dnc_stream.cpp

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// Copyright (C) 2026 Douglas Quigg (dstroy0) <dquigg123@gmail.com>
// SPDX-License-Identifier: AGPL-3.0-or-later
 
/**
 * @file dnc_stream.cpp
 * @brief DNC drip-feed engine (see dnc_stream.h). Frames a program with the dnc codec and paces it
 *        against reverse-channel XON/XOFF over a send/recv seam.
 */
 
#include "dnc_stream.h"
 
#if DETWS_ENABLE_DNC
 
#include <string.h>
 
// Drain any reverse-channel bytes into the flow state (non-blocking); false on a recv error.
static bool flow_drain(DncFlow *flow, DncRecvFn recv, void *ctx)
{
    uint8_t tmp[16];
    int r = recv(ctx, tmp, sizeof(tmp));
    if (r < 0)
        return false;
    for (int i = 0; i < r; i++)
        dnc_flow_feed(flow, tmp[i]);
    return true;
}
 
// Send @p n bytes, first honoring XOFF: update flow, and while paused poll recv for the XON.
static bool emit(DncFlow *flow, DncSendFn send, DncRecvFn recv, void *ctx, const uint8_t *data, size_t n)
{
    if (!flow_drain(flow, recv, ctx))
        return false;
    uint32_t polls = 0;
    while (!dnc_flow_can_send(flow))
    {
        if (++polls > DETWS_DNC_XOFF_MAX_POLLS)
            return false; // XOFF never cleared
        if (!flow_drain(flow, recv, ctx))
            return false;
    }
    return send(ctx, data, n) == (int)n;
}
 
// Emit @p count NUL runout (leader/trailer) bytes in chunks, honoring flow control.
static bool emit_runout(DncFlow *flow, DncSendFn send, DncRecvFn recv, void *ctx, uint16_t count)
{
    uint8_t zeros[32];
    memset(zeros, 0, sizeof(zeros));
    while (count)
    {
        uint16_t chunk = count < sizeof(zeros) ? count : (uint16_t)sizeof(zeros);
        if (!emit(flow, send, recv, ctx, zeros, chunk))
            return false;
        count -= chunk;
    }
    return true;
}
 
DncStreamResult dnc_stream(const DncCfg *cfg, const char *program, size_t prog_len, DncSendFn send, DncRecvFn recv,
                           void *ctx)
{
    if (!cfg || !send || !recv || (prog_len && !program))
        return DncStreamResult::DNC_STREAM_ERR_ARG;
 
    DncFlow flow;
    dnc_flow_init(&flow);
    uint8_t buf[DETWS_DNC_LINE_MAX + 8];
 
    // leader runout
    if (cfg->leader_len && !emit_runout(&flow, send, recv, ctx, cfg->leader_len))
        return DncStreamResult::DNC_STREAM_ERR_IO;
 
    // program-start marker
    size_t n = dnc_encode_marker(cfg, buf, sizeof(buf));
    if (n == 0)
        return DncStreamResult::DNC_STREAM_ERR_ENCODE;
    if (!emit(&flow, send, recv, ctx, buf, n))
        return DncStreamResult::DNC_STREAM_ERR_IO;
 
    // one block per source line
    size_t i = 0;
    while (i < prog_len)
    {
        size_t j = i;
        while (j < prog_len && program[j] != '\n')
            j++;
        size_t line_len = j - i;
        if (line_len && program[i + line_len - 1] == '\r')
            line_len--; // strip a trailing CR (CRLF sources)
        n = dnc_encode_block(cfg, program + i, line_len, buf, sizeof(buf));
        if (n == 0)
            return DncStreamResult::DNC_STREAM_ERR_ENCODE; // untranslatable char or over-long block - fail closed
        if (!emit(&flow, send, recv, ctx, buf, n))
            return DncStreamResult::DNC_STREAM_ERR_IO;
        i = j + 1; // skip the LF
    }
 
    // program-end marker (byte-identical to the start marker)
    n = dnc_encode_marker(cfg, buf, sizeof(buf));
    if (!emit(&flow, send, recv, ctx, buf, n))
        return DncStreamResult::DNC_STREAM_ERR_IO;
 
    // trailer runout
    if (cfg->leader_len && !emit_runout(&flow, send, recv, ctx, cfg->leader_len))
        return DncStreamResult::DNC_STREAM_ERR_IO;
 
    return DncStreamResult::DNC_STREAM_OK;
}
 
#endif // DETWS_ENABLE_DNC