backoff.hpp

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#pragma once
// Exponential backoff with jitter for reconnects: delay_n = cap_n * (1 - jitter) +
// U(0, cap_n * jitter) where cap_n = min(max_ms, base_ms * 2^n). jitter = 1.0 is the
// "full jitter" scheme (recommended: spreads a thundering herd of reconnects after a venue
// outage), jitter = 0 is deterministic (useful in tests).
#include <algorithm>
#include <cmath>
#include <cstdint>
#include <random>
 
namespace fastmm::net {
 
struct BackoffConfig {
  std::uint32_t base_ms = 250;
  std::uint32_t max_ms = 30'000;
  double jitter = 1.0;  // fraction of the capped delay that is randomised, in [0, 1]
};
 
class ExponentialBackoff {
 public:
  explicit ExponentialBackoff(BackoffConfig cfg = {}, std::uint64_t seed = 0x5eed) noexcept
      : cfg_(cfg), rng_(seed) {
    cfg_.jitter = std::clamp(cfg_.jitter, 0.0, 1.0);
    if (cfg_.max_ms < cfg_.base_ms) cfg_.max_ms = cfg_.base_ms;
  }
 
  // Delay for the current attempt in milliseconds; advances the attempt counter.
  std::uint32_t next_ms() noexcept {
    const std::uint32_t cap = capped_delay(attempt_);
    if (attempt_ < 31) ++attempt_;
    const double fixed = static_cast<double>(cap) * (1.0 - cfg_.jitter);
    const double random_span = static_cast<double>(cap) * cfg_.jitter;
    std::uniform_real_distribution<double> dist(0.0, 1.0);
    const double delay = fixed + random_span * dist(rng_);
    return static_cast<std::uint32_t>(std::llround(delay));
  }
 
  void reset() noexcept { attempt_ = 0; }
  std::uint32_t attempt() const noexcept { return attempt_; }
  const BackoffConfig& config() const noexcept { return cfg_; }
 
  // Upper bound of the delay for attempt n (before jitter) - exposed for tests/metrics.
  std::uint32_t capped_delay(std::uint32_t attempt) const noexcept {
    const std::uint64_t raw = static_cast<std::uint64_t>(cfg_.base_ms)
                              << std::min<std::uint32_t>(attempt, 31);
    return static_cast<std::uint32_t>(std::min<std::uint64_t>(raw, cfg_.max_ms));
  }
 
 private:
  BackoffConfig cfg_;
  std::mt19937_64 rng_;
  std::uint32_t attempt_ = 0;
};
 
}  // namespace fastmm::net