#include <stdio.h>
#include <limits.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdint.h>

int
read_f(int fd, void *buf, size_t size)
{
    char *p = buf;
    size_t s = 0;
    while(size && (s = read(fd, p, size))) {
        size -= s;
        p += s;
    }
    return s;
}

int
write_f(int fd, void *buf, size_t size)
{
    char *p = buf;
    size_t s = 0;
    while(size && (s = write(fd, p, size))) {
        size -= s;
        p += s;
    }
    return s;
}

struct FHeader
{
    char    magic[4];
    int32_t partnum;
};

struct FSection
{
    int32_t offset;
    int32_t size;
};

void
copy_section(int src, int dst, int32_t offset, int32_t size)
{
    size_t src_off = lseek(src, 0, SEEK_CUR);
    char *buf = calloc(size, sizeof(*buf));
    lseek(src, offset, SEEK_SET);
    read_f(src, buf, size);
    struct FHeader dsth;
    lseek(dst, 0, SEEK_SET);
    read_f(dst, &dsth, sizeof(dsth));
    struct FSection *desc = calloc(dsth.partnum, sizeof(*desc));
    int32_t new_offset = lseek(dst, 0, SEEK_END) - lseek(dst, 0, SEEK_SET) -
        dsth.partnum * sizeof(*desc);
    struct FSection new = { new_offset, size };
    lseek(dst, new_offset, SEEK_SET);
    read_f(dst, desc, dsth.partnum * sizeof(*desc));
    lseek(dst, 0, SEEK_SET);
    ++dsth.partnum;
    write_f(dst, &dsth, sizeof(dsth));
    lseek(dst, new_offset, SEEK_SET);
    write_f(dst, buf, size);
    write_f(dst, desc, (dsth.partnum - 1)* sizeof(*desc));
    write_f(dst, &new, sizeof(new));
    lseek(src, src_off, SEEK_SET);
}

int
main(int argc, char *argv[])
{
    enum { BASE = 10 };
    int src = open(argv[1], O_RDONLY, 0);
    struct FHeader h, mask = { {'C', 'M', 'C', '\x7f' }, 0};
    struct FSection sh;
    read_f(src, &h, sizeof(h));
    int filec = strtol(argv[2], NULL, BASE);
    char path[PATH_MAX];
    if(h.partnum < filec) {
        filec = 1;
    }
    int minor = h.partnum / filec, major = minor + 1;
    int c;
    lseek(src, -(h.partnum * sizeof(sh)), SEEK_END);
    for(int i = 0; i < filec; ++i) {
        if(i < h.partnum % filec) {
            c = major;
        } else {
            c = minor;
        }
        sprintf(path, "%s-%d", argv[1], i + 1);
        int dst = open(path, O_RDWR | O_CREAT | O_TRUNC, 0644);
        write_f(dst, &mask, sizeof(mask));
        for(int j = 0; j < c; ++j) {
            read_f(src, &sh, sizeof(sh));
            copy_section(src, dst, sh.offset, sh.size);
        }
        close(dst);
    }
    return 0;
}