#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>


int head(int fd, int lines, const char * name)
{
	static const size_t SIZE = 1024 * 4;
	char buffer[SIZE];
	while (lines > 0) {
		const ssize_t available = read(fd, buffer, SIZE);
		if (-1 == available) {
			perror(name);
			return 1;
		}
		if (0 == available)
			return 0;
		size_t offset = 0;
		for (ssize_t n = 0; n < available; ++n) {
			if (buffer[n] == '\n') {
				lines--;
				if (0 <= lines) {
					const ssize_t res = write(1, buffer + offset, 1 + n - offset);
					if (-1 == res) {
						perror(name);
						return 1;
					}
					offset = 1 + n;
				}
				if (0 >= lines)
					return 0;
			}
		}
		if (0 < lines) {
			const ssize_t res = write(1, buffer + offset, available - offset);
			if (-1 == res) {
				perror(name);
				return 1;
			}
		}
	}
	return 0;
}


int main(int argc, char * argv[])
{
	int opt;
	int lines = 10;
	while (-1 != (opt = getopt(argc, argv, "n:"))) {
		switch (opt) {
		case 'n':
			errno = 0;
			lines = strtol(optarg, (char **) NULL, 10);
			if (lines < 0)
				errno = EINVAL;
			if (0 != errno) {
				perror(optarg);
				return 1;
			}
			break;
		default:
			dprintf(2, "Usage: %s [-n lines] [file...]\n", argv[0]);
			return 1;
		}
	}
	const unsigned int file_count = argc - optind;
	int res = 0;
	if (optind >= argc)
		res |= head(0, lines, "-");
	else for (int i = optind; i < argc; ++i) {
		if (1 < file_count)
			printf("==> %s <==\n", argv[i]);
		const unsigned int is_stdin = '-' == argv[i][0] && 0 == argv[i][1];
		const int fd = is_stdin ? 0 : open(argv[i], O_RDONLY);
		if (-1 == fd) {
			perror(argv[i]);
			res = 1;
			continue;
		}
		res |= head(fd, lines, argv[i]);
		if (!is_stdin) {
			const int clr = close(fd);
			if (-1 == clr) {
				perror(argv[i]);
				res = 1;
			}
		}
	}
	return res;
}