/*
 * Copyright (c) 2023 Emma Tebibyte <emma@tebibyte.media>
 * SPDX-License-Identifier: AGPL-3.0-or-later
 *
 * This file is part of YAC coreutils.
 *
 * YAC coreutils is free software: you can redistribute it and/or modify it
 * under the terms of the GNU Affero General Public License as published by the
 * Free Software Foundation, either version 3 of the License, or (at your
 * option) any later version.
 * 
 * YAC coreutils is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
 * FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
 * details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program. If not, see https://www.gnu.org/licenses/.
 */

#include <errno.h>
#include <sysexits.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#include "yac.h"

void tailc(FILE *file, long num) {
	int byte;
	char buf[num];
	long offset = -num;

	fseek(file, offset, SEEK_END);
	for (int i = 0; (byte = fgetc(file)) != EOF; i++) {
		buf[i] = byte;
	}

	fputs(buf, stdout);
}

void tailf(FILE *file) {
	int byte;

	while(true) {
		if ((byte = fgetc(file)) != EOF) { putchar(byte); }
	}
}

void tailn(FILE *file, long num) {
	char *buf = calloc(4096, 1);
	char *lines[num];
	int byte;
	int lc = 0;

	for (int bc = 0; (byte = fgetc(file)) != EOF; bc++) {
		if (bc == sizeof(buf)) {
			int err;
			if (err = realloc(buf, sizeof(buf) * 2) == NULL) {
				// TODO: error handling
			}
		}

		buf[bc] = byte;

		if (byte == '\n') {
			lines[lc] = buf;
			bc = 0;
			lc++;
		}

		if (lc == num) {
			for (int i = 0; i < lc; i++) {
				lines[i] = lines[i + 1];
			}
			lc--;
		}
	}

	int i;
	if ((i = lc - num) < 0) { i = 0; }

	for (; i < lc; i++) {
		fputs(lines[i], stdout);
	}
}

int main(int argc, char *argv[]) {
	bool c = false;
	bool f = false;
	bool n = false;
	int i;
	int opt;
	long num;
	void (*fn)(FILE *, long) = tailn;

	extern int optind;
	while ((opt = getopt(argc, argv, "c:fn:")) != -1) {
		switch (opt) {
		/*
		 * From tail(1p):
		 *
		 * -c number The application shall ensure that the number option-argument
		 *           is a decimal integer, optionally including a sign.  The sign
		 *           shall affect the location in the file, measured in bytes, to
		 *           begin the copying:
		 *
		 *                  ┌─────┬────────────────────────────────────────┐
		 *                  │Sign │             Copying Starts             │
		 *                  ├─────┼────────────────────────────────────────┤
		 *                  │ +   │ Relative to the beginning of the file. │
		 *                  │ -   │ Relative to the end of the file.       │
		 *                  │none │ Relative to the end of the file.       │
		 *                  └─────┴────────────────────────────────────────┘
		 *           The application shall ensure that if the sign of the number
		 *           option-argument is '+', the number option-argument is a non-
		 *           zero decimal integer.
		 *
		 *           The origin for counting shall be 1; that is, -c +1 represents
		 *           the first byte of the file, -c -1 the last.
		 *
		 * -f        If the input file is a regular file or if the file operand
		 *           specifies a FIFO, do not terminate after the last line of the
		 *           input file has been copied, but read and copy further bytes
		 *           from the input file when they become available. If no file
		 *           operand is specified and standard input is a pipe or FIFO,
		 *           the -f option shall be ignored. If the input file is not a
		 *           FIFO, pipe, or regular file, it is unspecified whether or not
		 *           the -f option shall be ignored.
		 *
		 * -n number This option shall be equivalent to -c number, except the
		 *  starting location in the file shall be measured in lines
		 *  instead of bytes. The origin for counting shall be 1; that
		 *  is, -n +1 represents the first line of the file, -n -1 the
		 *  last.
		 *
		 *  If neither -c nor -n is specified, -n 10 shall be assumed.
		 */
			case 'c':
				c = true;
				fn = tailc;
				num = (long)optarg;
				break;
			case 'f':
				f = true;
			case 'n':
				n = true;
				num = (long)optarg;
				break;
			default:
				fprintf(
					stderr,
					"Usage: %s (-f) [-c characters] [-n lines] file...\n",
					argv[0]
				);
				return EX_USAGE;
		}
	}

	if (!n && !c) {
		num = 10;
	} else if (n && c) {
		fprintf(
			stderr,
			"Usage: %s (-f) [-c characters] [-n lines] file...\n",
			argv[0]
		);
		return EX_USAGE;
	}

	FILE *file;

	if (optind = argc) {
		fn(stdin, num);

		if (f) { tailf(stdin); }
	} else {
		for (i = optind; i < argc; i++) {
			if ((file = rpath(argv[0], argv[i])) != NULL) {
				fn(file, num);
				fclose(file);
			}

			if (f) { tailf(file); }
		}
	}
	return EX_OK;
}