// Copyright (C) 2022 RozK
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program.  If not, see <http://www.gnu.org/licenses/>.

#include "render_opengles.hpp"
#include <cstdio>

static rk_shader const * rk_current_shader = nullptr;
static rk_vertices const * rk_current_vertices = nullptr;

static void rk_printf(char const * messsage) {
    printf("[RK_ENGINE] %s\n", messsage);
}

#define rk_error(message) { if (glGetError() != GL_NO_ERROR) { rk_printf(message); } }

static void rk_debug_message_callback(
    GLenum source,
    GLenum type,
    GLuint id,
    GLenum severity,
    GLsizei length,
    GLchar const * message,
    void const * userParam) {
    printf("[RK_ENGINE][GL] (id=%d) %s\n", id, message);
}

rk_window_t rk_initialize(
    char const * name,
    rk_uint width,
    rk_uint height) {
    rk_window_t const window = rk_create_context(name, width, height);
    if (window) {
        GLubyte const * const vendor = glGetString(GL_VENDOR);
        GLubyte const * const renderer = glGetString(GL_RENDERER);
        printf("[RK_ENGINE] vendor: %s, renderer: %s\n", vendor, renderer);
        GLubyte const * const version = glGetString(GL_VERSION);
        GLubyte const * const language = glGetString(GL_SHADING_LANGUAGE_VERSION);
        printf("[RK_ENGINE] version: %s, language: %s\n", version, language);

        glDebugMessageCallback(rk_debug_message_callback, nullptr);
        glEnable(GL_DEBUG_OUTPUT);

        glDisable(GL_BLEND);
        glEnable(GL_DITHER);
        glEnable(GL_DEPTH_TEST);
        glDisable(GL_SCISSOR_TEST);
        glDisable(GL_STENCIL_TEST);
        glEnable(GL_CULL_FACE);
        glFrontFace(GL_CCW);
        glCullFace(GL_BACK);
        glHint(GL_GENERATE_MIPMAP_HINT, GL_NICEST);
    }
    return window;
}

static void rk_print_shader_infolog(GLuint shader) {
    int length;
    char infolog[1024];
    glGetShaderInfoLog(shader, sizeof(infolog), &length, infolog);
    if (length > 0) {
        rk_printf(infolog);
    }
}

static void rk_print_program_infolog(GLuint program) {
    int length;
    char infolog[1024];
    glGetProgramInfoLog(program, sizeof(infolog), &length, infolog);
    if (length > 0) {
        rk_printf(infolog);
    }
}

//TODO: error handling
rk_shader_t rk_load_shader(
    rk_uint const vert_nlines,
    char const ** const vert_lines,
    rk_uint const frag_nlines,
    char const ** const frag_lines) {
    rk_shader * const shader = new rk_shader;
    shader->vertex = glCreateShader(GL_VERTEX_SHADER);
    shader->fragment = glCreateShader(GL_FRAGMENT_SHADER);
    shader->program = glCreateProgram();
    if (!vert_nlines || !vert_lines) {
        rk_printf("Missing vertex shader.");
        return nullptr;
    }
    if (!frag_nlines || !frag_lines) {
        rk_printf("Missing fragment shader.");
        return nullptr;
    }
    rk_printf("Compiling vertex shader...");
    glShaderSource(shader->vertex, vert_nlines, vert_lines, nullptr);
    glCompileShader(shader->vertex);
    rk_error("glCompileShader() failed.");
    rk_print_shader_infolog(shader->vertex);
    rk_printf("Compiling fragment shader...");
    glShaderSource(shader->fragment, frag_nlines, frag_lines, nullptr);
    glCompileShader(shader->fragment);
    rk_error("glCompileShader() failed.");
    rk_print_shader_infolog(shader->fragment);
    rk_printf("Linking program...");
    glAttachShader(shader->program, shader->vertex);
    glAttachShader(shader->program, shader->fragment);
    glLinkProgram(shader->program);
    rk_error("glLinkProgram() failed.");
    rk_print_program_infolog(shader->program);
    rk_printf("Done.");
    glReleaseShaderCompiler();
    return shader;
}

void rk_select_shader(
    rk_shader_t _shader) {
    rk_shader * const shader = reinterpret_cast<rk_shader *>(_shader);
    if (shader) {
        rk_current_shader = shader;
        glUseProgram(shader->program);
    }
}

rk_input_t rk_resolve_input(
    char const * name) {
    if (!rk_current_shader || !name) {
        return nullptr;
    }
    GLint const uniform = glGetUniformLocation(rk_current_shader->program, name);
    return reinterpret_cast<rk_input_t>(uniform + 1);
}

void rk_set_input_float(
    rk_input_t _input,
    float value) {
    GLint const input = reinterpret_cast<intptr_t>(_input) - 1;
    if (rk_current_shader && input > -1) {
        glUniform1f(input, value);
    }
}

void rk_set_input_vec3(
    rk_input_t _input,
    rk_vec3 const & value) {
    GLint const input = reinterpret_cast<intptr_t>(_input) - 1;
    if (rk_current_shader && input > -1) {
        glUniform3fv(input, 1, glm::value_ptr(value));
    }
}

void rk_set_input_mat3(
    rk_input_t _input,
    rk_mat3 const & value) {
    GLint const input = reinterpret_cast<intptr_t>(_input) - 1;
    if (rk_current_shader && input > -1) {
        glUniformMatrix3fv(input, 1, GL_FALSE, glm::value_ptr(value));
    }
}

void rk_set_input_mat4(
    rk_input_t _input,
    rk_mat4 const & value) {
    GLint const input = reinterpret_cast<intptr_t>(_input) - 1;
    if (rk_current_shader && input > -1) {
        glUniformMatrix4fv(input, 1, GL_FALSE, glm::value_ptr(value));
    }
}

rk_param_t rk_resolve_param(
    char const * name) {
    if (!rk_current_shader || !name) {
        return nullptr;
    }
    GLint const location = glGetAttribLocation(rk_current_shader->program, name);
    return reinterpret_cast<rk_param_t>(location + 1);
}

void rk_set_param_vec3(
    rk_param_t _param,
    rk_vec3 const & value) {
    GLint const param = reinterpret_cast<intptr_t>(_param) - 1;
    if (rk_current_shader && param > -1) {
        glVertexAttrib3fv(param, glm::value_ptr(value));
    }
}

rk_texture_t rk_create_texture(
    rk_uint slot,
    char const * input,
    rk_texture_format format,
    rk_uint width,
    rk_uint height,
    rk_uint nlevels,
    rk_texture_flags flags,
    void const * pixels) {
    if (!input || width == 0 || height == 0 || !pixels || !rk_current_shader) {
        return nullptr;
    }
    GLint internal_format;
    GLenum source_format;
    GLenum source_type;
    switch (format) {
        case RK_TEXTURE_FORMAT_SRGB8_A8:
            internal_format = GL_SRGB8_ALPHA8;
            source_format = GL_RGBA;
            source_type = GL_UNSIGNED_BYTE;
            break;
        case RK_TEXTURE_FORMAT_RGBA8:
            internal_format = GL_RGBA8;
            source_format = GL_RGBA;
            source_type = GL_UNSIGNED_BYTE;
            break;
        case RK_TEXTURE_FORMAT_RGB10_A2:
            internal_format = GL_RGB10_A2;
            source_format = GL_RGBA;
            source_type = GL_UNSIGNED_INT_2_10_10_10_REV;
            break;
        case RK_TEXTURE_FORMAT_32F:
            internal_format = GL_R32F;
            source_format = GL_RED;
            source_type =  GL_FLOAT;
            break;
        default:
            return nullptr;
            break;
    }
    rk_texture * const texture = new rk_texture;
    glActiveTexture(GL_TEXTURE0 + slot);
    glGenTextures(1, &texture->texture);
    GLenum target;
    if (nlevels) {
        if (flags & RK_TEXTURE_FLAG_3D) {
            target = GL_TEXTURE_3D;
        } else {
            target = GL_TEXTURE_2D_ARRAY;
        }
        glBindTexture(target, texture->texture);
        //TODO: glTexStorage3D
        glTexImage3D(target, 0, internal_format, width, height, nlevels, 0, source_format, source_type, pixels);
    } else {
        target = GL_TEXTURE_2D;
        glBindTexture(target, texture->texture);
        //TODO: glTexStorage2D
        glTexImage2D(target, 0, internal_format, width, height, 0, source_format, source_type, pixels);
    }
    if (flags & RK_TEXTURE_FLAG_MIPMAPS) {
        if (flags & RK_TEXTURE_FLAG_MIN_LINEAR) {
            glTexParameteri(target, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
        } else {
            glTexParameteri(target, GL_TEXTURE_MIN_FILTER, GL_NEAREST_MIPMAP_LINEAR);
        }
    } else {
        if (flags & RK_TEXTURE_FLAG_MIN_LINEAR) {
            glTexParameteri(target, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
        } else {
            glTexParameteri(target, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
        }
    }
    if (flags & RK_TEXTURE_FLAG_MAG_LINEAR) {
        glTexParameteri(target, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    } else {
        glTexParameteri(target, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
    }
    glTexParameteri(target, GL_TEXTURE_WRAP_S, GL_REPEAT);
    glTexParameteri(target, GL_TEXTURE_WRAP_T, GL_REPEAT);
    glTexParameteri(target, GL_TEXTURE_WRAP_R, GL_REPEAT);
    if (flags & RK_TEXTURE_FLAG_MIPMAPS) {
        glGenerateMipmap(target);
    }
    texture->slot = slot;
    texture->nlevels = nlevels;
    texture->sampler = glGetUniformLocation(rk_current_shader->program, input);
    if (texture->sampler == -1) {
        printf("[RK_ENGINE] glGetUniformLocation(%s) failed\n", input);
    }
    glBindTexture(target, 0);
    return texture;
}

rk_triangles_t rk_create_triangles(
    rk_uint nvertices,
    rk_vec3 const * vertices) {
    if (nvertices == 0 || !vertices || !rk_current_shader) {
        return nullptr;
    }
    rk_triangles * const triangles = new rk_triangles;
    triangles->size = nvertices;
    glGenVertexArrays(1, &triangles->array);
    glBindVertexArray(triangles->array);
    glGenBuffers(1, &triangles->vertices);
    glBindBuffer(GL_ARRAY_BUFFER, triangles->vertices);
    glBufferData(GL_ARRAY_BUFFER, nvertices * sizeof(rk_vec3), vertices, GL_STATIC_DRAW);
    glEnableVertexAttribArray(0);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);
    glBindBuffer(GL_ARRAY_BUFFER, 0);
    glBindVertexArray(0);
    return triangles;
}

rk_vertices_t rk_create_vertices(
    rk_vertex_format const * format,
    rk_uint nvertices,
    void const * _vertices,
    rk_uint nindices,
    rk_ushort const * indices) {
    if (!format || !nvertices || !_vertices || !nindices || !indices || !rk_current_shader) {
        return nullptr;
    }
    rk_uint vertex_size = 0;
    for (rk_vertex_format const * f = format; *f; ++f) {
        switch (*f & RK_VERTEX_FORMAT_MASK) {
            case RK_VERTEX_FORMAT_VEC3_FLOAT:
                vertex_size += sizeof(rk_vec3);
                break;
            case RK_VERTEX_FORMAT_VEC3_INT10:
                vertex_size += sizeof(rk_int);
                break;
            case RK_VERTEX_FORMAT_VEC3_UINT10:
                vertex_size += sizeof(rk_uint);
                break;
            default:
                rk_printf("rk_create_vertices(): invalid format.");
                return nullptr;
                break;
        }
    }
    if (!vertex_size) {
        rk_printf("rk_create_vertices(): empty format.");
        return nullptr;
    }
    rk_vertices * const vertices = new rk_vertices;
    glGenVertexArrays(1, &vertices->array);
    glBindVertexArray(vertices->array);
    glGenBuffers(1, &vertices->vertices);
    glBindBuffer(GL_ARRAY_BUFFER, vertices->vertices);
    glBufferData(GL_ARRAY_BUFFER, nvertices * vertex_size, _vertices, GL_STATIC_DRAW);
    glBindBuffer(GL_ARRAY_BUFFER, 0);
    vertices->vertex_size = vertex_size;
    vertices->layout = 0;
    rk_uint offset = 0;
    for (rk_vertex_format const * f = format; *f; ++f, ++vertices->layout) {
        glEnableVertexAttribArray(vertices->layout);
        GLboolean const normalize = (*f & RK_VERTEX_FORMAT_NORMALIZE) != 0;
        switch (*f & RK_VERTEX_FORMAT_MASK) {
            case RK_VERTEX_FORMAT_VEC3_FLOAT:
                glVertexAttribFormat(vertices->layout, 3, GL_FLOAT, normalize, offset);
                offset += sizeof(rk_vec3);
                break;
            case RK_VERTEX_FORMAT_VEC3_INT10:
                glVertexAttribFormat(vertices->layout, 4, GL_INT_2_10_10_10_REV, normalize, offset);
                offset += sizeof(rk_int);
                break;
            case RK_VERTEX_FORMAT_VEC3_UINT10:
                glVertexAttribFormat(vertices->layout, 4, GL_UNSIGNED_INT_2_10_10_10_REV, normalize, offset);
                offset += sizeof(rk_uint);
                break;
        }
        glVertexAttribBinding(vertices->layout, RK_VERTICES_BINDING);
    }
    glBindVertexBuffer(RK_VERTICES_BINDING, vertices->vertices, 0, vertices->vertex_size);
    glGenBuffers(1, &vertices->indices);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, vertices->indices);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, nindices * sizeof(rk_ushort), indices, GL_STATIC_DRAW);
    glBindVertexArray(0);
    return vertices;
}

static void rk_pack_vec3_float(
    rk_vec3 * const __restrict dst,
    rk_vec3 const * const __restrict src) {
    *dst = *src;
}

static void rk_pack_vec3_short(
    rk_vec3_short * const __restrict dst,
    rk_vec3 const * const __restrict src) {
    dst->x = static_cast<rk_short>(src->x);
    dst->y = static_cast<rk_short>(src->y);
    dst->z = static_cast<rk_short>(src->z);
}

static void rk_pack_vec3_short_norm(
    rk_vec3_short * const __restrict dst,
    rk_vec3 const * const __restrict src) {
    #define _convert(s) (static_cast<rk_short>((s) * ((s) < 0.f ? 32768.f : 32767.f)))
    dst->x = _convert(src->x);
    dst->y = _convert(src->y);
    dst->z = _convert(src->z);
    #undef _convert
}

static void rk_pack_vec3_int10(
    rk_int * const __restrict dst,
    rk_vec3 const * const __restrict src) {
    #define _convert(s) (static_cast<rk_int>((s) * ((s) < 0.f ? 512.f : 511.f)) & 1023)
    *dst = _convert(src->x) | (_convert(src->y) << 10) | (_convert(src->z) << 20);
    #undef _convert
}

rk_batch_t rk_create_batch(
    rk_uint max_size,
    rk_param_format const * params_format) {
    if (!max_size || !params_format || max_size > RK_BATCH_MAX_SIZE || !rk_current_shader || !rk_current_vertices) {
        rk_printf("rk_create_batch(): invalid parameters.");
        return nullptr;
    }
    rk_uint nparams = 0;
    rk_uint params_size = 0;
    rk_uint packed_size = 0;
    for (rk_param_format const * f = params_format; *f; ++f, ++nparams) {
        switch (*f & RK_PARAM_FORMAT_MASK) {
            case RK_PARAM_FORMAT_VEC3_FLOAT:
                params_size += sizeof(rk_vec3);
                packed_size += sizeof(rk_vec3);
                break;
            case RK_PARAM_FORMAT_VEC3_SHORT:
                params_size += sizeof(rk_vec3);
                packed_size += sizeof(rk_vec3_short);
                break;
            case RK_PARAM_FORMAT_VEC3_INT10:
                params_size += sizeof(rk_vec3);
                packed_size += sizeof(rk_int);
                break;
            default:
                rk_printf("rk_create_batch(): invalid param format.");
                return nullptr;
                break;
        }
    }
    rk_batch * batch = new rk_batch;
    batch->size = max_size;
    batch->nparams = nparams;
    batch->params_size = params_size;
    batch->packed_size = packed_size;
    batch->indices = new rk_ushort[max_size];
    batch->commands = new rk_command[max_size * sizeof(rk_command)];
    if (nparams) {
        batch->packers = new rk_packer[nparams];
        batch->params = new rk_ubyte[max_size * packed_size];
        glGenBuffers(1, &batch->params_buffer);
        rk_uint layout = rk_current_vertices->layout;
        rk_packer * packer = batch->packers;
        rk_uint offset = 0;
        for (rk_param_format const * f = params_format; *f; ++f, ++layout, ++packer) {
            GLboolean const normalize = (*f & RK_PARAM_FORMAT_NORMALIZE) != 0;
            glEnableVertexAttribArray(layout);
            switch (*f & RK_PARAM_FORMAT_MASK) {
                case RK_PARAM_FORMAT_VEC3_FLOAT:
                    glVertexAttribFormat(layout, 3, GL_FLOAT, normalize, offset);
                    packer->pack = reinterpret_cast<rk_packer_fn>(rk_pack_vec3_float);
                    packer->dst_incr = sizeof(rk_vec3);
                    packer->src_incr = sizeof(rk_vec3);
                    break;
                case RK_PARAM_FORMAT_VEC3_SHORT:
                    glVertexAttribFormat(layout, 3, GL_SHORT, normalize, offset);
                    if (normalize) {
                        packer->pack = reinterpret_cast<rk_packer_fn>(rk_pack_vec3_short_norm);
                    } else {
                        packer->pack = reinterpret_cast<rk_packer_fn>(rk_pack_vec3_short);
                    }
                    packer->dst_incr = sizeof(rk_vec3_short);
                    packer->src_incr = sizeof(rk_vec3);
                    break;
                case RK_PARAM_FORMAT_VEC3_INT10:
                    glVertexAttribFormat(layout, 4, GL_INT_2_10_10_10_REV, normalize, offset);
                    packer->pack = reinterpret_cast<rk_packer_fn>(rk_pack_vec3_int10);
                    packer->dst_incr = sizeof(rk_int);
                    packer->src_incr = sizeof(rk_vec3);
                    break;
            }
            offset += packer->dst_incr;
            glVertexAttribBinding(layout, RK_PARAMS_BINDING);
        }
        glVertexBindingDivisor(RK_PARAMS_BINDING, 1);
        glBindVertexBuffer(RK_PARAMS_BINDING, batch->params_buffer, 0, batch->packed_size);
    }
    if (rk_MultiDrawElementsIndirect) {
        glGenBuffers(1, &batch->commands_buffer);
    }
    return batch;
}

void rk_begin_frame() {
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
}

void rk_select_texture(
    rk_texture_t _texture) {
    rk_texture const * const texture = reinterpret_cast<rk_texture const *>(_texture);
    if (texture) {
        glActiveTexture(GL_TEXTURE0 + texture->slot);
        if (texture->nlevels) {
            glBindTexture(GL_TEXTURE_2D_ARRAY, texture->texture);
        } else {
            glBindTexture(GL_TEXTURE_2D, texture->texture);
        }
        if (texture->sampler > -1) {
            glUniform1i(texture->sampler, texture->slot);
        }
    }
}

RK_EXPORT void rk_draw_triangles(
    rk_triangles_t _triangles) {
    rk_triangles const * const triangles = reinterpret_cast<rk_triangles const *>(_triangles);
    if (triangles && rk_current_shader && !rk_current_vertices) {
        glBindVertexArray(triangles->array);
        glDrawArrays(GL_TRIANGLES, 0, triangles->size);
        glBindVertexArray(0);
    }
}

void rk_select_vertices(
    rk_vertices_t _vertices) {
    rk_vertices * const vertices = reinterpret_cast<rk_vertices *>(_vertices);
    if (vertices && rk_current_shader) {
        glBindVertexArray(vertices->array);
        rk_current_vertices = vertices;
    }
}

static rk_uint rk_batch_filter(
    rk_batch & batch,
    rk_uint const size,
    rk_instance_flags const * flags) {
    rk_ushort * indices = batch.indices;
    for (rk_ushort index = 0; index < size; ++index, ++flags) {
        if ((*flags & RK_INSTANCE_FLAGS_SPAWNED_VISIBLE) == RK_INSTANCE_FLAGS_SPAWNED_VISIBLE) {
            *indices++ = index;
        }
    }
    return indices - batch.indices;
}

static rk_uint rk_batch_build_commands(
    rk_batch & batch,
    rk_uint const ninstances,
    rk_mesh const * const meshes) {
    rk_command * commands = batch.commands;
    rk_ushort * base = batch.indices;
    rk_ushort * const last = batch.indices + ninstances;
    for (rk_ushort * first = batch.indices; first < last; base = first, ++commands) {
        rk_mesh const & mesh = meshes[*first++];
        for ( ; first < last && meshes[*first].packed == mesh.packed; ++first) {
        }
        for (rk_ushort * second = first; second < last; ++second) {
            rk_ushort const index = *second;
            if (meshes[index].packed == mesh.packed) {
                *second = *first;
                *first++ = index;
            }
        }
        commands->nvertices = static_cast<GLuint>(mesh.ntriangles) * 3;
        commands->ninstances = first - base;
        commands->base_index = mesh.base_index;
        commands->base_vertex = 0;
        commands->base_instance = base - batch.indices;
    }
    return commands - batch.commands;
}

static void rk_batch_convert_params(
    rk_batch & batch,
    rk_uint const ninstances,
    rk_ubyte const * const params) {
    rk_ubyte * __restrict dst = batch.params;
    rk_ushort const * const last_index = batch.indices + ninstances;
    rk_packer const * const last_packer = batch.packers + batch.nparams;
    for (rk_ushort const * index = batch.indices; index < last_index; ++index) {
        rk_ubyte const * __restrict src = &params[batch.params_size * (*index)];
        for (rk_packer const * packer = batch.packers; packer < last_packer; ++packer) {
            packer->pack(dst, src);
            dst += packer->dst_incr;
            src += packer->src_incr;
        }
    }
}

void rk_draw_batch(
    rk_batch_t _batch,
    rk_uint size,
    rk_instance_flags const * flags,
    rk_mesh const * meshes,
    rk_ubyte const * params) {
    rk_batch & batch = *reinterpret_cast<rk_batch *>(_batch);
    if (!size || size > batch.size || !flags || !meshes || !rk_current_shader || !rk_current_vertices) {
        return;
    }
    rk_uint const ninstances = rk_batch_filter(batch, size, flags);
    if (!ninstances) {
        return;
    }
    rk_uint const ncommands = rk_batch_build_commands(batch, ninstances, meshes);
    if (rk_MultiDrawElementsIndirect) {
        glBindBuffer(GL_DRAW_INDIRECT_BUFFER, batch.commands_buffer);
        glBufferData(GL_DRAW_INDIRECT_BUFFER, ncommands * sizeof(rk_command), batch.commands, GL_STREAM_DRAW);
    }
    if (batch.nparams) {
        rk_batch_convert_params(batch, ninstances, params);
        glBindBuffer(GL_ARRAY_BUFFER, batch.params_buffer);
        glBufferData(GL_ARRAY_BUFFER, ninstances * batch.packed_size, batch.params, GL_STREAM_DRAW);
        glBindBuffer(GL_ARRAY_BUFFER, 0);
    }
    if (rk_DrawElementsInstancedBaseInstance) {
        if (rk_MultiDrawElementsIndirect) {
            rk_MultiDrawElementsIndirect(GL_TRIANGLES, GL_UNSIGNED_SHORT, nullptr, ncommands, sizeof(rk_command));
            glBindBuffer(GL_DRAW_INDIRECT_BUFFER, 0);
        } else {
            rk_command const * const last_command = batch.commands + ncommands;
            for (rk_command const * command = batch.commands; command < last_command; ++command) {
                rk_DrawElementsInstancedBaseInstance(
                    GL_TRIANGLES, command->nvertices, GL_UNSIGNED_SHORT,
                    reinterpret_cast<void const *>(command->base_index << 1),
                    command->ninstances, command->base_instance);
            }
        }
    } else {
        rk_uint params_offset = 0;
        rk_command const * const last_command = batch.commands + ncommands;
        for (rk_command const * command = batch.commands; command < last_command; ++command) {
            if (batch.nparams) {
                glBindVertexBuffer(RK_PARAMS_BINDING, batch.params_buffer, params_offset, batch.packed_size);
                params_offset += command->ninstances * batch.packed_size;
            }
            glDrawElementsInstanced(
                GL_TRIANGLES, command->nvertices, GL_UNSIGNED_SHORT,
                reinterpret_cast<void const *>(command->base_index << 1),
                command->ninstances);
        }
    }
}

void rk_unselect_vertices(
    rk_vertices_t _vertices) {
    rk_current_vertices = nullptr;
    glBindVertexArray(0);
}

void rk_unselect_texture(
    rk_texture_t _texture) {
    rk_texture const * const texture = reinterpret_cast<rk_texture const *>(_texture);
    if (texture) {
        glActiveTexture(GL_TEXTURE0 + texture->slot);
        if (texture->nlevels) {
            glBindTexture(GL_TEXTURE_2D, 0);
        } else {
            glBindTexture(GL_TEXTURE_2D_ARRAY, 0);
        }
    }
}

void rk_unselect_shader(
    rk_shader_t _shader) {
    rk_current_shader = nullptr;
    glUseProgram(0);
}

void rk_end_frame() {
    rk_swap_buffers();
}

void rk_destroy_batch(
    rk_batch_t _batch) {
    rk_batch * const batch = reinterpret_cast<rk_batch *>(_batch);
    if (batch) {
        delete[] batch->indices;
        delete[] batch->commands;
        if (batch->nparams) {
            delete[] batch->packers;
            delete[] batch->params;
            glDeleteBuffers(1, &batch->params_buffer);
        }
        if (rk_MultiDrawElementsIndirect) {
            glDeleteBuffers(1, &batch->commands_buffer);
        }
        delete batch;
    }
}

void rk_destroy_triangles(
    rk_triangles_t _triangles) {
    rk_triangles * const triangles = reinterpret_cast<rk_triangles *>(_triangles);
    if (triangles) {
        glDeleteBuffers(1, &triangles->vertices);
        glDeleteVertexArrays(1, &triangles->array);
        delete triangles;
    }
}

void rk_destroy_vertices(
    rk_vertices_t _vertices) {
    rk_vertices * const vertices = reinterpret_cast<rk_vertices *>(_vertices);
    if (vertices) {
        glDeleteBuffers(1, &vertices->indices);
        glDeleteBuffers(1, &vertices->vertices);
        glDeleteVertexArrays(1, &vertices->array);
        delete vertices;
    }
}

void rk_destroy_texture(
    rk_texture_t _texture) {
    rk_texture * const texture = reinterpret_cast<rk_texture *>(_texture);
    if (texture) {
        glDeleteTextures(1, &texture->texture);
        delete texture;
    }
}

void rk_destroy_shader(
    rk_shader_t _shader) {
    rk_shader * const shader = reinterpret_cast<rk_shader *>(_shader);
    if (shader) {
        glDeleteShader(shader->vertex);
        glDeleteShader(shader->fragment);
        glDeleteProgram(shader->program);
        delete shader;
    }
}

void rk_terminate() {
    rk_destroy_context();
}