plotter/adapters/glfw/glfw_host.hpp

#pragma once

#include <functional>
#include <memory>
#include <optional>
#include <vector>

#include <plot/core/rect.hpp>
#include <plot/core/status.hpp>
#include <plot/core/types.hpp>
#include <plot/input/controller.hpp>
#include <plot/plot/figure.hpp>
#include <plot/plot/plot.hpp>
#include <plot/render/renderer.hpp>

/// GLFW's window type. Forward-declared on purpose: this header is
/// self-contained and needs no GLFW include directory, so a consumer of the
/// adapter does not inherit the toolkit's headers (and their `<GL/gl.h>`, whose
/// macros would rewrite this library's enumerators).
struct GLFWwindow;

namespace plot::glfw {

/// What `Host::run()` opens the window with.
struct RunConfig {
  int width = 1280;
  int height = 720;
  const char* title = "plot";
  bool vsync = true;
};

/// One frame of application code, called by `Host::run()` after the viewport is
/// set and before the frame is staged, so it is where data is refreshed.
/// `dt` is the wall time since the previous call, in seconds.
using FrameFn = std::function<void(plot::Plot&, f64 dt)>;

/// The reference host: owns the GLFW window and its OpenGL 3.3 core context, the
/// GL entry points, the `Renderer`, the `Plot`, the `input::Controller` and the
/// frame loop, so nothing above it names GLFW or GL.
///
/// Per-frame sequence, in this order:
///
///   poll events -> controller.set_viewport(framebuffer-pixel rect) ->
///   frame(plot, dt) -> begin_frame -> draw -> [screenshot] -> end_frame ->
///   swap buffers
///
/// Two pixel spaces meet here and must not be confused. GLFW reports the cursor
/// and the window in *window* coordinates, while the framebuffer -- what
/// `begin_frame` takes and what every `Rect` in this library speaks -- can be
/// larger under a fractional or HiDPI scale, so `glfwGetFramebufferSize()` and
/// `glfwGetWindowSize()` disagree. The host calls `begin_frame` with the
/// framebuffer size and scales every pointer coordinate by the ratio
/// `framebuffer / window`, so a gesture lands on the pixels the plot was laid out
/// in. The same ratio states the display to the renderer
/// (`Style::ui_scale`), which is what makes the chrome -- labels, strokes, paddings
/// -- one physical size at any scale. On an unscaled display the ratio is 1.0 and
/// nothing changes.
///
/// Headless smoke mode, read once per `run()`:
///
///   `PLOT_DEMO_EXIT_AFTER_MS=<n>`  stop the loop after n ms and return
///                                  (default 0: run until the window is closed)
///   `PLOT_DEMO_SCREENSHOT=<path>`  on that final frame, write it with
///                                  `Renderer::dump_frame_ppm()` before the swap
///
/// That is what lets a CI run a GUI library under `xvfb-run`: no display access
/// and no screen capture, just a PPM on disk.
///
/// A run with a deadline is that automated path, so its window is created hidden
/// (`GLFW_VISIBLE` false): an automated run must not flash a window on whatever
/// display it lands on. It also means the loop is not throttled by a display
/// refresh, so the frame times it reports are the CPU's, not a vblank's. A run
/// without a deadline shows its window, as an interactive application must.
///
/// Not thread-safe, and confined to the thread that runs `run()`.
class Host {
 public:
  Host() noexcept;
  ~Host();
  Host(const Host&) = delete;
  Host& operator=(const Host&) = delete;

  /// Opens the window, makes a 3.3 core context current, resolves the GL entry
  /// points and initialises the renderer, then runs the loop until the window is
  /// closed or the exit deadline passes. Returns false with `st` filled in when
  /// GLFW, the context or `Renderer::init()` fails; a failed run leaves nothing
  /// open.
  ///
  /// On success the window *stays open*, so a caller can stop and join whatever
  /// feeds the plot before the GL resources go away: `close()` (or the
  /// destructor) releases the renderer, the context and the window, in that
  /// order, on this thread. Calling `run()` again reopens the window and keeps
  /// the plot.
  bool run(const RunConfig& cfg, const FrameFn& frame, Status* st = nullptr);

  /// Releases the renderer and the window. Idempotent, and safe on a host that
  /// never ran.
  void close() noexcept;

  /// The plot the frame callback populates. Valid for the host's whole life.
  Plot& plot() noexcept { return plot_; }
  /// Draw a whole `Figure` instead of the host's own plot: the multi-plot shape,
  /// several rows sharing one x axis, which one `Plot` cannot express. While a
  /// figure is set the run loop lays it out over the framebuffer, routes each
  /// pointer, wheel and key event to the row the event lands in (one `Controller`
  /// per row, created here, each bound to that row's own `Plot`), and draws it with
  /// `Renderer::draw(Figure&)`. The frame callback is handed the figure's **row 0**
  /// as its `Plot&`, so `plot()` and the frame argument agree.
  ///
  /// The figure's **crosshair** follows the pointer as one crosshair and not one per row: the run
  /// loop reads the hover out of the row under the pointer and applies it to the whole figure
  /// (`sync_figure_crosshair()`), so every row's readout lists its own signals at the one x the
  /// pointer is over. That is what "synced if multiple plots available" means, and the host is the
  /// layer that can do it -- it is the one that knows which row the pointer is in.
  ///
  /// The figure is borrowed and must outlive the run. Passing null restores the
  /// single-plot loop, which is what every other caller uses; the host's own
  /// `plot_` is then untouched by any of this.
  void set_figure(Figure* figure) noexcept;
  Figure* figure() noexcept { return figure_; }
  /// One figure row's controller -- the one a gesture on that row reaches -- or
  /// null while no figure is set or the row does not exist. A host reads a drag
  /// mode or a hover out of it (`controller()` is the single-plot loop's, and is
  /// not the one figure gestures go to).
  input::Controller* row_controller(u32 row) noexcept;
  /// The controller an event at framebuffer pixel (x, y) belongs to: `controller_` --
  /// the single-plot loop -- while no figure is set, and the controller of the figure
  /// row that pixel lands in otherwise. The row rects come from `Figure::layout()`, the
  /// same ones the renderer draws, so a gesture lands on the row it was aimed at; a
  /// pixel inside no row -- the row gap a host asked for with `Figure::set_row_gap()`,
  /// or a margin -- is the *nearest* row rather than dropped or defaulted, so a drag
  /// that leaves the data area still ends where it should.
  ///
  /// It also hands that row its own viewport, which is what keeps the controller's
  /// hit-test geometry in step with the layout between two frames. The event callbacks
  /// route through it, and so can a host that wants to know where a synthetic gesture
  /// would land.
  input::Controller& controller_at(f32 x, f32 y) noexcept;

  /// The gesture controller the host feeds. Valid for the host's whole life.
  input::Controller& controller() noexcept { return controller_; }

  /// The renderer, or null while no window is open. Valid from inside the frame
  /// callback and after a successful `run()`; `close()` releases it.
  Renderer* renderer() noexcept { return renderer_ ? &*renderer_ : nullptr; }

  /// Retitles the open window -- from the frame callback, for a live
  /// samples/second and frame-time readout. A no-op while no window is open.
  void set_title(const char* title) noexcept;

  /// Framebuffer pixels per window pixel, one ratio per axis, as of the last
  /// event or frame. 1.0 on an unscaled display.
  f32 framebuffer_scale_x() const noexcept { return fb_scale_x_; }
  f32 framebuffer_scale_y() const noexcept { return fb_scale_y_; }
  /// The window's content scale, one per axis, as of the last frame.
  f32 content_scale_x() const noexcept { return content_scale_x_; }
  f32 content_scale_y() const noexcept { return content_scale_y_; }

 private:
  /// Reads the framebuffer and window sizes and the content scale, refreshing
  /// the window -> framebuffer ratios the pointer translation uses.
  void refresh_sizes() noexcept;

  /// Callbacks find their host through GLFW's per-window user pointer.
  static Host* from(GLFWwindow* w) noexcept;
  static void cursor_pos_cb(GLFWwindow* w, double x, double y);
  static void mouse_button_cb(GLFWwindow* w, int button, int action, int mods);
  static void scroll_cb(GLFWwindow* w, double dx, double dy);
  static void key_cb(GLFWwindow* w, int key, int scancode, int action, int mods);
  static void framebuffer_size_cb(GLFWwindow* w, int width, int height);

  void on_cursor_pos(f64 wx, f64 wy) noexcept;
  void on_mouse_button(int button, int action, int mods) noexcept;
  void on_scroll(f64 dx, f64 dy) noexcept;
  void on_key(int key, int action, int mods) noexcept;

  /// The index of the row of `rows` a framebuffer y belongs to: the row it is in, or the nearest
  /// row when it is in none (a row gap, a margin outside the figure). One rule, shared by the event
  /// routing and the crosshair sync -- and taking the rects rather than reading them, so a caller
  /// that has just laid the figure out does not make it lay out again for this.
  static u32 row_in(const std::vector<Rect>& rows, f32 y) noexcept;

  /// Spreads the hover over a whole `Figure` (spec §15).
  ///
  /// A figure's rows share one x, so the crosshair one row's controller placed is read back here and
  /// applied to every row: the controller turns a pointer position into a data coordinate -- which
  /// is its job, and the same one it does for a single plot -- and the figure spreads that one
  /// coordinate over its rows. Reading it back rather than deriving a second coordinate here is what
  /// keeps the hover the row shows and the hover the figure shows the same number.
  ///
  /// The row it reads is the one under the host's pointer, or -- with no pointer of the host's own,
  /// which is how a scripted run injects its events -- the row whose crosshair is not the value this
  /// host spread last (`spread_x_`/`spread_active_`), i.e. the row something outside changed. A row
  /// whose controller has no crosshair clears the figure's.
  ///
  /// Called once per frame from the run loop, so a scripted run that bypasses the GLFW callbacks
  /// gets the same synced figure a real pointer does. `rows` is the figure's row rects, which the
  /// run loop has just read -- the one layout per frame this shares with the row viewports.
  void sync_figure_crosshair(const std::vector<Rect>& rows) noexcept;

  Plot plot_;
  input::Controller controller_;  ///< forwards its viewport to `plot_`
  std::optional<Renderer> renderer_;
  GLFWwindow* window_ = nullptr;
  /// The figure the host draws instead of `plot_`, and one controller per row for
  /// it -- one per `Plot`, because a `Controller` is bound to a plot at construction
  /// and this is what makes a gesture on any row reach that row. Both are empty
  /// until `set_figure()` is called, and the run loop's single-plot path reads
  /// neither.
  Figure* figure_ = nullptr;
  std::vector<std::unique_ptr<input::Controller>> row_controllers_;
  /// The row the last event landed in, which is where a key event goes: a key has
  /// no position of its own, and the row the pointer is over is the one the user is
  /// looking at.
  u32 last_row_ = 0u;
  /// The crosshair this host last spread over the figure (see
  /// `sync_figure_crosshair()`): what a row is compared against to find the one a
  /// scripted run touched, since the figure writes one value to every row.
  /// `spread_active_` false means "spread nothing" -- the state before the first
  /// sync, and the one a clear leaves.
  f64 spread_x_ = 0.0;
  bool spread_active_ = false;

  u32 fb_width_ = 0;   ///< framebuffer size of the last refresh_sizes()
  u32 fb_height_ = 0;
  /// Framebuffer pixels per window pixel. Taken from the two sizes GLFW reports
  /// rather than from the content scale, which under a fractional scale does not
  /// round back to the framebuffer size.
  f32 fb_scale_x_ = 1.0f;
  f32 fb_scale_y_ = 1.0f;
  f32 content_scale_x_ = 1.0f;
  f32 content_scale_y_ = 1.0f;
  /// Last pointer position in framebuffer pixels: the deltas the controller pans
  /// with are the movements since the previous event, which GLFW does not report.
  f32 last_pointer_x_ = 0.0f;
  f32 last_pointer_y_ = 0.0f;
  bool have_pointer_ = false;
};

}  // namespace plot::glfw