Day 8: Resonant Collinearity
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C
Not hard but a little fiddly.
Code
#include "common.h"
#define GZ 52
static char g[GZ][GZ];
#define ANTI_P1 1
#define ANTI_P2 2
static uint8_t anti[GZ][GZ];
static int w,h;
int
main(int argc, char **argv)
{
int p1=0,p2=0, x,y, x1,y1, ax,ay, i;
char *lf;
if (argc > 1)
DISCARD(freopen(argv[1], "r", stdin));
for (h=0; h<GZ && fgets(g[h], GZ, stdin); h++)
;
assert(feof(stdin));
lf = strchr(g[0], '\n');
assert(lf);
w = lf - g[0];
/*
* Find antenna pairs, then project backwards from the first,
* forwards from the second. Don't like the repetition but it
* makes for easy code.
*/
for (y=0; y<h; y++)
for (x=0; x<w; x++) {
if (!isalnum(g[y][x]))
continue;
for (y1=y; y1<h; y1++)
for (x1=(y==y1?x+1:0); x1<w; x1++) {
if (g[y][x] != g[y1][x1])
continue;
for (i=0; ; i++) {
if ((ax = x-(x1-x)*i) <0 || ax>w ||
(ay = y-(y1-y)*i) <0 || ay>h)
break;
anti[ay][ax] |= ANTI_P1 * i==1;
anti[ay][ax] |= ANTI_P2;
}
for (i=0; ; i++) {
if ((ax = x1+(x1-x)*i) <0 || ax>w ||
(ay = y1+(y1-y)*i) <0 || ay>h)
break;
anti[ay][ax] |= ANTI_P1 * i==1;
anti[ay][ax] |= ANTI_P2;
}
}
}
for (y=0; y<h; y++)
for (x=0; x<w; x++) {
p1 += !!(anti[y][x] & ANTI_P1);
p2 += !!(anti[y][x] & ANTI_P2);
}
printf("08: %d %d\n", p1, p2);
return 0;
}
