// SPDX-License-Identifier: MIT
// SPDX-FileCopyrightText: 2026 Martino Ferrari

#pragma once
#include <vector>

#include <plot/core/types.hpp>
#include <plot/data/minmax_pyramid.hpp>

namespace plot::data {

/// What geometry a gather level produces for its buckets. The LOD *level* is
/// chosen exactly as before; the mode decides what each bucket contributes.
enum class GatherMode {
  /// One (min, max) envelope pair per bucket: the default, and what a dense
  /// time-series trace wants -- the eye reads the extremes.
  Envelope,
  /// One raw sample per bucket (the pyramid's `snap` pick), x at the sample:
  /// a thinner geometry for traces whose *shape* matters more than extremes.
  Nearest,
  /// One point per bucket, y = the bucket mean (`sum` / count), x at the
  /// bucket centre: a smoothed trace for noisy data.
  Mean,
};

/// A uniform-grid series of y samples: sample i lives at x = x0() + i*dx().
///
/// Every concrete series answers range queries exactly, and never by scanning the
/// whole range: the bucket-aligned interior comes from the pyramid (O(log n)
/// bucket aggregates) and at most two partial edge buckets are read through
/// copy_raw() and scanned. Cost is independent of series length and no query
/// allocates.
///
/// Indexing is over the series' own window: index 0 is the oldest sample the
/// series currently holds (for a ring-backed series that is the oldest retained
/// one), so reduce(start, end) and gather() are in the same coordinates as
/// size(), x0() and pyramid().
class Series {
 public:
  virtual ~Series() = default;

  virtual u64 size() const = 0;
  /// x of sample 0, i.e. of the oldest sample currently held.
  virtual f64 x0() const = 0;
  /// Uniform x spacing between consecutive samples.
  virtual f64 dx() const = 0;
  virtual Range x_bounds() const = 0;
  virtual Range y_bounds() const = 0;
  virtual const MinMaxPyramid& pyramid() const = 0;
  /// True when the retained window slides as samples are produced, so that
  /// `x_bounds()` reports the window the series currently holds rather than an
  /// extent that only changes when the caller replaces the data. Only such a
  /// series has a *live edge* for `XFollow::Live` to follow -- the newest x it
  /// holds, which moves forward on its own -- so the follow path asks this rather
  /// than guessing from a type: `StreamSeries` says true, and every other series
  /// keeps the default. Virtual rather than a flag a caller could disagree with,
  /// so the series that owns the sliding window is the one that answers.
  virtual bool streaming() const noexcept { return false; }

  /// Appends interleaved (x, y) pairs for level `level`.
  ///
  /// **x is relative to `x_origin`**: the emitted value is `x_at(i) - x_origin` as f32, never
  /// the absolute coordinate. A consumer reconstructs the absolute x in f64
  /// (`x_origin + emitted`) before mapping it to the screen; staging an absolute epoch-time x
  /// as f32 would quantise it to a grid coarser than the pixel (f32 carries 24 bits -- 1 s at
  /// 1.7e9). y is unchanged.
  ///
  /// **The caller names the origin because only the caller knows the span the f32 has to be
  /// exact against.** The origin was `x0()` -- the start of the series' *retained window* --
  /// and a relative value's ULP is then a fraction of the whole window however little of it is
  /// on screen: 1.9 us over 20 s of retained data, which at a view 10 us wide is more than a
  /// hundred pixels between where a sample is *drawn* and the x a readout quotes for the same
  /// sample (`x_at()`, exact f64). Passing the visible range's minimum instead makes the error
  /// 2^-24 of the *visible span* -- a fixed 4e-5 of a pixel over a 700 px axis -- at every
  /// zoom. A caller with no view in mind may pass `x0()` and get the old behaviour.
  ///
  /// Level 0 emits the raw samples of [begin, end). Level >= 1 emits one
  /// geometry element per *complete* level bucket wholly inside the retained
  /// window, bounded by the two partial buckets at the edges of the requested
  /// range -- the bucket the window starts inside and the bucket the newest
  /// samples are in -- which it emits as raw samples through copy_raw(). A
  /// bucket that is only partly retained holds aggregates of samples the
  /// window no longer has and a partial trailing bucket is not stored at all,
  /// so neither may be drawn from aggregates. `mode` picks what a bucket
  /// contributes (see `GatherMode`); the edge raw samples are the same for
  /// every mode. A bucket whose samples are all non-finite is empty, not
  /// zero-valued (the library-wide NaN policy): Envelope and Mean emit nothing
  /// for it, Nearest draws the raw centre sample. `level` is clamped to
  /// `kMaxPyramidLevel`.
  ///
  /// The emitted geometry therefore always spans [begin, end) exactly, for at
  /// most two buckets of extra raw work (about two pixels at the level
  /// level_for() picks), and never reaches outside the retained window -- nor
  /// outside [begin, end). Pairs are appended to out_xy, which is never
  /// cleared.
  virtual void gather(u32 level, u64 begin, u64 end, f64 x_origin, std::vector<f32>& out_xy,
                      GatherMode mode = GatherMode::Envelope) const = 0;

  /// Absolute x of sample `i` (`x0() + i*dx()` on a uniform-grid series).
  /// Clamping is the caller's business: out-of-range `i` is the caller's bug.
  virtual f64 x_at(u64 i) const;

  /// The samples bracketing data-space `x`: sets `i0` to the last index with
  /// `x_at(i0) <= x` and `i1` to the first with `x_at(i1) >= x` (equal when x
  /// lands exactly on a sample). False when x lies outside the series' x
  /// bounds; `i0`/`i1` are then untouched. The crosshair and measurement
  /// paths read coordinates only through this and `x_at()`, so a series with
  /// its own x array brackets exactly.
  virtual bool neighbours(f64 x, u64& i0, u64& i1) const;

  /// Index of the sample nearest data-space `x`, clamped to `[0, size()-1]`;
  /// 0 for an empty series.
  virtual u64 snap_index(f64 x) const;

  /// Exact min/max, sum and sum-of-squares over [begin, end).
  /// Preconditions: begin < end, end <= size(); a degenerate range yields a
  /// zero-span Range and zero sums. Implemented once, non-virtually, over
  /// copy_raw() in src/data/series.cpp.
  Range reduce(u64 begin, u64 end) const;
  f64 sum(u64 begin, u64 end) const;
  f64 sumsq(u64 begin, u64 end) const;

 protected:
  /// Copies raw y samples for [begin, end) into `out`, which the caller sized to
  /// (end - begin). Handles ring wraparound. Only ever called with the small edge
  /// spans of the queries above, never on a per-sample path. Out-of-range spans
  /// are clamped and the unwritten tail is zero-filled.
  virtual void copy_raw(u64 begin, u64 end, f32* out) const = 0;

  /// The shared body of the concrete gather() overrides: raw samples at level 0,
  /// bucket geometry above it (per `mode`).
  void gather_impl(u32 level, u64 begin, u64 end, f64 x_origin, std::vector<f32>& out_xy,
                   GatherMode mode) const;

 private:
  /// [begin, end) split into the bucket-aligned interior the pyramid can answer
  /// exactly, plus the at most two partial edge spans it cannot.
  struct QueryParts {
    u64 interior_begin = 0, interior_end = 0;   // whole level-0 buckets, may be empty
    u64 left_begin = 0, left_end = 0;           // partial head bucket
    u64 right_begin = 0, right_end = 0;         // partial tail bucket
  };
  /// Needs the pyramid's window base: the bucket grid it aligns to is absolute,
  /// while begin/end are window coordinates.
  QueryParts split_query(u64 begin, u64 end) const;
  /// Reads both edge spans through copy_raw() into `out` (which must hold at
  /// least 2*kBucket samples) and returns how many were written.
  u64 copy_edges(const QueryParts& parts, f32* out) const;
  /// Appends the raw samples of [begin, end) as (x, y) pairs, in copy_raw()'s
  /// bounded chunks. The one path that reads samples rather than aggregates: level
  /// 0 is entirely this, and a level >= 1 gather uses it for the two partial edge
  /// buckets a pyramid of complete buckets cannot answer for. Precondition:
  /// begin < end, end <= size().
  void gather_raw(u64 begin, u64 end, f64 x_origin, std::vector<f32>& out_xy) const;
};

}  // namespace plot::data