The fastest Longest Line Length finding code in the West.

mmap_lll.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/mman.h>


void find_longest(const char *file, unsigned file_length,
                  const char **longest_start, unsigned *longest_len) {

    const char *cur_start = file;
    const char *cur = cur_start;
    unsigned file_off = 0;

    const char *longest_start_l;
    unsigned longest_len_l = 0;

    for (; file_off < file_length; ++cur, ++file_off) {
        if (__builtin_expect(*cur == '\n', 0)) {
            unsigned cur_length = cur - cur_start;
            if (cur_length > longest_len_l) {
                longest_start_l = cur_start;
                longest_len_l = cur_length;
            }
            cur_start = cur+1;
        }
    }

    unsigned cur_length = cur - cur_start;
    if (cur_length > *longest_len) {
        longest_start_l = cur_start;
        longest_len_l = cur_length;
    }

    *longest_len = longest_len_l;
    *longest_start = longest_start_l;
}

int main(int argc, char** argv) {
    if (argc != 2) {
        printf("Usage: %s <file>\n", argv[0]);
        return 2;
    }

    struct stat finfo;
    if (stat(argv[1], &finfo) < 0) {
        perror("stat");
        return 1;
    }

    int fd = open(argv[1], O_RDONLY);
    if (fd < 0) {
        perror("open");
        return 1;
    }

    char* filebuf = mmap(0, finfo.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
    if (filebuf == MAP_FAILED) {
        perror("mmap");
        return 1;
    }

    if (madvise(filebuf, finfo.st_size, MADV_SEQUENTIAL) < 0) {
        perror("madvise");
        return 1;
    }

    unsigned longest_len = 0;
    const char *longest_start = 0;
    find_longest(filebuf, finfo.st_size, &longest_start, &longest_len);

    printf("Longest line length: %d\n", longest_len);
    char *linebuffer = malloc(longest_len + 1);
    memcpy(linebuffer, longest_start, longest_len);
    linebuffer[longest_len] = '\0';
    printf("Line: %s\n", linebuffer);
}