// lab1_var12.c - Программа из лабораторной работы №1
// Вариант 12: замена символа первого в строке на пробелы
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>

#define RBUFSZ 4096
#define WBUFSZ 4096

static void die_perror(const char *what, const char *path, int exit_code) {
    int saved = errno;
    char msg[512];
    if (path) {
        snprintf(msg, sizeof(msg), "%s: %s: %s\n", what, path, strerror(saved));
    } else {
        snprintf(msg, sizeof(msg), "%s: %s\n", what, strerror(saved));
    }
    (void) write(STDERR_FILENO, msg, strlen(msg));
    _exit(exit_code);
}

static long long parse_ll(const char *s) {
    char *end = NULL;
    errno = 0;
    long long v = strtoll(s, &end, 10);
    if (errno != 0 || end == s || *end != '\0' || v < 0) {
        errno = EINVAL;
        return -1;
    }
    return v;
}

int main(int argc, char *argv[]) {
    if (argc != 4) {
        const char *usage =
                "Usage: lab1_var12 <input.txt> <output.txt> <max_replacements>\n"
                "Variant 12: per line, take the first character as key and replace its matches with spaces,\n"
                "            stopping after <max_replacements> replacements globally.\n";
        (void) write(STDERR_FILENO, usage, strlen(usage));
        return -1;
    }

    const char *in_path = argv[1];
    const char *out_path = argv[2];
    long long cap = parse_ll(argv[3]);
    if (cap < 0) {
        die_perror("invalid max_replacements", argv[3], -1);
    }

    int in_fd = open(in_path, O_RDONLY);
    if (in_fd < 0) {
        die_perror("open input failed", in_path, -1);
    }

    mode_t filePerms = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH; /* rw-rw-rw- */
    int out_fd = open(out_path, O_CREAT | O_WRONLY | O_TRUNC, filePerms);
    if (out_fd < 0) {
        die_perror("open output failed", out_path, -1);
    }

    char rbuf[RBUFSZ];
    char wbuf[WBUFSZ];
    size_t wlen = 0;

    long long total_replacements = 0;
    int at_line_start = 1;
    unsigned char line_key = 0;
    int replacing_enabled = 1; /* flips to 0 when cap reached */

    for (;;) {
        ssize_t n = read(in_fd, rbuf, sizeof(rbuf));
        if (n > 0) {
            for (ssize_t i = 0; i < n; i++) {
                unsigned char c = (unsigned char) rbuf[i];

                if (at_line_start) {
                    line_key = c;
                    at_line_start = 0;
                }

                unsigned char outc = c;
                if (c == '\n') {
                    at_line_start = 1;
                } else if (replacing_enabled && c == line_key) {
                    if (total_replacements < cap) {
                        outc = ' ';
                        total_replacements++;
                        if (total_replacements == cap) {
                            replacing_enabled = 0; /* hit the cap; from now on just copy */
                        }
                    } else {
                        replacing_enabled = 0;
                    }
                }

                if (wlen == sizeof(wbuf)) {
                    ssize_t wrote = write(out_fd, wbuf, wlen);
                    if (wrote < 0 || (size_t) wrote != wlen) {
                        die_perror("write failed", out_path, -1);
                    }
                    wlen = 0;
                }
                wbuf[wlen++] = (char) outc;
            }
        } else if (n == 0) {
            if (wlen > 0) {
                ssize_t wrote = write(out_fd, wbuf, wlen);
                if (wrote < 0 || (size_t) wrote != wlen) {
                    die_perror("write failed", out_path, -1);
                }
                wlen = 0;
            }
            break;
        } else {
            die_perror("read failed", in_path, -1);
        }
    }

    if (close(in_fd) < 0) {
        die_perror("close input failed", in_path, -1);
    }
    if (close(out_fd) < 0) {
        die_perror("close output failed", out_path, -1);
    }

    char res[64];
    int m = snprintf(res, sizeof(res), "%lld\n", total_replacements);
    if (m > 0) {
        (void) write(STDOUT_FILENO, res, (size_t) m);
    }

    return 0;
}