#define _POSIX_C_SOURCE 200809L

#include <sys/types.h>

#include <err.h>
#include <errno.h>
#include <fcntl.h>
#include <inttypes.h>
#include <limits.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

static uint64_t
parse_number(const char *text, const char *what)
{
	char *end;
	uintmax_t value;

	errno = 0;
	value = strtoumax(text, &end, 0);
	if (errno != 0 || *text == '\0' || *end != '\0' || value > INT64_MAX)
		errx(2, "invalid %s: %s", what, text);
	return ((uint64_t)value);
}

static void
write_all(int fd, const unsigned char *buffer, size_t length)
{
	size_t done;
	ssize_t written;

	for (done = 0; done < length; done += (size_t)written) {
		written = write(fd, buffer + done, length - done);
		if (written < 0)
			err(1, "write output");
	}
}

static void
pread_range(const char *path, uint64_t raw_offset, uint64_t raw_length,
    const char *output)
{
	unsigned char buffer[65536];
	uint64_t done;
	ssize_t count;
	size_t request;
	int fd, outfd;

	if (raw_length > INT64_MAX - raw_offset)
		errx(2, "offset plus length overflows off_t");
	fd = open(path, O_RDONLY);
	if (fd < 0)
		err(1, "open %s", path);
	outfd = open(output, O_WRONLY | O_CREAT | O_TRUNC, 0600);
	if (outfd < 0)
		err(1, "open %s", output);
	for (done = 0; done < raw_length; done += (uint64_t)count) {
		request = raw_length - done < sizeof(buffer) ?
		    (size_t)(raw_length - done) : sizeof(buffer);
		count = pread(fd, buffer, request, (off_t)(raw_offset + done));
		if (count < 0)
			err(1, "pread %s at %ju", path,
			    (uintmax_t)(raw_offset + done));
		if (count == 0)
			errx(1, "unexpected EOF at %ju", (uintmax_t)(raw_offset + done));
		write_all(outfd, buffer, (size_t)count);
	}
	if (close(outfd) != 0 || close(fd) != 0)
		err(1, "close");
	printf("bytes=%ju offset=%ju\n", (uintmax_t)raw_length,
	    (uintmax_t)raw_offset);
}

static void
expect_error(const char *path, int expected)
{
	unsigned char buffer[65536];
	ssize_t count;
	int fd;

	fd = open(path, O_RDONLY);
	if (fd < 0) {
		if (errno != expected)
			errx(1, "open returned %s, expected errno %d",
			    strerror(errno), expected);
		printf("expected_errno=%d stage=open\n", expected);
		return;
	}
	for (;;) {
		errno = 0;
		count = read(fd, buffer, sizeof(buffer));
		if (count < 0)
			break;
		if (count == 0)
			errx(1, "read reached EOF without expected errno %d", expected);
	}
	if (errno != expected)
		errx(1, "read returned %s, expected errno %d",
		    strerror(errno), expected);
	if (close(fd) != 0)
		err(1, "close");
	printf("expected_errno=%d stage=read\n", expected);
}

int
main(int argc, char **argv)
{
	uint64_t offset, length, error_number;

	if (argc == 6 && strcmp(argv[1], "pread") == 0) {
		offset = parse_number(argv[3], "offset");
		length = parse_number(argv[4], "length");
		pread_range(argv[2], offset, length, argv[5]);
		return (0);
	}
	if (argc == 4 && strcmp(argv[1], "expect-error") == 0) {
		error_number = parse_number(argv[3], "errno");
		if (error_number == 0 || error_number > INT_MAX)
			errx(2, "errno is out of range");
		expect_error(argv[2], (int)error_number);
		return (0);
	}
	errx(2, "usage: %s pread FILE OFFSET LENGTH OUTPUT | "
	    "expect-error FILE ERRNO", argv[0]);
}