2017/advent20.c

   1 #define _GNU_SOURCE 1
   2 #include <stdio.h>
   3 #include <string.h>
   4 #include <stdlib.h>
   5 #include <unistd.h>
   6 #include <stdbool.h>
   7 #include <ctype.h>
   8
   9 #define MAX 1000 // determined by wc on the input
  10
  11 typedef struct particle particle;
  12 struct particle {
  13   int px;
  14   int py;
  15   int pz;
  16   int vx;
  17   int vy;
  18   int vz;
  19   int ax;
  20   int ay;
  21   int az;
  22   bool dead;
  23 } field[MAX];
  24
  25 int
  26 main (int argc, char **argv)
  27 {
  28   int minrow;
  29   int minvalue = 999999;
  30   particle *p = field;
  31   while (scanf ("p=<%d,%d,%d>, v=<%d,%d,%d>, a=<%d,%d,%d>\n", &p->px, &p->py,
  32                 &p->pz, &p->vx, &p->vy, &p->vz, &p->ax, &p->ay, &p->az) == 9) {
  33     int value = abs (p->ax) + abs (p->ay) + abs (p->az);
  34     if (value < minvalue) {
  35       minvalue = value;
  36       minrow = p - field;
  37     }
  38     p++;
  39   }
  40   printf ("closest particle among %d is %d\n", (int) (p - field), minrow);
  41   int gen = 0, i, j;
  42   bool done = false;
  43   while (!done) {
  44     // remove collisions
  45     done = true;
  46     for (i = 0; i < MAX - 1; i++) {
  47       bool dead = false;
  48       if (field[i].dead)
  49         continue;
  50       for (j = i + 1; j < MAX; j++)
  51         if (!field[j].dead && field[i].px == field[j].px &&
  52             field[i].py == field[j].py && field[i].pz == field[j].pz)
  53           dead = field[j].dead = true;
  54       field[i].dead = dead;
  55     }
  56     // move particles; keep iterating if position or velocity decreases
  57     int alive = 0;
  58     for (i = 0; i < MAX; i++) {
  59       p = &field[i];
  60       if (p->dead)
  61         continue;
  62       alive++;
  63       int oldp = abs (p->px) + abs (p->py) + abs (p->pz);
  64       int oldv = abs (p->vx) + abs (p->vy) + abs (p->vz);
  65       p->vx += p->ax;
  66       p->vy += p->ay;
  67       p->vz += p->az;
  68       p->px += p->vx;
  69       p->py += p->vy;
  70       p->pz += p->vz;
  71       if (oldp > abs (p->px) + abs (p->py) + abs (p->pz) ||
  72           oldv > abs (p->vx) + abs (p->vy) + abs (p->vz))
  73         done = false;
  74     }
  75     printf ("at time %d with %d particles\n", gen, alive);
  76     gen++;
  77   }
  78   printf ("all particles resolved\n");
  79   return 0;
  80 }