#include "opengl.h"
#include <stdio.h>
#include <stdlib.h>

GLuint genComputeProg(GLuint texHandle) {
	// Creating the compute shader, and the program object containing the shader
    GLuint progHandle = glCreateProgram();
    GLuint cs = glCreateShader(GL_COMPUTE_SHADER);

	// In order to write to a texture, we have to introduce it as image2D.
	// local_size_x/y/z layout variables define the work group size.
	// gl_GlobalInvocationID is a uvec3 variable giving the global ID of the thread,
	// gl_LocalInvocationID is the local index within the work group, and
	// gl_WorkGroupID is the work group's index
	const char *csSrc[] = {
		"#version 430\n",
		"uniform float roll;\
		 uniform image2D destTex;\
		 layout (local_size_x = 16, local_size_y = 16) in;\
		 void main() {\
			 ivec2 storePos = ivec2(gl_GlobalInvocationID.xy);\
			 float localCoef = length(vec2(ivec2(gl_LocalInvocationID.xy)-8)/8.0);\
			 float globalCoef = sin(float(gl_WorkGroupID.x+gl_WorkGroupID.y)*0.1 + roll)*0.5;\
			 imageStore(destTex, storePos, vec4(1.0-globalCoef*localCoef, 0.0, 0.0, 0.0));\
		 }"
	};

    glShaderSource(cs, 2, csSrc, NULL);
	glCompileShader(cs);
    int rvalue;
    glGetShaderiv(cs, GL_COMPILE_STATUS, &rvalue);
    if (!rvalue) {
        fprintf(stderr, "Error in compiling the compute shader\n");
        GLchar log[10240];
        GLsizei length;
        glGetShaderInfoLog(cs, 10239, &length, log);
        fprintf(stderr, "Compiler log:\n%s\n", log);
        exit(40);
    }
    glAttachShader(progHandle, cs);

    glLinkProgram(progHandle);
    glGetProgramiv(progHandle, GL_LINK_STATUS, &rvalue);
    if (!rvalue) {
        fprintf(stderr, "Error in linking compute shader program\n");
        GLchar log[10240];
        GLsizei length;
        glGetProgramInfoLog(progHandle, 10239, &length, log);
        fprintf(stderr, "Linker log:\n%s\n", log);
        exit(41);
    }   
	glUseProgram(progHandle);
    
	glUniform1i(glGetUniformLocation(progHandle, "destTex"), 0);

	checkErrors("Compute shader");
	return progHandle;
}