#pragma once
#include <algorithm>
#include <atomic>
#include <bit>
#include <cstddef>
#include <vector>

#include <plot/core/types.hpp>

namespace plot::data {

/// Single-producer / single-consumer ring of at most `capacity` elements.
///
/// The capacity is rounded up to a power of two so slot addressing is a mask
/// instead of a modulo. The producer writes a slot and then publishes the new
/// sample count with a release store; a consumer that observes that count with an
/// acquire load is guaranteed to see the slot contents written before it.
///
/// A push into a full ring silently discards the oldest element: for a live
/// plotting stream the newest sample is the interesting one and the retained
/// window slides forward.
///
/// Two indexing conventions, deliberately named apart:
///  - retained-window (operator[], index 0 = oldest element still held) is a
///    single-threaded view: it re-derives the window from the live count, so a
///    producer pushing concurrently would move it under the reader's feet.
///  - absolute (base(), copy_from()) fixes one count for a whole read, which is
///    what a consumer holding a snapshot must use. StreamSeries::acquire() takes
///    such a snapshot and then reads only absolute indices.
template <typename T>
class RingBuffer {
 public:
  explicit RingBuffer(u64 capacity)
      : cap_(std::bit_ceil(capacity < 1 ? u64{1} : capacity)),
        mask_(cap_ - 1),
        buf_(static_cast<usize>(cap_)) {}

  /// Producer thread only.
  void push(T v) {
    const u64 i = produced_.load(std::memory_order_relaxed);
    buf_[static_cast<usize>(i & mask_)] = v;
    produced_.store(i + 1, std::memory_order_release);
  }

  /// Producer thread only. Equivalent to n calls to push().
  void push(const T* vs, u64 n) {
    for (u64 i = 0; i < n; ++i) push(vs[i]);
  }

  /// Consumer: retained element count, saturating at capacity().
  u64 size() const noexcept {
    const u64 p = produced_.load(std::memory_order_acquire);
    return p < cap_ ? p : cap_;
  }

  /// Consumer: total number of elements ever pushed (monotonic).
  u64 produced() const noexcept { return produced_.load(std::memory_order_acquire); }

  u64 capacity() const noexcept { return cap_; }

  /// Consumer: absolute index of the oldest element still retained. 0 until the
  /// ring has wrapped.
  u64 base() const noexcept {
    const u64 p = produced_.load(std::memory_order_acquire);
    return p > cap_ ? p - cap_ : 0;
  }

  /// Single-threaded: retained-window index (0 = oldest retained). `i` must be
  /// < size().
  const T& operator[](u64 i) const noexcept {
    return buf_[static_cast<usize>((base() + i) & mask_)];
  }

  /// Consumer: copies the `n` elements at absolute indices [absolute, absolute+n)
  /// into `out`, splitting the copy at the ring's wrap point. The caller must hold
  /// a snapshot that retains those indices; this never consults the live count, so
  /// a pushing producer cannot shift the read.
  void copy_from(u64 absolute, u64 n, T* out) const {
    if (n == 0) return;
    const u64 s = absolute & mask_;
    const u64 run = std::min(n, cap_ - s);
    const auto first = buf_.begin() + static_cast<std::ptrdiff_t>(s);
    std::copy(first, first + static_cast<std::ptrdiff_t>(run), out);
    if (run < n) {
      std::copy(buf_.begin(), buf_.begin() + static_cast<std::ptrdiff_t>(n - run),
                out + static_cast<std::ptrdiff_t>(run));
    }
  }

  /// Consumer thread only: discards every element (not safe while a producer is
  /// pushing).
  void clear() noexcept { produced_.store(0, std::memory_order_relaxed); }

 private:
  u64 cap_;
  u64 mask_;
  std::vector<T> buf_;
  std::atomic<u64> produced_{0};
};

}  // namespace plot::data