Stucky dithering as custom CIKernel does not work - ios

Question

I'm trying to implement Stucky dithering (error diffusion) as a CIKernel but I am a bit lost. I don't find a way to debug the filter (I m new to CIKernel).
Here is what I have came up so far, but the kernel does not compile, plus, I wonder how I could store the spreaded error to the destination pixels.
Help welcome. Is there a tool/way to trace or debug the code?
float mDiffCoef = [
0, 0 0, 8/42, 4/42,
2/42, 4/42, 8/42, 4/42, 2/42,
1/42, 2/42, 4/42, 2/42, 1/42
];
kernel vec4 dither(__sample image) {
vec2 coord = samplerCoord(image);
vec4 pixel = sample(image, coord);
// decompress pixel into rgb
float red = pixel.r;
float color = 0;
if (red >= 127) color = 255;
float err = red - color;
// Spread the error according to the matrix
for (int x = -2; x <= 2; x++) {
for (int y = 0; y <= 3; y++) {
vec2 workingSpaceCoordinate = destCoord() + vec2(x,y);
vec2 imageSpaceCoordinate = samplerTransform(image, workingSpaceCoordinate);
vec3 color = sample(image, imageSpaceCoordinate).rgb;
color += err * mDiffCoef[(2+x)+5*y];
}
}
return vec4(color, 1.0);
}
[Updated code to remove syntax errors]:
kernel vec4 dither(__sample image) {
float mDiffCoef[3 * 5];
int i = 0;
mDiffCoef[i++] = 0.;
mDiffCoef[i++] = 0.;
mDiffCoef[i++] = 0.;
mDiffCoef[i++] = 8./42.;
mDiffCoef[i++] = 4./42.;
mDiffCoef[i++] = 2./42.;
mDiffCoef[i++] = 4./42.;
mDiffCoef[i++] = 8./42.;
mDiffCoef[i++] = 4./42.;
mDiffCoef[i++] = 2./42.;
mDiffCoef[i++] = 1./42.;
mDiffCoef[i++] = 2./42.;
mDiffCoef[i++] = 4./42.;
mDiffCoef[i++] = 2./42.;
mDiffCoef[i++] = 1./42.;
vec2 coord = samplerCoord(image);
vec4 pixel = sample(image, coord);
// decompress pixel into rgb
float red = pixel.r;
float c = 0.;
if (red >= 127.) c = 255.;
float err = red - c;
float color = 0.;
// Spread the error according to the matrix
for (int x = -2; x <= 2; x++) {
for (int y = 0; y < 3; y++) {
vec2 workingSpaceCoordinate = destCoord() + vec2(x,y);
vec2 imageSpaceCoordinate = samplerTransform(image, workingSpaceCoordinate);
float c = sample(image, imageSpaceCoordinate).r; //gb;
color = c + err * mDiffCoef[(2+x)+5*y];
}
}
color = clamp(color, 0., 255.);
return vec4(255., color, color, 1.);
My main issue is how can I write the outpu pixel buffers by cumulating portion of value from the error diffusion?