#include <stddef.h>

int replace_char(char *buf, int key, int *replacements_left) {
    (void)key;
    int replaced = 0;
    size_t i = 0;

    while (buf[i] != '\0') {
        if (buf[i] == '\n') {
            i++;
            continue;
        }

        // Detect start of a symbol run
        char symbol = buf[i];
        size_t run_start = i;
        size_t run_len = 1;

        // Count length of run
        while (buf[i + run_len] == symbol && buf[i + run_len] != '\n' && buf[i + run_len] != '\0') {
            run_len++;
        }

        // For pairs and longer runs: replace every 2nd, 4th, ...
        for (size_t j = 1; j < run_len && *replacements_left > 0; j += 2) {
            buf[run_start + j] = ' ';
            replaced++;
            (*replacements_left)--;
            if (*replacements_left == 0) break;
        }

        i += run_len;
    }
    return replaced;
}