102 lines
3.6 KiB
C++
102 lines
3.6 KiB
C++
#pragma once
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#include <algorithm>
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#include <atomic>
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#include <bit>
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#include <cstddef>
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#include <vector>
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#include <plot/core/types.hpp>
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namespace plot::data {
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/// Single-producer / single-consumer ring of at most `capacity` elements.
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///
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/// The capacity is rounded up to a power of two so slot addressing is a mask
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/// instead of a modulo. The producer writes a slot and then publishes the new
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/// sample count with a release store; a consumer that observes that count with an
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/// acquire load is guaranteed to see the slot contents written before it.
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///
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/// A push into a full ring silently discards the oldest element: for a live
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/// plotting stream the newest sample is the interesting one and the retained
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/// window slides forward.
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///
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/// Two indexing conventions, deliberately named apart:
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/// - retained-window (operator[], index 0 = oldest element still held) is a
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/// single-threaded view: it re-derives the window from the live count, so a
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/// producer pushing concurrently would move it under the reader's feet.
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/// - absolute (base(), copy_from()) fixes one count for a whole read, which is
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/// what a consumer holding a snapshot must use. StreamSeries::acquire() takes
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/// such a snapshot and then reads only absolute indices.
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template <typename T>
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class RingBuffer {
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public:
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explicit RingBuffer(u64 capacity)
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: cap_(std::bit_ceil(capacity < 1 ? u64{1} : capacity)),
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mask_(cap_ - 1),
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buf_(static_cast<usize>(cap_)) {}
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/// Producer thread only.
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void push(T v) {
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const u64 i = produced_.load(std::memory_order_relaxed);
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buf_[static_cast<usize>(i & mask_)] = v;
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produced_.store(i + 1, std::memory_order_release);
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}
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/// Producer thread only. Equivalent to n calls to push().
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void push(const T* vs, u64 n) {
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for (u64 i = 0; i < n; ++i) push(vs[i]);
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}
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/// Consumer: retained element count, saturating at capacity().
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u64 size() const noexcept {
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const u64 p = produced_.load(std::memory_order_acquire);
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return p < cap_ ? p : cap_;
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}
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/// Consumer: total number of elements ever pushed (monotonic).
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u64 produced() const noexcept { return produced_.load(std::memory_order_acquire); }
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u64 capacity() const noexcept { return cap_; }
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/// Consumer: absolute index of the oldest element still retained. 0 until the
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/// ring has wrapped.
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u64 base() const noexcept {
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const u64 p = produced_.load(std::memory_order_acquire);
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return p > cap_ ? p - cap_ : 0;
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}
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/// Single-threaded: retained-window index (0 = oldest retained). `i` must be
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/// < size().
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const T& operator[](u64 i) const noexcept {
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return buf_[static_cast<usize>((base() + i) & mask_)];
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}
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/// Consumer: copies the `n` elements at absolute indices [absolute, absolute+n)
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/// into `out`, splitting the copy at the ring's wrap point. The caller must hold
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/// a snapshot that retains those indices; this never consults the live count, so
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/// a pushing producer cannot shift the read.
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void copy_from(u64 absolute, u64 n, T* out) const {
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if (n == 0) return;
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const u64 s = absolute & mask_;
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const u64 run = std::min(n, cap_ - s);
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const auto first = buf_.begin() + static_cast<std::ptrdiff_t>(s);
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std::copy(first, first + static_cast<std::ptrdiff_t>(run), out);
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if (run < n) {
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std::copy(buf_.begin(), buf_.begin() + static_cast<std::ptrdiff_t>(n - run),
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out + static_cast<std::ptrdiff_t>(run));
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}
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}
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/// Consumer thread only: discards every element (not safe while a producer is
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/// pushing).
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void clear() noexcept { produced_.store(0, std::memory_order_relaxed); }
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private:
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u64 cap_;
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u64 mask_;
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std::vector<T> buf_;
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std::atomic<u64> produced_{0};
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};
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} // namespace plot::data
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