#include "twn_gl_any_rendering_c.h"
#include "twn_draw_c.h"
#include "twn_engine_context_c.h"

#include <stb_ds.h>

#ifdef EMSCRIPTEN
#include <GLES2/gl2.h>
#else
#include <glad/glad.h>
#endif


void setup_viewport(int x, int y, int width, int height) {
    glViewport(x, y, width, height);
}


VertexBuffer create_vertex_buffer(void) {
    GLuint result;
    glGenBuffers(1, &result);
    return result;
}


void delete_vertex_buffer(VertexBuffer buffer) {
    glDeleteBuffers(1, &buffer);
}


void specify_vertex_buffer(VertexBuffer buffer, void const *data, size_t bytes) {
    glBindBuffer(GL_ARRAY_BUFFER, buffer);
    glBufferData(GL_ARRAY_BUFFER, bytes, data, GL_STREAM_DRAW);
}


VertexBuffer get_quad_element_buffer(void) {
    static VertexBuffer buffer = 0;

    /* it's only generated once at runtime */
    /* TODO: use builder interface, not direct calls (glMapBuffer isn't portable) */
    if (buffer == 0) {
        buffer = create_vertex_buffer();
        VertexBufferBuilder builder = build_vertex_buffer(buffer, sizeof (GLshort) * QUAD_ELEMENT_BUFFER_LENGTH * 6 );

        for (size_t i = 0; i < QUAD_ELEMENT_BUFFER_LENGTH; ++i) {
            GLshort indices[6];
            indices[0] = (GLshort)(i * 4 + 0);
            indices[1] = (GLshort)(i * 4 + 1);
            indices[2] = (GLshort)(i * 4 + 2);
            indices[3] = (GLshort)(i * 4 + 2);
            indices[4] = (GLshort)(i * 4 + 3);
            indices[5] = (GLshort)(i * 4 + 0);

            push_to_vertex_buffer_builder(&builder, indices, sizeof indices);
        }
    }

    SDL_assert_always(buffer);

    return buffer;
}


VertexBuffer get_circle_element_buffer(void) {
    static VertexBuffer buffer = 0;

    if (buffer == 0) {
        buffer = create_vertex_buffer();
        VertexBufferBuilder builder = build_vertex_buffer(buffer, sizeof (GLshort) * (CIRCLE_VERTICES_MAX - 2) * 3);

        for (size_t i = 1; i < CIRCLE_VERTICES_MAX - 1; ++i) {
            /* first one is center point index, always zero */
            GLshort indices[3];

            indices[0] = 0;

            /* generated point index */
            indices[1] = (GLshort)i;

            indices[2] = (GLshort)i + 1;

            push_to_vertex_buffer_builder(&builder, indices, sizeof indices);
        }
    }

    SDL_assert_always(buffer);

    return buffer;
}


/* potentially double buffered array of vertex array handles */
/* we assume they will be refilled fully each frame */
static size_t scratch_va_front_used, scratch_va_back_used;
static GLuint *front_scratch_vertex_arrays, *back_scratch_vertex_arrays;
static GLuint **current_scratch_vertex_array = &front_scratch_vertex_arrays;

void restart_scratch_vertex_arrays(void) {
    scratch_va_front_used = 0;
    scratch_va_back_used = 0;

    if (ctx.render_double_buffered) {
        current_scratch_vertex_array = current_scratch_vertex_array == &front_scratch_vertex_arrays ?
                &back_scratch_vertex_arrays : &front_scratch_vertex_arrays;
    }
}


GLuint get_scratch_vertex_array(void) {
    size_t *used = current_scratch_vertex_array == &front_scratch_vertex_arrays ?
            &scratch_va_front_used : &scratch_va_back_used;

    if (arrlenu(*current_scratch_vertex_array) <= *used) {
        GLuint handle;
        glGenBuffers(1, &handle);
        arrpush(*current_scratch_vertex_array, handle);
    }

    (*used)++;
    return (*current_scratch_vertex_array)[*used - 1];
}