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#include <stdio.h>
#include <stdlib.h>
#define C_EMPTY 0x00
#define C_BLZ_N 0x01
#define C_BLZ_E 0x02
#define C_BLZ_S 0x04
#define C_BLZ_W 0x08
#define C_BLZ (C_BLZ_N | C_BLZ_E | C_BLZ_S | C_BLZ_W)
#define C_WALL 0x10
#define C_ELF 0x20
unsigned char *
pos(unsigned char * buf, int cols, int rows, int i, int j)
{
i = ((i % cols) + cols) % cols;
j = ((j % rows) + rows) % rows;
return buf + i + j * cols;
}
int
main()
{
unsigned char * buf = NULL, * next = NULL;
int bufsiz = 0, buflen = 0;
int cols = -1, rows = -1, exit_col = -1, exit_row = -1;
int i, j, round;
// Read input
while (1) {
if (bufsiz - buflen < BUFSIZ) {
void * p;
bufsiz += BUFSIZ;
if ((p = realloc(buf, bufsiz)) == NULL) {
fprintf(stderr, "Bad realloc(%d)\n", bufsiz);
free(buf);
return -1;
}
buf = p;
}
if ((i = fgetc(stdin)) == EOF)
break;
switch (i) {
case '#':
buf[buflen++] = C_WALL;
break;
case '.':
if (cols == -1)
buf[buflen++] = C_ELF;
else
buf[buflen++] = C_EMPTY;
break;
case '^':
buf[buflen++] = C_BLZ_N;
break;
case '>':
buf[buflen++] = C_BLZ_E;
break;
case 'v':
buf[buflen++] = C_BLZ_S;
break;
case '<':
buf[buflen++] = C_BLZ_W;
break;
case '\n':
if (cols == -1)
cols = buflen;
break;
default:
fprintf(stderr, "Unexpected input char %c\n", i);
return -1;
}
}
if (cols == -1 || buflen % cols != 0) {
fprintf(stderr, "Input size %d is not a multiple of row length %d\n",
buflen, cols);
free(buf);
return -1;
}
rows = buflen / cols;
// Find the exit position
exit_row = rows - 1;
for (i = 0; i < cols; ++i) {
if (*pos(buf, cols, rows, i, exit_row) == C_EMPTY)
exit_col = i;
}
if (exit_col == -1) {
fprintf(stderr, "No exit position found!\n");
free(buf);
return -1;
}
// Create the "next" buffer, and place the walls in it.
if ((next = calloc(buflen, 1)) == NULL) {
fprintf(stderr, "Bad next alloc(%d)\n", buflen);
free(buf);
return -1;
}
for (i = 0; i < buflen; ++i) {
if (buf[i] == C_WALL)
next[i] = C_WALL;
}
for (round = 0; (*pos(buf, cols, rows, exit_col, exit_row) & C_ELF) == 0; ++round) {
int elves = 0;
unsigned char * psrc, * pdest, * tmp;
// Work out the possible positions of elves and blizzards in `next`
for (j = 0; j < rows; ++j) {
for (i = 0; i < cols; ++i) {
int o;
psrc = pos(buf, cols, rows, i, j);
if (*psrc & C_ELF) {
*pos(next, cols, rows, i, j) |= C_ELF;
if ((*(pdest = pos(next, cols, rows, i, j - 1)) & C_WALL) == 0)
*pdest |= C_ELF;
if ((*(pdest = pos(next, cols, rows, i + 1, j)) & C_WALL) == 0)
*pdest |= C_ELF;
if ((*(pdest = pos(next, cols, rows, i, j + 1)) & C_WALL) == 0)
*pdest |= C_ELF;
if ((*(pdest = pos(next, cols, rows, i - 1, j)) & C_WALL) == 0)
*pdest |= C_ELF;
++elves;
}
if (*psrc & C_BLZ_N) {
for (o = 1; (*(pdest = pos(next, cols, rows, i, j - o)) & C_WALL) != 0; ++o)
;
*pdest |= C_BLZ_N;
}
if (*psrc & C_BLZ_E) {
for (o = 1; (*(pdest = pos(next, cols, rows, i + o, j)) & C_WALL) != 0; ++o)
;
*pdest |= C_BLZ_E;
}
if (*psrc & C_BLZ_S) {
for (o = 1; (*(pdest = pos(next, cols, rows, i, j + o)) & C_WALL) != 0; ++o)
;
*pdest |= C_BLZ_S;
}
if (*psrc & C_BLZ_W) {
for (o = 1; (*(pdest = pos(next, cols, rows, i - o, j)) & C_WALL) != 0; ++o)
;
*pdest |= C_BLZ_W;
}
}
}
if (elves == 0) {
fprintf(stderr, "Warning, ran out of elves after %d rounds!\n", round);
free(next);
free(buf);
return -1;
}
// Whereever there is a blizzard and and elf together, remove the elf
for (j = 0; j < rows; ++j) {
for (i = 0; i < cols; ++i) {
if ((*(pdest = pos(next, cols, rows, i, j)) & C_ELF) != 0
&& (*pdest & C_BLZ) != 0)
{
*pdest &= ~C_ELF;
}
}
}
// Swap next and buf, and clear `next` for next round
tmp = buf;
buf = next;
next = tmp;
for (j = 0; j < rows; ++j) {
for (i = 0; i < cols; ++i) {
if ((*(pdest = pos(next, cols, rows, i, j)) & C_WALL) == 0)
*pdest = C_EMPTY;
}
}
}
printf("Found exit after %d rounds\n", round);
// Tidy and exit
free(next);
free(buf);
return 0;
}
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