#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <stdint.h>
#include <inttypes.h>
#include <errno.h>
#include <fcntl.h>
#include <err.h>


static inline uint64_t
rdtsc(void)
{
    uint32_t low, high;
    __asm__ __volatile__("rdtsc" : "=a"(low), "=d"(high));
    return low | ((uint64_t) high) << 32;
}


void *
mmapfile(
    const char * filename,
    uint64_t len
)
{
    int perms = 0666;
    int open_flag = O_RDWR | O_CREAT;
    int mmap_flags = PROT_READ | PROT_WRITE;

    const int fd = open(filename, open_flag, perms);
    if (fd < 0)
	goto fail;

    // Ensure that the file is empty and the right size
    if (ftruncate(fd, 0) < 0)
	goto fail;

    if (ftruncate(fd, len) < 0)
	goto fail;

    // Map the entire actual length of the file
    void * const base = mmap(
	NULL,
	len,
	mmap_flags,
	MAP_SHARED | MAP_POPULATE,
	fd,
	0
    );
    if (base == MAP_FAILED)
	goto fail;

    close(fd);
    return base;

fail:
    err(1, "%s: Unable to map %"PRIu64" bytes", filename, len);
}


int main(
    int argc,
    char ** argv
)
{
    const char * filename = argv[1];
    const uint64_t len = argc > 2 ? strtoul(argv[2], NULL, 0) : (5ul << 30);
    const uint64_t max_index = len / sizeof(uint64_t);

    uint64_t mmap_time = -rdtsc();
    uint64_t * const buf = mmapfile(filename, len);
    mmap_time += rdtsc();
    fprintf(stderr, "%s: mapped %"PRIu64" bytes in %"PRIu64" ticks\n",
	filename,
	len,
	mmap_time
    );

    while (1)
    {
	uint64_t max = 0;
	uint64_t sum = 0;
	uint64_t i;
	const uint64_t loop_start = rdtsc();
	const uint64_t iters = 1 << 30;

	uint64_t start = loop_start;
	for (i = 0 ; i < iters ; i++)
	{
	    uint64_t i = lrand48() % max_index;
	    buf[i] += start;

	    uint64_t end = rdtsc();
	    const uint64_t delta = end - start;
	    start = end;

	    sum += delta;
	    if (delta > max)
		max = delta;

	    // Force a report every 10 billion ticks ~= 3 seconds
	    if (end - loop_start > 10e9)
		break;
	}

	printf("%"PRIu64": avg %"PRIu64" max %"PRIu64" ticks\n",
	    i,
	    i ? sum / i : 0,
	    max
	);
    }

    return 0;
}