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#define _GNU_SOURCE
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
// Proper modulo operator
#define modulo(n, m) ((((n) % (m)) + (m)) % (m))
enum direction {
D_NORTH = 3,
D_EAST = 0,
D_SOUTH = 1,
D_WEST = 2,
};
char const *
direction_name(enum direction dir)
{
switch (dir) {
case D_NORTH: return "North";
case D_EAST: return "East";
case D_SOUTH: return "South";
case D_WEST: return "West";
}
return NULL;
}
struct elf {
long pos;
enum direction dir;
};
int
elf_turn(struct elf * elf, char turn)
{
#define direction_go(d, t) (1000 * (d) + (t))
switch (direction_go(elf->dir, turn)) {
case direction_go(D_NORTH, 'R'):
case direction_go(D_SOUTH, 'L'):
elf->dir = D_EAST;
break;
case direction_go(D_NORTH, 'L'):
case direction_go(D_SOUTH, 'R'):
elf->dir = D_WEST;
break;
case direction_go(D_EAST, 'L'):
case direction_go(D_WEST, 'R'):
elf->dir = D_NORTH;
break;
case direction_go(D_EAST, 'R'):
case direction_go(D_WEST, 'L'):
elf->dir = D_SOUTH;
break;
default:
fprintf(stderr, "Unexpected elf_turn(%d, %c)\n", elf->dir, turn);
return -1;
}
return 0;
#undef direction_go
}
int
elf_move(struct elf * elf, char const * map, int cols, int rows, int distance)
{
int skip, wrap, base;
switch (elf->dir) {
case D_NORTH:
skip = -cols;
wrap = cols * rows;
break;
case D_EAST:
skip = 1;
wrap = cols;
break;
case D_SOUTH:
skip = cols;
wrap = cols * rows;
break;
case D_WEST:
skip = -1;
wrap = cols;
break;
default:
fprintf(stderr, "Unexpected direction %d\n", elf->dir);
return -1;
}
base = (elf->pos / wrap) * wrap;
while (distance > 0) {
int newpos = base + modulo(elf->pos + skip, wrap);
while (map[newpos] == ' ' || map[newpos] == '\n')
newpos = base + modulo(newpos + skip, wrap);
switch (map[newpos]) {
case '.':
elf->pos = newpos;
--distance;
break;
case '#':
distance = 0;
break;
default:
fprintf(stderr, "Unexpected map character %c\n", map[newpos]);
return -1;
}
}
return 0;
}
int
main()
{
char * buf = NULL, * beg, * end;
long bufsiz = 0, buflen = 0, maxline = 0, lines = 0;
long pos;
struct elf elf;
char distance[8], c;
int dlen = 0;
// Read map data
// Don't try and figure out lines yet, in case we get a line longer than
// our read buffer, which could make things awkward.
while (1) {
if (bufsiz - buflen < BUFSIZ / 2) {
void * p;
bufsiz += BUFSIZ;
if ((p = realloc(buf, bufsiz)) == NULL) {
fprintf(stderr, "Bad realloc(%ld)\n", bufsiz);
free(buf);
return -1;
}
buf = p;
}
if (!fgets(buf + buflen, bufsiz - buflen, stdin))
// End of file!
break;
if (buflen > 0 && buf[buflen] == '\n' && buf[buflen - 1] == '\n')
// End of map input.
break;
buflen += strlen(buf + buflen);
}
// Work out max lines and longest line.
for (beg = buf, lines = 0; beg < buf + buflen; beg = end + 1, ++lines) {
end = memchr(beg, '\n', buf + buflen - beg);
if (end - beg > maxline)
maxline = end - beg;
}
if (maxline == 0 || lines == 0) {
fprintf(stderr, "Unexpected map size (%ld,%ld)\n", maxline, lines);
free(buf);
return -1;
}
++maxline; // Also count the newline on the end of each line.
// Resize the map data buffer to hold a full grid of data
if ((beg = realloc(buf, lines * maxline)) == NULL) {
fprintf(stderr, "Bad realloc(%ld)\n", bufsiz);
free(buf);
return -1;
}
buf = beg;
memset(buf + buflen, ' ', lines * maxline - buflen);
// Move the map data so that it fits the grid properly
for (end = buf + buflen, pos = lines - 1; pos >= 0; end = beg, --pos) {
beg = memrchr(buf, '\n', (end - buf) - 1);
beg = beg ? beg + 1 : buf;
memmove(buf + pos * maxline, beg, (end - beg));
memset(buf + (pos * maxline) + (end - beg) - 1, ' ', maxline - (end - beg) + 1);
buf[pos * maxline + maxline - 1] = '\n';
}
// Set initial position
elf.pos = 0;
elf.dir = D_EAST;
while (buf[elf.pos] != '.')
++elf.pos;
// Read and follow the movement instructions.
while (elf.pos >= 0 && (c = fgetc(stdin)) != EOF) {
if (isdigit(c)) {
distance[dlen++] = c;
}
else if (c == 'L' || c == 'R') {
distance[dlen] = '\0';
if (elf_move(&elf, buf, maxline, lines, atoi(distance)) != 0)
elf.pos = -1;
if (elf_turn(&elf, c) != 0)
elf.pos = -1;
dlen = 0;
}
else if (c == '\n') {
// Cover last bit of distance, if any
distance[dlen] = '\0';
if (elf_move(&elf, buf, maxline, lines, atoi(distance)) != 0)
elf.pos = -1;
break;
}
else {
fprintf(stderr, "Unexpected movement instruction (%c)\n", c);
free(buf);
return -1;
}
}
if (elf.pos < 0) {
free(buf);
return -1;
}
// Done.
printf("Password is %ld (%ld,%ld,%d)\n",
1000 * (1 + elf.pos / maxline) + 4 * (1 + elf.pos % maxline) + elf.dir,
(1 + elf.pos % maxline), (1 + elf.pos / maxline), elf.dir);
// Tidy and exit.
free(buf);
return 0;
}
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