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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct pos {
int x;
int y;
struct pos * next;
};
struct pos *
pos_create(int x, int y, struct pos * next)
{
struct pos * p = malloc(sizeof(struct pos));
if (!p)
return NULL;
p->x = x;
p->y = y;
p->next = next;
return p;
}
void
pos_free(struct pos * p)
{
if (!p)
return;
pos_free(p->next);
free(p);
}
int
visited(unsigned char c)
{
return c & 0x80;
}
int
height(unsigned char c)
{
c &= 0x7F;
switch (c) {
case 'S': return 'a';
case 'E': return 'z';
}
return c;
}
int
tryvisit(unsigned char * range, int cols, int rows, struct pos const * from, int dx, int dy)
{
int x, y;
unsigned char src, dst;
x = from->x + dx;
y = from->y + dy;
// Check bounds.
if (x < 0 || x >= cols - 1
|| y < 0 || y >= rows)
return 0;
// Check if already visited
dst = range[x + y * cols];
if (visited(dst))
return 0;
// Check height
src = range[from->x + from->y * cols];
if (height(dst) > height(src) + 1)
return 0;
// OK. We can visit, so mark the new position as visited.
range[x + y * cols] |= 0x80;
return 1;
}
int
main()
{
unsigned char * range, * p;
size_t rangesize, rangelen, bytes;
int cols, rows;
struct pos * front;
int steps;
// Read hill range
rangesize = BUFSIZ;
rangelen = 0;
range = malloc(rangesize);
while ((bytes = fread(range + rangelen, 1, rangesize - rangelen, stdin)) > 0) {
rangelen += bytes;
if (rangelen == rangesize) {
rangesize += BUFSIZ;
if ((p = realloc(range, rangesize)) == NULL) {
fprintf(stderr, "realloc(%zu) failed\n", rangesize);
free(range);
return -1;
}
range = p;
}
}
// Get cols/rows in hill range
if ((p = memchr(range, '\n', rangelen)) == NULL) {
fprintf(stderr, "No lines found in input!\n");
free(range);
return -1;
}
cols = p - range + 1;
if (rangelen % cols != 0) {
fprintf(stderr, "Input length %zu is not a multiple of line length %d\n",
rangelen, cols);
free(range);
return -1;
}
rows = rangelen / cols;
// Find start, and mark it as visited
if ((p = memchr(range, 'S', rangelen)) == NULL) {
fprintf(stderr, "No start position!\n");
free(range);
return -1;
}
front = pos_create((p - range) % cols, (p - range) / cols, NULL);
range[front->x + front->y * cols] |= 0x80;
// Take steps, widening the search front each iteration.
for (steps = 0; 1; ++steps) {
struct pos * nextfront = NULL;
struct pos * loc;
for (loc = front; loc != NULL; loc = loc->next) {
if ((range[loc->x + loc->y * cols] & 0x7F) == 'E')
// We found the end point!
break;
if (tryvisit(range, cols, rows, loc, -1, 0))
nextfront = pos_create(loc->x - 1, loc->y, nextfront);
if (tryvisit(range, cols, rows, loc, +1, 0))
nextfront = pos_create(loc->x + 1, loc->y, nextfront);
if (tryvisit(range, cols, rows, loc, 0, -1))
nextfront = pos_create(loc->x, loc->y - 1, nextfront);
if (tryvisit(range, cols, rows, loc, 0, +1))
nextfront = pos_create(loc->x, loc->y + 1, nextfront);
}
if (loc) {
// Create a duplicate of the final location at the start
// of the current search front
front = pos_create(loc->x, loc->y, front);
pos_free(nextfront);
break;
}
pos_free(front);
front = nextfront;
}
printf("Found end at %d,%d in %d steps\n", front->x, front->y, steps);
pos_free(front);
free(range);
return 0;
}
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