plotter/include/plot/data/minmax_pyramid.hpp

#pragma once
#include <deque>
#include <vector>

#include <plot/core/types.hpp>

namespace plot::data {

/// Aggregate of one bucket: the extremes of the samples it covers.
struct MinMax {
  f32 min = 0.0f;
  f32 max = 0.0f;
};

/// Samples per level-0 bucket. Fixed so the LOD geometry is identical for every
/// series and the level maths is a shift.
inline constexpr u32 kBucket = 64;

/// Incremental min/max LOD pyramid, indexed in absolute bucket space.
///
/// Level `l` bucket `k` covers the absolute samples
/// `[k * (kBucket << l), (k + 1) * (kBucket << l))`. Bucket boundaries are
/// therefore fixed for the pyramid's whole life: appending never re-buckets an
/// existing aggregate, and the window a caller reads can slide over the same
/// buckets instead of being rebuilt from the raw samples.
///
/// Each level holds its complete buckets in a deque (O(1) push_back *and*
/// pop_front) plus a `base`, the absolute level-`l` bucket index of its first
/// entry. `append()` folds a sample into the in-progress level-0 bucket and, when
/// that bucket completes, pushes it and walks up -- stopping at the first level
/// that gains no new entry. Level `l` gains one entry per `kBucket << l` samples,
/// so an append costs amortised O(1) and never walks every live level.
///
/// `drop_before(u)` slides the window's leading edge to absolute sample `u`: each
/// level discards the buckets whose span starts before `u`, so the memory is
/// bounded by the retained window and not by the number of samples ever appended.
/// A bucket entry is 24 bytes -- `MinMax` plus the exact `sum`/`sumsq` the exact
/// range queries fold over -- so level 0 costs 0.375n bytes and the geometric tail
/// brings the whole pyramid to about 0.75n bytes, ~19% of the 4n bytes of raw
/// samples. (A min/max-only index would be 0.25n, ~6%: the two doubles are the
/// price of answering mean/RMS/integral from whole buckets instead of raw
/// samples.) Every stored entry is the complete aggregate of its own span and
/// every stored span lies inside the retained window, so no aggregate is ever
/// recomputed or invalidated.
///
/// Queries are in *window* coordinates: index 0 is the oldest retained sample,
/// `size()` is the retained sample count, and `window_base()` is the absolute
/// index window index 0 refers to. That is the same window `Series` publishes,
/// which is why a `StreamSeries` can hand the pyramid a moving base and keep
/// answering in its own coordinates.
///
/// A pyramid stores *aggregates*, so it can only answer exactly for whole
/// buckets; a range with partial edge buckets would silently pull in samples
/// outside the range. Every query here is therefore bucket-grained and named
/// *_buckets, and exact range queries live on Series, which owns the raw samples.
class MinMaxPyramid {
 public:
  MinMaxPyramid() = default;

  /// Sizes the level table for a window of `n` samples so a following run of
  /// appends never grows the level vector. The bucket deques themselves grow in
  /// chunks: a sliding window cannot know their final length up front.
  void reserve(u64 n);
  void append(f32 v);
  /// Slides the retained window's leading edge to absolute sample index
  /// `absolute_sample`, dropping every aggregate that covered a sample before it.
  /// Monotonic, and a no-op only when `absolute_sample <= window_base()`: an empty
  /// pyramid is not exempt, because a caller whose window already starts past
  /// everything appended so far has to be able to say so (that is exactly what a
  /// ring that saturated before the first acquire() passes here).
  ///
  /// An argument past `window_end()` -- a producer that outran the consumer by
  /// more than the ring held -- drops every aggregate and restarts the window at
  /// `absolute_sample`: the samples in between were never appended, so they are
  /// not representable, and the window the caller reads is the new one.
  void drop_before(u64 absolute_sample);
  void clear();

  /// Samples in the retained window.
  u64 size() const noexcept { return count_ - base_; }
  /// Absolute index of the sample window index 0 refers to.
  u64 window_base() const noexcept { return base_; }
  /// Absolute index one past the last appended sample: the first sample index a
  /// following append() will occupy.
  u64 window_end() const noexcept { return count_; }
  /// At least 1, even when empty, so level_count() - 1 is always valid.
  u32 level_count() const noexcept { return live_levels_; }

  /// Absolute level-`l` bucket index of level `l`'s first entry, and how many
  /// entries it holds. They cover `[level_base(l), level_base(l) + level_size(l))`:
  /// entry `i` is the aggregate of the absolute samples
  /// `[i * (kBucket << l), (i + 1) * (kBucket << l))`. A level that exists but
  /// holds nothing, or a level past level_count(), reports size 0 (its base is
  /// then meaningless).
  u64 level_base(u32 l) const noexcept;
  u64 level_size(u32 l) const noexcept;
  /// Aggregate of the absolute level-`l` bucket `i`. Precondition:
  /// level_base(l) <= i < level_base(l) + level_size(l).
  MinMax bucket(u32 l, u64 i) const;

  /// Min/max over the whole level-0 buckets fully contained in the window range
  /// [begin, end). Partial edge buckets are excluded, never approximated; an
  /// empty result is a degenerate Range with span() == 0.
  Range reduce_buckets(u64 begin, u64 end) const;
  /// Sum of the samples in those same whole buckets, accumulated in double.
  f64 sum_buckets(u64 begin, u64 end) const;
  /// Sum of squares of the samples in those same whole buckets.
  f64 sumsq_buckets(u64 begin, u64 end) const;
  /// Pyramid level whose buckets hold about `samples_per_pixel` samples, for a
  /// window of `n` samples: 0 when a pixel covers at most one bucket. Clamped to
  /// the levels that exist, to levels with more than one bucket, and to levels
  /// that give the window at least two buckets.
  u32 level_for(u64 n, f64 samples_per_pixel) const;
  /// Index of the sample nearest x on the uniform grid (x0 + i*dx), clamped to
  /// [0, size() - 1]. Returns 0 for an empty pyramid or a non-positive dx.
  u64 snap(f64 x, f64 x0, f64 dx) const;

 private:
  /// One bucket: its extremes plus its own exact sum and sum of squares. The two
  /// doubles take a bucket from 8 bytes to 24, which is what lets mean/RMS/
  /// integral come from whole buckets instead of raw samples.
  struct Entry {
    MinMax mm;
    f64 sum = 0.0;
    f64 sumsq = 0.0;
  };
  /// One level's complete buckets, in absolute bucket-index order.
  struct Level {
    std::deque<Entry> entries;
    u64 base = 0;   ///< absolute level-l bucket index of entries.front()
  };

  void add_level();
  /// Appends `e` at absolute level-`l` bucket `k`, which must be one past the
  /// level's last entry.
  void push(u32 l, u64 k, const Entry& e);
  /// Entry for absolute level-`l` bucket `i`; the caller has checked the range.
  const Entry& at(u32 l, u64 i) const {
    return levels_[l].entries[static_cast<usize>(i - levels_[l].base)];
  }

  u64 count_ = 0;   ///< absolute index one past the last appended sample
  u64 base_ = 0;    ///< absolute index of the window's oldest sample
  u32 live_levels_ = 1;
  std::vector<Level> levels_{1};
  Entry pending_;   ///< the in-progress level-0 bucket, not yet an entry
};

}  // namespace plot::data