duty_cycle.hpp

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// SPDX-License-Identifier: Apache-2.0
// Copyright 2026 Chris Leishman
 
#pragma once
 
#include <idfxx/radio/airtime.hpp>
#include <idfxx/radio/types.hpp>
 
#include <chrono>
#include <cstdint>
#include <optional>
#include <utility>
 
namespace idfxx::radio {
 
inline constexpr std::chrono::microseconds default_min_rx_sleep{1016};
 
[[nodiscard]] constexpr std::optional<rx_duty_cycle> rx_duty_cycle_for(
    const lora_modulation& mod,
    uint16_t sender_preamble,
    uint16_t min_symbols = 8,
    std::chrono::microseconds min_sleep = default_min_rx_sleep
) noexcept {
    if (min_symbols == 0 || 2 * static_cast<uint32_t>(min_symbols) >= sender_preamble) {
        return std::nullopt;
    }
 
    // Symbol duration Tsym = 2^SF / BW. Durations are computed as
    // n·2^SF·1e6 / BW in integer arithmetic, rounding the sleep window down
    // (wake early) and the listen window up (listen longer).
    const int64_t bw = airtime_detail::bandwidth_hz(mod.bw);
    const int64_t sym_x_1e6 = (int64_t{1} << std::to_underlying(mod.sf)) * 1'000'000;
    const auto symbols_floor = [&](int64_t n) { return n * sym_x_1e6 / bw; };
    const auto symbols_ceil = [&](int64_t n) { return (n * sym_x_1e6 + bw - 1) / bw; };
 
    const int64_t sleep_us = symbols_floor(sender_preamble - 2 * min_symbols);
    if (sleep_us < min_sleep.count()) {
        return std::nullopt;
    }
 
    constexpr int64_t margin_us = 1'000;
    const int64_t worst_case_us = (symbols_ceil(sender_preamble + 1) - (sleep_us - margin_us) + 1) / 2;
    const int64_t detect_us = symbols_ceil(min_symbols + 1);
    const int64_t rx_us = worst_case_us > detect_us ? worst_case_us : detect_us;
 
    return rx_duty_cycle{std::chrono::microseconds{rx_us}, std::chrono::microseconds{sleep_us}};
}
 
} // namespace idfxx::radio