#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>

struct thread_arg {
  const float *A;
  const float *B;
  float *C;
  int M;
  int N;
  int K;
  int num_threads;
  int rank; /* id of this thread */
} args[256];

static pthread_t threads[256];

static void *matmul_kernel(void *arg) {
  struct thread_arg *_arg = (struct thread_arg *)arg;
  const float *A = _arg->A;
  const float *B = _arg->B;
  float *C = _arg->C;
  int M = _arg->M;
  int N = _arg->N;
  int K = _arg->K;
  int num_threads = _arg->num_threads;
  int rank = _arg->rank;

  int start = ((M + num_threads - 1) / num_threads) * rank;
  int end = ((M + num_threads - 1) / num_threads) * (rank + 1);

  if (end >= M)
    end = M;
  for (int i = start; i < end; ++i) {
    for (int k = 0; k < K; ++k) {
      for (int j = 0; j < N; ++j) {
        C[i * N + j] += A[i * K + k] * B[k * N + j];
      }
    }
  }
  return NULL;
}

void matmul(const float *A, const float *B, float *C, int M, int N, int K,
            int num_threads) {
  int err;
  for (int t = 0; t < num_threads; ++t) {
    args[t].A = A, args[t].B = B, args[t].C = C, args[t].M = M, args[t].N = N,
    args[t].K = K, args[t].num_threads = num_threads, args[t].rank = t;
    err = pthread_create(&threads[t], NULL, matmul_kernel, (void *)&args[t]);
    if (err) {
      printf("pthread_create(%d) failed with err %d\n", t, err);
      exit(EXIT_FAILURE);
    }
  }

  for (int t = 0; t < num_threads; ++t) {
    err = pthread_join(threads[t], NULL);
    if (err) {
      printf("pthread_join(%d) failed with err %d\n", t, err);
      exit(EXIT_FAILURE);
    }
  }
}