plotter/include/plot/gl/handle.hpp

#pragma once
#include <plot/core/color.hpp>
#include <plot/core/status.hpp>
#include <plot/core/types.hpp>
#include <plot/gl/api.hpp>
#include <plot/math/vec2.hpp>

namespace plot::gl {

/// Move-only ownership of one GL object name (`GLuint`).
///
/// `Derived` supplies `static void destroy(GLuint) noexcept`. Copying is
/// deleted and moving transfers the name, leaving the source empty.
/// Destruction and `reset()` are no-ops when `id() == 0`, so an empty handle is
/// safe to destroy at any point — including after its context is gone. A
/// non-empty handle must not outlive the context that owns the name; call
/// `reset()` first when the context is about to be destroyed.
template <class Derived>
class Handle {
 public:
  Handle() noexcept = default;
  Handle(const Handle&) = delete;
  Handle& operator=(const Handle&) = delete;
  ~Handle() { reset(); }

  /// GL object name, or 0 when the handle is empty.
  GLuint id() const noexcept { return id_; }
  explicit operator bool() const noexcept { return id_ != 0; }

  /// Identity of the object this handle names, as a counter that only ever
  /// advances: every `reset()` that deleted a name, every allocating `create()`
  /// and every move advances it. A driver is free to hand out a name it has
  /// already used, so state that captured the old object -- the attribute
  /// pointers of a vertex array above all -- must compare this counter rather
  /// than the name (see `render::detail::InstanceStore::epoch()`).
  u32 generation() const noexcept { return generation_; }

  /// Deletes the GL object, if any, and empties the handle.
  void reset() noexcept {
    if (id_ != 0) {
      Derived::destroy(id_);
      id_ = 0;
      ++generation_;
    }
  }

 protected:
  Handle(Handle&& other) noexcept : id_(other.id_), generation_(other.generation_) {
    other.id_ = 0;
    ++generation_;         // the object is this handle's now
    ++other.generation_;   // and no longer the other's
  }
  Handle& operator=(Handle&& other) noexcept {
    if (this != &other) {
      reset();  // advances the generation when it deleted a name
      id_ = other.id_;
      other.id_ = 0;
      ++generation_;
      ++other.generation_;
    }
    return *this;
  }

  /// Advances the generation for a name the derived class has just allocated and
  /// stored: every allocating `create()` calls it, so the counter is a complete
  /// record of the handle's identity. A failed allocation needs no call -- it has
  /// already gone through `reset()`, which advanced it.
  void note_allocated() noexcept { ++generation_; }

  GLuint id_ = 0;

 private:
  u32 generation_ = 0;
};

/// Vertex buffer object. All fallible operations report through `Status`;
/// the void ones assume that `plot::gl::load()` ran and a context is current.
class Buffer final : public Handle<Buffer> {
 public:
  /// Allocates a buffer name. Fails with `Unsupported` when the entry points
  /// are not resolved yet and `GLFailure` when the driver returns name 0.
  bool create(Status* st = nullptr) noexcept;

  void bind(GLenum target = GL_ARRAY_BUFFER) const noexcept;

  /// Replaces the whole store with `bytes` bytes of `data` (`data` may be null
  /// to allocate without initialising).
  void upload(const void* data, usize bytes, GLenum usage) const noexcept;

  /// Overwrites `bytes` bytes starting at `offset`; the store must already be
  /// at least `offset + bytes` large.
  void upload_sub(const void* data, usize bytes, usize offset) const noexcept;
  /// Replaces the store with `bytes` bytes of immutable storage carrying `flags`
  /// (`GL_MAP_WRITE_BIT` and friends). GL 4.4 / ARB_buffer_storage: only legal on
  /// a context whose caps() report `buffer_storage`, and the entry point may be
  /// null even on such a context (it is resolved best-effort, see api.hpp), so a
  /// caller checks `glBufferStorage != nullptr` first. The contents start
  /// undefined.
  void storage(usize bytes, GLbitfield flags) const noexcept;

  /// Maps `bytes` bytes from `offset` with `access`, returning the pointer or
  /// null. GL 3.0 core, but a `GL_MAP_PERSISTENT_BIT` map needs the same
  /// capability as `storage()` above.
  void* map_range(usize offset, usize bytes, GLbitfield access) const noexcept;

  /// Unmaps the store's data; the pointer from map_range() is invalid after this.
  bool unmap() const noexcept;

  /// Exposed for the CRTP base. Deleting an unknown name is a GL no-op.
  static void destroy(GLuint id) noexcept;
};

/// Vertex array object holding the attribute layout of the bound buffer.
class VertexArray final : public Handle<VertexArray> {
 public:
  bool create(Status* st = nullptr) noexcept;

  void bind() const noexcept;

  /// Enables attribute `index` as `components` floats read from the currently
  /// bound `GL_ARRAY_BUFFER`, advancing `stride` bytes per vertex and starting
  /// `offset` bytes into it. Requires this vertex array to be bound.
  void attrib(u32 index, i32 components, usize stride, usize offset) const noexcept;

  /// Exposed for the CRTP base.
  static void destroy(GLuint id) noexcept;
};

/// Linked shader program. Constructed only through `from_source()`.
class Program final : public Handle<Program> {
 public:
  /// Compiles and links a vertex/fragment source pair. On failure the returned
  /// program is empty and `st` (when non-null) carries `GLFailure` plus the
  /// driver's info log, or `Unsupported` when `load()` was never called.
  static Program from_source(const char* vertex_src, const char* fragment_src,
                             Status* st = nullptr) noexcept;

  void use() const noexcept;

  /// Sets a float uniform; a name that is not an active uniform of this program
  /// is a silent no-op.
  void set_uniform(const char* name, f32 value) const noexcept;

  /// Sets a two-component float uniform.
  void set_uniform(const char* name, Vec2 value) const noexcept;
  /// Sets a four-component float uniform, e.g. the colour of a pipeline.
  void set_uniform(const char* name, Color value) const noexcept;

  /// Sets an integer uniform, e.g. a sampler's texture unit. Deliberately not
  /// an overload of the float setter: an untyped literal such as `0` must not
  /// turn a float uniform into a `glUniform1i` call.
  void set_uniform_int(const char* name, i32 value) const noexcept;

  /// Exposed for the CRTP base.
  static void destroy(GLuint id) noexcept;
};

/// 2D texture. Creation only allocates storage; `upload_*` replaces the whole
/// image (`glTexImage2D`), which is what the plot pipelines need for atlases
/// and one-shot uploads.
class Texture final : public Handle<Texture> {
 public:
  /// Allocates `width` x `height` 8-bit RGBA storage (`GL_RGBA8`), linear
  /// filtering, clamped edges.
  bool create_rgba(u32 width, u32 height, Status* st = nullptr) noexcept;

  /// Allocates `width` x `height` single-channel 8-bit storage (`GL_R8`) for
  /// coverage masks and narrow series data.
  bool create_r8(u32 width, u32 height, Status* st = nullptr) noexcept;

  /// Replaces the whole image with `width * height * 4` bytes of RGBA data.
  void upload_rgba(u32 width, u32 height, const void* pixels) const noexcept;

  /// Replaces the whole image with `width * height` bytes of single-channel
  /// data (row padding is disabled, so any width is valid). It sets
  /// `GL_UNPACK_ALIGNMENT` to 1 to do so and leaves it there -- a caller that
  /// shares the context with other uploaders saves and restores it, as
  /// `Renderer` does around its atlas and its frames.
  void upload_r8(u32 width, u32 height, const void* pixels) const noexcept;

  /// Binds to `unit` (e.g. `GL_TEXTURE0`).
  void bind(GLenum unit = GL_TEXTURE0) const noexcept;

  void set_filter(GLenum min, GLenum mag) const noexcept;

  /// Exposed for the CRTP base.
  static void destroy(GLuint id) noexcept;
};

/// Framebuffer object, the render target of the off-screen paths.
class Framebuffer final : public Handle<Framebuffer> {
 public:
  bool create(Status* st = nullptr) noexcept;

  void bind(GLenum target = GL_FRAMEBUFFER) const noexcept;

  /// Attaches `texture`'s level 0 to `GL_COLOR_ATTACHMENT0`.
  void attach_color(const Texture& texture) const noexcept;

  /// True when the attachment set is `GL_FRAMEBUFFER_COMPLETE`.
  bool complete() const noexcept;

  /// Exposed for the CRTP base.
  static void destroy(GLuint id) noexcept;
};

/// Move-only GL sync object (`GLsync`, GL 3.2) used to guard reuse of buffers
/// that the GPU may still be reading.
class Fence {
 public:
  Fence() noexcept = default;
  Fence(const Fence&) = delete;
  Fence& operator=(const Fence&) = delete;
  Fence(Fence&& other) noexcept : id_(other.id_) { other.id_ = nullptr; }
  Fence& operator=(Fence&& other) noexcept {
    if (this != &other) {
      reset();
      id_ = other.id_;
      other.id_ = nullptr;
    }
    return *this;
  }
  /// Deleting no fence is a no-op, so an empty fence is always safe.
  ~Fence() { reset(); }

  /// Inserts a fence in the GL command stream.
  bool create(Status* st = nullptr) noexcept;

  /// Waits up to `timeout_ns` nanoseconds for the fence to be signalled,
  /// flushing the command stream first. True when it signalled in time.
  bool wait(u64 timeout_ns) const noexcept;

  GLsync id() const noexcept { return id_; }
  explicit operator bool() const noexcept { return id_ != nullptr; }

  void reset() noexcept;

 private:
  GLsync id_ = nullptr;
};

}  // namespace plot::gl