Fast FizzBuzz

main.cpp

// compiled with gcc main.cpp -O3
#include<stdio.h>
#include<memory.h>

const int BUFFER_SIZE = 8092 * 4;
char BUFFER[BUFFER_SIZE];
short BUFFER_INDEX = 0;
const int BIGINT_SIZE = 2048;
char BIGINT[BIGINT_SIZE];
short BIGINT_INDEX = BIGINT_SIZE - 1;
short BIGINT_HEAD = BIGINT_SIZE - 1;

char FIZZ[5] = "fizz";
char BUZZ[5] = "buzz";
char FIZZBUZZ[9] = "fizzbuzz";

void incr() {
  BIGINT[BIGINT_INDEX] += 1;
  if (BIGINT[BIGINT_INDEX] == ':') {
    while (BIGINT[BIGINT_INDEX] == ':') {
      BIGINT[BIGINT_INDEX] = '0';
      BIGINT_INDEX -= 1;
      BIGINT[BIGINT_INDEX] += 1;
    }
    BIGINT_HEAD = BIGINT_INDEX < BIGINT_HEAD ? BIGINT_INDEX : BIGINT_HEAD;
    BIGINT_INDEX = BIGINT_SIZE - 1;
  }
}

void write_to_buffer(char * source, int start, int end) {

  int len = end - start;

  if (BUFFER_INDEX + len > BUFFER_SIZE - 2) {
    BUFFER_INDEX = 0;
    printf("%s", BUFFER);
    // only wipe the maximum size we'll write to the buffer
    // so we always have a terminator, but don't have to zero-out
    int reset_size = 10 > BIGINT_SIZE - BIGINT_HEAD ? 10 : BIGINT_SIZE - BIGINT_HEAD;
    memset(BUFFER + BUFFER_SIZE - reset_size, 0, reset_size);
  }

  memcpy(BUFFER + BUFFER_INDEX, source + start, len);
  BUFFER_INDEX += len;
  BUFFER[BUFFER_INDEX] = '\n';
  BUFFER_INDEX += 1;
}

int fizzbuzz_no_conditionals() {
  // initialize BIGINT
  memset(BIGINT, '0', BIGINT_SIZE);
  BIGINT[BIGINT_SIZE - 1] = '\0';
  memset(BUFFER, 0, BUFFER_SIZE);

  incr();
  while (true) {
    write_to_buffer(BIGINT, BIGINT_HEAD, BIGINT_SIZE);
    incr();
    write_to_buffer(BIGINT, BIGINT_HEAD, BIGINT_SIZE);
    incr();
    write_to_buffer(FIZZ, 0, 4);
    incr();
    write_to_buffer(BIGINT, BIGINT_HEAD, BIGINT_SIZE);
    incr();
    write_to_buffer(BUZZ, 0, 4);
    incr();
    write_to_buffer(FIZZ, 0, 4);
    incr();
    write_to_buffer(BIGINT, BIGINT_HEAD, BIGINT_SIZE);
    incr();
    write_to_buffer(BIGINT, BIGINT_HEAD, BIGINT_SIZE);
    incr();
    write_to_buffer(FIZZ, 0, 4);
    incr();
    write_to_buffer(BUZZ, 0, 4);
    incr();
    write_to_buffer(BIGINT, BIGINT_HEAD, BIGINT_SIZE);
    incr();
    write_to_buffer(FIZZ, 0, 4);
    incr();
    write_to_buffer(BIGINT, BIGINT_HEAD, BIGINT_SIZE);
    incr();
    write_to_buffer(BIGINT, BIGINT_HEAD, BIGINT_SIZE);
    incr();
    write_to_buffer(FIZZBUZZ, 0, 8);
    incr();
  }
  return 0;
}

int main() {
  fizzbuzz_no_conditionals();
  return 0;
}

peaks at 2 GiB/s on a core i7-9850H