#include "gbos.h"
/* tail [-n N] [N] [file]: last N lines (default 10) of a file or stdin.
 *
 * There is no seek syscall, so this cannot jump to the end: it streams the
 * whole input through a ring of N lines and prints the ring at EOF. The ring
 * is the memory budget - N lines of up to LINEW chars each - so N is capped. */

#define MAXN  12 /* 12 * 42 = 504 bytes of ring, fits a task */
#define LINEW 42

void main(void) {
    char *argv[8];
    char ring[MAXN][LINEW];
    unsigned char len[MAXN];
    unsigned char argc = argv_parse(argv, 8);
    char *nv = optval(argv, argc, 'n');
    unsigned char n = 10, i, cur = 0, filled = 0, fd = NOFD, l = 0;
    char *fname = 0;
    int ch;

    if (nv) n = atou(nv);
    for (i = 0; i < argc; i++) {
        if (argv[i][0] == '-') continue;
        if (nv == argv[i]) continue;
        if (!nv && argv[i][0] >= '0' && argv[i][0] <= '9') {
            n = atou(argv[i]);
            continue;
        }
        fname = argv[i];
        break;
    }
    if (n == 0) return;
    if (n > MAXN) n = MAXN;

    if (fname) {
        fd = open(fname, O_READ);
        if (fd == EISDIR) {
            puts("tail: is a directory");
            nl();
            sexit(2);
        }
        if (fd == NOFD) {
            puts("tail: no such file");
            nl();
            sexit(2);
        }
    }
    for (i = 0; i < MAXN; i++) len[i] = 0;

    for (;;) {
        if (fd == NOFD) {
            char c = readc();
            ch = (c == EOF) ? -1 : c;
        } else ch = fgetc(fd);
        if (ch == '\r') continue;
        if (ch >= 0 && ch != '\n') {
            if (len[cur] < LINEW) ring[cur][len[cur]++] = (char)ch;
            continue;
        }
        if (ch < 0 && len[cur] == 0) break;   /* EOF, nothing pending */
        cur = (unsigned char)((cur + 1) % n); /* keep only the last n */
        if (filled < n) filled++;
        len[cur] = 0;
        if (ch < 0) break;
    }
    if (fd != NOFD) close(fd);

    /* oldest kept line first: cur points at the (empty) next slot */
    i = (unsigned char)((cur + n - filled) % n);
    while (filled--) {
        writes(ring[i], len[i]);
        nl();
        i = (unsigned char)((i + 1) % n);
    }
}