not allow ECM or T.6 coding */
                                        span_log(&s->logging, SPAN_LOG_FLOW, "Inhibiting ECM\n");
                                        t->buffer[6] &= ~(DISBIT3 | DISBIT7);
                                        break;
                                }
                                }
                                break;
                            }
                        }
                        s->rx_data[s->rx_data_ptr++] = (s->corrupt_the_frame)  ?  0  :  bit_reverse8(t->buffer[t->len - 2]);
                        if (s->rx_data_ptr >= s->octets_per_data_packet)
                        {
                            t38_core_send_data(&s->t38, s->current_tx_data_type, T38_FIELD_HDLC_DATA, s->rx_data, s->rx_data_ptr);
                            /* Since we delay transmission by 2 octets, we should now have sent the last of the data octets when
                               we have just received the last of the CRC octets. */
                            s->rx_data_ptr = 0;
                            s->samples_since_last_tx_packet = 0;
                        }
                    }
                    t->len++;
                }
                t->num_bits = 0;
            }
        }
    }
    t->raw_bit_stream <<= 1;
}
/*- End of function --------------------------------------------------------*/

static int restart_rx_modem(t38_gateway_state_t *s)
{
    put_bit_func_t put_bit_func;
    void *put_bit_user_data;

    span_log(&s->logging, SPAN_LOG_FLOW, "Restart modem - short = %d, ECM = %d\n", s->short_train,  s->ecm_mode);

    hdlc_rx_init(&(s->hdlcrx), FALSE, TRUE, 5, NULL, s);
    s->crc = 0xFFFF;
    s->rx_signal_present = FALSE;
    /* Default to the transmit data being V.21, unless a faster modem pops up trained. */
    s->current_tx_data_type = T38_DATA_V21;
    fsk_rx_init(&(s->v21rx), &preset_fsk_specs[FSK_V21CH2], TRUE, (put_bit_func_t) t38_hdlc_rx_put_bit, &(s->hdlcrx));
    s->v21_rx_active = TRUE;
    if (s->short_train  &&  s->ecm_mode)
    {
        put_bit_func = (put_bit_func_t) t38_hdlc_rx_put_bit;
        put_bit_user_data = (void *) &(s->hdlcrx);
    }
    else
    {
        put_bit_func = non_ecm_put_bit;
        put_bit_user_data = (void *) s;
    }
    s->rx_data_ptr = 0;
    s->octets_per_data_packet = 1;
    switch (s->fast_modem)
    {
#if defined(ENABLE_V17)
    case T38_V17_RX:
        v17_rx_restart(&(s->v17rx), s->fast_bit_rate, s->short_train);
        v17_rx_set_put_bit(&(s->v17rx), put_bit_func, put_bit_user_data);
        s->fast_rx_active = T38_V17_RX;
        break;
#endif
    case T38_V27TER_RX:
        v27ter_rx_restart(&(s->v27ter_rx), s->fast_bit_rate, FALSE);
        v27ter_rx_set_put_bit(&(s->v27ter_rx), put_bit_func, put_bit_user_data);
        s->fast_rx_active = T38_V27TER_RX;
        break;
    case T38_V29_RX:
        v29_rx_restart(&(s->v29rx), s->fast_bit_rate, FALSE);
        v29_rx_set_put_bit(&(s->v29rx), put_bit_func, put_bit_user_data);
        s->fast_rx_active = T38_V29_RX;
        break;
    }
    return 0;
}
/*- End of function --------------------------------------------------------*/

int t38_gateway_rx(t38_gateway_state_t *s, const int16_t *amp, int len)
{
    s->samples_since_last_tx_packet += len;
    if (s->v21_rx_active)
        fsk_rx(&(s->v21rx), amp, len);
    switch (s->fast_rx_active)
    {
#if defined(ENABLE_V17)
    case T38_V17_RX:
        v17_rx(&(s->v17rx), amp, len);
        break;
#endif
    case T38_V27TER_RX:
        v27ter_rx(&(s->v27ter_rx), amp, len);
        break;
    case T38_V29_RX:
        v29_rx(&(s->v29rx), amp, len);
        break;
    }
    return  0;
}
/*- End of function --------------------------------------------------------*/

int t38_gateway_tx(t38_gateway_state_t *s, int16_t *amp, int max_len)
{
    int len;

    if ((len = s->tx_handler(s->tx_user_data, amp, max_len)) < max_len)
    {
        if (set_next_tx_type(s))
        {
            /* Give the new handler a chance to file the remaining buffer space */
            len += s->tx_handler(s->tx_user_data, amp + len, max_len - len);
            if (len < max_len)
            {
                silence_gen_set(&(s->silence_gen), 0);
                set_next_tx_type(s);
            }
        }
    }
    return len;
}
/*- End of function --------------------------------------------------------*/

void t38_gateway_ecm_control(t38_gateway_state_t *s, int ecm_allowed)
{
    s->ecm_allowed = ecm_allowed;
}
/*- End of function --------------------------------------------------------*/

t38_gateway_state_t *t38_gateway_init(t38_gateway_state_t *s,
                                      t38_tx_packet_handler_t *tx_packet_handler,
                                      void *tx_packet_user_data)
{
    if (tx_packet_handler == NULL)
        return NULL;

    memset(s, 0, sizeof(*s));
    span_log_init(&s->logging, SPAN_LOG_NONE, NULL);
    span_log_set_protocol(&s->logging, "T.38G");
#if defined(ENABLE_V17)
    v17_rx_init(&(s->v17rx), 14400, non_ecm_put_bit, s);
    v17_tx_init(&(s->v17tx), 14400, FALSE, non_ecm_get_bit, s);
#endif
    v29_rx_init(&(s->v29rx), 9600, non_ecm_put_bit, s);
    v29_tx_init(&(s->v29tx), 9600, FALSE, non_ecm_get_bit, s);
    v27ter_rx_init(&(s->v27ter_rx), 4800, non_ecm_put_bit, s);
    v27ter_tx_init(&(s->v27ter_tx), 4800, FALSE, non_ecm_get_bit, s);
    s->octets_per_data_packet = 1;
    silence_gen_init(&(s->silence_gen), 0);
    hdlc_tx_init(&s->hdlctx, FALSE, 2, TRUE, hdlc_underflow_handler, s);
    s->rx_signal_present = FALSE;
    s->tx_handler = (span_tx_handler_t *) &(silence_gen);
    s->tx_user_data = &(s->silence_gen);
    t38_core_init(&s->t38, process_rx_indicator, process_rx_data, process_rx_missing, (void *) s);
    s->t38.iaf = FALSE;
    s->t38.tx_packet_handler = tx_packet_handler;
    s->t38.tx_packet_user_data = tx_packet_user_data;
#if defined(ENABLE_V17)
    s->t38.fastest_image_data_rate = 14400;
#else
    s->t38.fastest_image_data_rate = 9600;
#endif
    s->ecm_allowed = FALSE;
    restart_rx_modem(s);
    return s;
}
/*- End of function --------------------------------------------------------*/
/*- End of file ------------------------------------------------------------*/