Halide: Reduction over a domain for the specific values - halide

Question

I got a func f(x, y, z) in which the values is either 1 and 0, and I need to get the the first 100 coordinates of the values which equals to 1, to reduction/update them to 0.
This is very simple to realize in c and other languages, However, I've been trying to solve it with Halide for a couple of days. Is there any Function or Algorithm that I can use to solve it in Halide Generators?

Answer 1

The question amounts to "How do I implement stream compaction in Halide?" There is much written on parallel stream compaction and it is somewhat non-trivial to do well. See this Stack Overflow answer on doing it in cuda for some discussion and references: CUDA stream compaction algorithm
An quick implementation of simple stream compaction in Halide using a prefix sum looks like so:
#include "Halide.h"
#include <iostream>
using namespace Halide;
static void print_1d(const Buffer<int32_t> &result) {
std::cout << "{ ";
const char *prefix = "";
for (int i = 0; i < result.dim(0).extent(); i++) {
std::cout << prefix << result(i);
prefix = ", ";
}
std::cout << "}\n";
}
int main(int argc, char **argv) {
uint8_t vals[] = {0, 10, 99, 76, 5, 200, 88, 15};
Buffer<uint8_t> in(vals);
Var x;
Func prefix_sum;
RDom range(1, in.dim(0).extent() - 1);
prefix_sum(x) = (int32_t)0;
prefix_sum(range) = select(in(range - 1) > 42, prefix_sum(range - 1) + 1, prefix_sum(range - 1));
RDom in_range(0, in.dim(0).extent());
Func compacted_indices;
compacted_indices(x) = -1;
compacted_indices(clamp(prefix_sum(in_range), 0, in.dim(0).extent() - 1)) = select(in(in_range) > 42, in_range, - 1);
Buffer<int32_t> sum = prefix_sum.realize(8);
Buffer<int32_t> indices = compacted_indices.realize(8);
print_1d(sum);
print_1d(indices);
return 0;
}