Day 18

days/day18.st

@@ -0,0 +1,83 @@
+Object subclass: MemorySpace [
+    | corruption origin dirs end
+      grid steps size
+      queue seen |
+
+    initialize
+    [ origin := Posn x: 1 y: 1.
+      dirs := {Posn up. Posn down. Posn left. Posn right} ]
+
+    corruption: n [corruption := n collect: [:it | it + origin]]
+    corruption [^corruption]
+
+    grid [^grid]
+
+    plotAtTime: time
+    [ | g | g := Grid width: size height: size initWith: [:i | $.].
+      g rows: (g rows collect: [:it | it asString]).
+      1 to: time do: [:i | g at: (corruption at: i) put: $#].
+      g printNl ]
+
+    findExitWithInitial: initial
+    [ | stepsInitial |
+      size := 71.
+      grid := Grid width: size height: size initWith: [
+                       :i | {"blocked at: "FloatD infinity.
+                             "last reachable before: "nil}].
+      corruption keysAndValuesDo: [
+          :time :pos | (grid at: pos) at: 1 put: time ].
+
+      queue := OrderedCollection new.
+      self enqueue: origin withLastTime: ((grid at: origin) at: 1).
+
+      steps := 0.
+      end := Posn x: size y: size.
+      [ queue isEmpty ] whileFalse: [
+          self stepOnce.
+          steps := steps + 1.
+          (stepsInitial isNil and: [
+              ((grid at: end) at: 2)
+                  ifNil: [false]
+                  ifNotNil: [:it | it > initial]]) ifTrue: [
+              stepsInitial := steps.
+          ].
+      ].
+      ^stepsInitial ]
+
+    posnThatBlocksEnd [ ^(corruption at: ((grid at: end) at: 2)) - origin ]
+
+    stepOnce
+    [ | oldQueue |
+      oldQueue := queue.
+      queue := OrderedCollection new.
+      oldQueue do: [
+          :posn :time | dirs do: [
+              :dir |
+              self enqueue: (posn + dir)
+                   withLastTime: time ]]
+                       asSpreader ]
+
+    enqueue: posn withLastTime: lastTime
+    [ | entry bestTime |
+      entry := grid at: posn.
+      entry ifNil: [^false].
+      bestTime := (entry at: 1)
+                      ifNil: [lastTime]
+                      ifNotNil: [:e | lastTime min: e].
+      (entry at: 2) ifNotNil: [
+          :prevLastReachable |
+          bestTime <= prevLastReachable ifTrue: [^false]].
+      entry at: 2 put: bestTime.
+      queue add: {posn . bestTime}.
+      ^true ]
+]
+
+AOC input: [ | posns |
+             posns := stdin toLines asArray collect: [
+                 :it | it scanf: '%d,%d' with: [:x :y | Posn x: x y: y]].
+             MemorySpace new corruption: posns ];
+    part1: [ :space | space findExitWithInitial: 1024 ];
+    part2: [ :space | | p | p := space posnThatBlocksEnd.
+             '%1,%2' % {p x . p y} ];
+    result: [ :space :part | part value: space ];
+    finish