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//#define _GNU_SOURCE
#include <ctype.h>
#include <search.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
struct point {
int x;
int y;
};
struct point *
point_create(int x, int y)
{
struct point * p = malloc(sizeof(struct point));
if (!p)
return NULL;
p->x = x;
p->y = y;
return p;
}
struct point *
point_copy(struct point const * other)
{
return point_create(other->x, other->y);
}
int
point_cmp(struct point const * a, struct point const * b)
{
return a->x - b->x != 0
? a->x - b->x
: a->y - b->y;
}
int
point_tcmp(void const * a, void const * b)
{
return point_cmp(a, b);
}
int
main(int argc, char ** argv)
{
int len = 2, i;
char buf[BUFSIZ];
struct point * rope;
void * tails = NULL;
int ntails = 0;
while ((i = getopt(argc, argv, "p:l:")) != -1) {
switch (i) {
case 'p':
switch (atoi(optarg)) {
case 1: len = 2; break;
case 2: len = 10; break;
default:
fprintf(stderr, "Unexpected part: %s\n", optarg);
return -1;
}
break;
case 'l':
len = atoi(optarg);
if (len < 2) {
fprintf(stderr, "Unexpected length: %d\n", len);
return -1;
}
break;
default:
return -1;
}
}
rope = malloc(len * sizeof(struct point));
for (i = 0; i < len; ++i) {
rope[i].x = rope[i].y = 0;
}
tsearch(point_create(0, 0), &tails, point_tcmp);
++ntails;
while (fgets(buf, sizeof(buf), stdin)) {
int dist;
if (!isalpha(buf[0])
|| buf[1] != ' '
|| !isdigit(buf[2]))
{
fprintf(stderr, "Unexpected input: %s\n", buf);
return -1;
}
dist = atoi(buf + 2);
if (dist == 0) {
fprintf(stderr, "Unexpected distance in: %s\n", buf);
// Allow 0 for now...
}
while (dist > 0) {
int seg;
switch (buf[0]) {
case 'L': --rope[0].x; break;
case 'R': ++rope[0].x; break;
case 'U': ++rope[0].y; break;
case 'D': --rope[0].y; break;
default:
fprintf(stderr, "Unexpected input: %s\n", buf);
return -1;
}
for (seg = 1; seg < len; ++seg) {
int dx, dy;
dx = rope[seg - 1].x - rope[seg].x;
dy = rope[seg - 1].y - rope[seg].y;
if (dx * dx + dy * dy <= 2)
break;
if (dx > 0)
++rope[seg].x;
if (dx < 0)
--rope[seg].x;
if (dy > 0)
++rope[seg].y;
if (dy < 0)
--rope[seg].y;
}
if (seg == len) {
// Had to move all segments, including tail.
// Check if it's a new tail pos
struct point * tail, ** pp;
// Search/insert current tail in visited tails
tail = point_copy(rope + len - 1);
pp = tsearch(tail, &tails, point_tcmp);
if (*pp == tail) {
// This was a new one, tail is now in the tree
++ntails;
}
else {
// Tail already in tree, free the one we just created.
free(tail);
}
}
--dist;
}
}
printf("Unique tail spots visited: %d\n", ntails);
free(rope);
#ifdef _GNU_SOURCE
tdestroy(tails, free);
#endif
return 0;
}
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