This December I participated in Advent of Code.

The day I didn’t finish within 24 hours.

Ah yes. Shortest path. Dijkstra. My first program for part 1 brute forced, but when I saw part 2, I rewrote part 1. And good thing too! I needed that later in the month.

Even then part 2 needed a long think. I ended up building a model of the map, where every cell occurred 4 times. The 4 parts of each cell represented the direction, I was headed. Going from “left” to “up” was expensive. On the other hand, going from my “up” to the cell above me, to its “up” part, was rather cheap. Anyway. Once I broke down and coded this the right way, the program ran fine. It took some time, but it ran. But this was still just part 1! The program I had now written 3 times.

Part 2 was replacing “<” with “<=”. Don’t just register a shortest path, register them all. And remember to register where this path was going. Which neighbor visited me, resulting in a shortest path. Because later, I needed to retrace all these shortest paths. Recursively.

Part 1:

<?php
function print_map() {
  global $map, $mapwid, $maphei;
  for($y=0;$y<$maphei;$y++) {
    for($x=0;$x<$mapwid;$x++) {
      if(isset($map[$y][$x])) {
        echo $map[$y][$x];
      } else {
        echo ".";
      }
    }
    echo "\n";
  }
  echo "\n";
}
function find($char) {
  global $map, $mapwid, $maphei;
  for($x=0;$x<$mapwid;$x++) {
    for($y=0;$y<$maphei;$y++) {
      if($map[$y][$x] == $char) {
        return array($x,$y);
      }
    }
  }
}
function find_short() {
  global $unvis, $maphei, $mapwid;
  $dist = 10000000;
  $newx = -1;
  $newy = -1;
  $newdir = "";
  for($x=0;$x<$mapwid;$x++) {
    if(!isset($unvis[$x])) {
      continue;
    }
    for($y=0;$y<$maphei;$y++) {
      if(!isset($unvis[$x][$y])) {
        continue;
      }
      foreach($unvis[$x][$y] as $dir => $val) {
        if($unvis[$x][$y][$dir][1] < $dist) {
          $newx = $x;
          $newy = $y;
          $newdir = $dir;
          $dist = $unvis[$x][$y][$dir][1];
        }
      }
    }
  }
  return array($newx, $newy, $newdir);
}
function isempty($map, $x, $y) {
  if($map[$y][$x] == ".") {
    return 1;
  } else {
    return 0;
  }
}
////////////////////////////////////////////////////////////////
$input = file_get_contents('./d16input2.txt', true);
foreach(preg_split("/((\r?\n)|(\r\n?))/", $input) as $line) {
  //print("$line\n");
  if(strlen($line)>2) {
    $map[] = str_split($line);
  }
}
$maphei = sizeof($map);
$mapwid = sizeof($map[0]);
// Assign to every node a distance from start value.
// explode map - every cell turns into 4 cells
// $map[$y][$x]
// $dimap[$x][$y][dir][type,dist,neigh]
for($x=0;$x<$mapwid;$x++) {
  for($y=0;$y<$maphei;$y++) {
    $dimap[$x][$y]["^"] = array($map[$y][$x], 10000000, array());
    $dimap[$x][$y][">"] = array($map[$y][$x], 10000000, array());
    $dimap[$x][$y]["v"] = array($map[$y][$x], 10000000, array());
    $dimap[$x][$y]["<"] = array($map[$y][$x], 10000000, array());
  }
}
// neigh: (x, y, type, dist)
// vertical neigh
for($x=0;$x<$mapwid;$x++) {
  for($y=1;$y<$maphei-1;$y++) {
    // neigh above?
    if($dimap[$x][$y-1]["^"][0] != "#") {
      $dimap[$x][$y]["^"][2][] = array($x, $y-1, "^", 1);
    }
    // neigh below?
    if($dimap[$x][$y+1]["v"][0] != "#") {
      $dimap[$x][$y]["v"][2][] = array($x, $y+1, "v", 1);
    }
  }
}
// horizontal neigh
for($x=1;$x<$mapwid-1;$x++) {
  for($y=0;$y<$maphei;$y++) {
    // neigh to the right?
    if($dimap[$x+1][$y][">"][0] != "#") {
      $dimap[$x][$y][">"][2][] = array($x+1, $y, ">", 1);
    }
    // neigh to the left?
    if($dimap[$x-1][$y]["<"][0] != "#") {
      $dimap[$x][$y]["<"][2][] = array($x-1, $y, "<", 1);
    }
  }
}
// internal neigh
for($x=0;$x<$mapwid;$x++) {
  for($y=0;$y<$maphei;$y++) {
    $dimap[$x][$y]["^"][2][] = array($x, $y, ">", 1000);
    $dimap[$x][$y]["^"][2][] = array($x, $y, "<", 1000);
    $dimap[$x][$y][">"][2][] = array($x, $y, "^", 1000);
    $dimap[$x][$y][">"][2][] = array($x, $y, "v", 1000);
    $dimap[$x][$y]["v"][2][] = array($x, $y, "<", 1000);
    $dimap[$x][$y]["v"][2][] = array($x, $y, ">", 1000);
    $dimap[$x][$y]["<"][2][] = array($x, $y, "^", 1000);
    $dimap[$x][$y]["<"][2][] = array($x, $y, "v", 1000);
  }
}
// Create a set of all unvisited nodes: the unvisited set.
$unvis = $dimap;
// Assign to every node a distance from start value: for the starting node, it is zero.
$coords = find("S");
$x = $coords[0];
$y = $coords[1];
$unvis[$x][$y]["^"][1] = 1000;
$unvis[$x][$y][">"][1] = 0;
$unvis[$x][$y]["v"][1] = 1000;
$unvis[$x][$y]["<"][1] = 10000000;
$dist = 0;
// begin loop
//for($i=0;$i<2;$i++) {
while($dist<83500) {
  $coords = find_short();
  $x = $coords[0];
  $y = $coords[1];
  if($x == -1 || $y == -1) {
    break;
  }
  $dir = $coords[2];
  $dist = $unvis[$x][$y][$dir][1];
if($dist % 100 == 0) {
  echo "$dist ";
}
  if(sizeof($unvis[$x][$y][$dir][2]) > 0) {  
    // For the current node, consider all of its unvisited neighbors
    // and update their distances through the current node
    foreach($unvis[$x][$y][$dir][2] as $num => $neigh) {
      $neighx = $neigh[0];
      $neighy = $neigh[1];
      $neighdir = $neigh[2];
      $dist2neigh = $neigh[3];
      if(isset($unvis[$neighx][$neighy][$neighdir])) {
        $neighcurdist = $unvis[$neighx][$neighy][$neighdir][1];
        if($dist + $dist2neigh < $neighcurdist) {
          $unvis[$neighx][$neighy][$neighdir][1] = $dist + $dist2neigh;
        }
      } else {
        unset($unvis[$x][$y][$dir][2][$num]);
      }
    }
  }
  
  // After considering all of the current node's unvisited neighbors,
  // the current node is removed from the unvisited set.
  $dimap[$x][$y][$dir][1] = $dist;
  if($dimap[$x][$y][$dir][0] == "E") {
    break;
  }
  unset($unvis[$x][$y][$dir]);
// end loop
}
$coords = find("E");
$x = $coords[0];
$y = $coords[1];
print_r($dimap[$x][$y]);
?>

Part 2:

<?php
function print_map() {
  global $map, $mapwid, $maphei;
  for($y=0;$y<$maphei;$y++) {
    for($x=0;$x<$mapwid;$x++) {
      if(isset($map[$y][$x])) {
        echo $map[$y][$x];
      } else {
        echo ".";
      }
    }
    echo "\n";
  }
  echo "\n";
}
function find($char) {
  global $map, $mapwid, $maphei;
  for($x=0;$x<$mapwid;$x++) {
    for($y=0;$y<$maphei;$y++) {
      if($map[$y][$x] == $char) {
        return array($x,$y);
      }
    }
  }
}
function find_short() {
  global $unvis, $maphei, $mapwid;
  $dist = 10000000;
  $newx = -1;
  $newy = -1;
  $newdir = "";
  for($x=0;$x<$mapwid;$x++) {
    if(!isset($unvis[$x])) {
      continue;
    }
    for($y=0;$y<$maphei;$y++) {
      if(!isset($unvis[$x][$y])) {
        continue;
      }
      foreach($unvis[$x][$y] as $dir => $val) {
        if($unvis[$x][$y][$dir][1] < $dist) {
          $newx = $x;
          $newy = $y;
          $newdir = $dir;
          $dist = $unvis[$x][$y][$dir][1];
        }
      }
    }
  }
  return array($newx, $newy, $newdir);
}
////////////////////////////////////////////////////////////////
$input = file_get_contents('./d16input2.txt', true);
foreach(preg_split("/((\r?\n)|(\r\n?))/", $input) as $line) {
  //print("$line\n");
  if(strlen($line)>2) {
    $map[] = str_split($line);
  }
}
$maphei = sizeof($map);
$mapwid = sizeof($map[0]);
// Assign to every node a distance from start value.
// explode map - every cell turns into 4 cells
// $map[$y][$x]
// $dimap[$x][$y][dir][type,dist,prev,neigh]
for($x=0;$x<$mapwid;$x++) {
  for($y=0;$y<$maphei;$y++) {
    $dimap[$x][$y]["^"] = array($map[$y][$x], 10000000, array(), array());
    $dimap[$x][$y][">"] = array($map[$y][$x], 10000000, array(), array());
    $dimap[$x][$y]["v"] = array($map[$y][$x], 10000000, array(), array());
    $dimap[$x][$y]["<"] = array($map[$y][$x], 10000000, array(), array());
  }
}
// neigh: (x, y, type, dist)
// vertical neigh
for($x=0;$x<$mapwid;$x++) {
  for($y=1;$y<$maphei-1;$y++) {
    // neigh above?
    if($dimap[$x][$y-1]["^"][0] != "#") {
      $dimap[$x][$y]["^"][3][] = array($x, $y-1, "^", 1);
    }
    // neigh below?
    if($dimap[$x][$y+1]["v"][0] != "#") {
      $dimap[$x][$y]["v"][3][] = array($x, $y+1, "v", 1);
    }
  }
}
// horizontal neigh
for($x=1;$x<$mapwid-1;$x++) {
  for($y=0;$y<$maphei;$y++) {
    // neigh to the right?
    if($dimap[$x+1][$y][">"][0] != "#") {
      $dimap[$x][$y][">"][3][] = array($x+1, $y, ">", 1);
    }
    // neigh to the left?
    if($dimap[$x-1][$y]["<"][0] != "#") {
      $dimap[$x][$y]["<"][3][] = array($x-1, $y, "<", 1);
    }
  }
}
// internal neigh
for($x=0;$x<$mapwid;$x++) {
  for($y=0;$y<$maphei;$y++) {
    $dimap[$x][$y]["^"][3][] = array($x, $y, ">", 1000);
    $dimap[$x][$y]["^"][3][] = array($x, $y, "<", 1000);
    $dimap[$x][$y][">"][3][] = array($x, $y, "^", 1000);
    $dimap[$x][$y][">"][3][] = array($x, $y, "v", 1000);
    $dimap[$x][$y]["v"][3][] = array($x, $y, "<", 1000);
    $dimap[$x][$y]["v"][3][] = array($x, $y, ">", 1000);
    $dimap[$x][$y]["<"][3][] = array($x, $y, "^", 1000);
    $dimap[$x][$y]["<"][3][] = array($x, $y, "v", 1000);
  }
}
// Create a set of all unvisited nodes: the unvisited set.
$unvis = $dimap;
// Assign to every node a distance from start value: for the starting node, it is zero.
$coords = find("S");
$x = $coords[0];
$y = $coords[1];
$unvis[$x][$y]["^"][1] = 1000;
$unvis[$x][$y][">"][1] = 0;
$unvis[$x][$y]["v"][1] = 1000;
$unvis[$x][$y]["<"][1] = 10000000;
$dist = 0;
// begin loop
while(0 < 1) {
  $coords = find_short();
  $x = $coords[0];
  $y = $coords[1];
  if($x == -1 || $y == -1) {
    break;
  }
  $dir = $coords[2];
  $dist = $unvis[$x][$y][$dir][1];
if($dist % 100 == 0) {
  echo "$dist ";
}
  if(sizeof($unvis[$x][$y][$dir][3]) > 0) {  
    // For the current node, consider all of its unvisited neighbors
    // and update their distances through the current node
    foreach($unvis[$x][$y][$dir][3] as $num => $neigh) {
      $neighx = $neigh[0];
      $neighy = $neigh[1];
      $neighdir = $neigh[2];
      $dist2neigh = $neigh[3];
      if(isset($unvis[$neighx][$neighy][$neighdir])) {
        $neighcurdist = $unvis[$neighx][$neighy][$neighdir][1];
        if($dist + $dist2neigh <= $neighcurdist) {
          $unvis[$neighx][$neighy][$neighdir][1] = $dist + $dist2neigh;
          // remember previous
          $unvis[$neighx][$neighy][$neighdir][2][] = array($x, $y, $dir);
        }
      } else {
        unset($unvis[$x][$y][$dir][3][$num]);
      }
    }
  }
  
  // After considering all of the current node's unvisited neighbors,
  // the current node is removed from the unvisited set.
  $dimap[$x][$y][$dir][1] = $dist;
  $dimap[$x][$y][$dir][2] = $unvis[$x][$y][$dir][2];
  if($dimap[$x][$y][$dir][0] == "E") {
    break;
  }
  unset($unvis[$x][$y][$dir]);
// end loop
}
// output distance
$coords = find("E");
$x = $coords[0];
$y = $coords[1];
$min = 10000000;
foreach($dimap[$x][$y] as $key => $distarr) {
  if($distarr[1] < $min) {
    $min = $distarr[1];
    $dir = $key;
  }
}
echo "$min\n";
// traverse all the shortest paths
$points = array( array($x, $y, $dir));
// begin loop
while(sizeof($points) > 0) {
  // i just need the first
  foreach($points as $key => $point) {
    $x = $point[0];
    $y = $point[1];
    $dir = $point[2];
    $prev = $dimap[$x][$y][$dir][2];
    break;
  }
  unset($points[$key]);
  $map[$y][$x] = "O";
  foreach($prev as $pre) {
    $points[] = array($pre[0], $pre[1], $pre[2]);
  }
// end loop
}
print_map();
$cells = 0;
for($i=0;$i<sizeof($map);$i++) {
  for($j=0;$j<sizeof($map[$i]);$j++) {
    if($map[$i][$j] == "O") {
      $cells++;
    }
  }
}
print("$cells\n");
?>