Day 20

days/day20.st

@@ -0,0 +1,96 @@
+Object subclass: Racetrack [
+    | grid distToEnd start end trackLen allCheats |
+    rows: r
+    [ grid := Grid new rows: r.
+      self initTrack ]
+
+    initTrack
+    [ | pos dir step |
+      trackLen := 1.
+      grid allPosnsDo: [
+        :p | | c | c := grid at: p.
+        (c = $S or: [c = $E]) ifTrue: [
+            grid at: p put: $..
+            c = $S ifTrue: [start := p].
+            c = $E ifTrue: [end := p].
+        ].
+        c = $. ifTrue: [ trackLen := trackLen + 1 ]].
+      distToEnd := Grid width: grid width
+                        height: grid height
+                        initWith: [:p | nil].
+
+      pos := start.
+      dir := {Posn up. Posn down. Posn left. Posn right} findFirstElt: [
+          :dir | self isEmpty: start + dir ].
+      step := 0.
+      [ pos = end ] whileFalse: [
+          distToEnd at: pos put: trackLen - step.
+          dir := {dir. dir rotateCw. dir rotateCcw} findFirstElt: [
+              :dir | self isEmpty: pos + dir ].
+          pos := pos + dir.
+          step := step + 1.
+      ].
+      distToEnd at: end put: 0.
+    ]
+
+    isEmpty: pos [ ^(grid at: pos) = $. ]
+
+    countCheatsWithTime: time
+    [ | total |
+      total := 0.
+      "allCheats := Bag new."
+      grid allPosnsDo: [
+          :pos | total := total + (self countCheatsFrom: pos withTime: time) ].
+      "allCheats printNl."
+      ^total ]
+
+    countCheatsFrom: pos withTime: time
+    [ | dist |
+      dist := distToEnd at: pos.
+      dist ifNil: [^0].
+      "dist < 100 ifTrue: [^0]."
+      ^self rangeCount: [
+          :dpos :ddist |
+          self validCheatFrom: pos dist: dist
+               to: dpos dist: ddist
+               withTime: time
+      ]
+            inManhDistance: time
+            from: pos
+            maxDist: dist - 100. ]
+
+    validCheatFrom: pos dist: dist
+    to: dpos dist: ddist
+    withTime: time
+    [ | ctime saved |
+      ctime := (dpos x - pos x) abs +
+               (dpos y - pos y) abs.
+      ctime > time ifTrue: [^false].
+      saved := dist - ddist - ctime.
+      "saved > 0 ifTrue: [allCheats add: saved]."
+      ^saved >= 100 ]
+
+    rangeCount: block
+    inManhDistance: distmh
+    from: pos
+    maxDist: mscore
+    [ | ddist dpos count px py doit |
+      count := 0.
+      dpos := Posn new.
+      px := pos x. py := pos y.
+      distmh negated to: distmh do: [
+          :dx | distmh negated to: distmh do: [
+              :dy | 
+              (dx abs + dy abs) <= distmh ifTrue: [
+                  dpos x: px + dx; y: py + dy.
+                  ddist := distToEnd at: dpos.
+                  ddist ifNotNil: [
+                      (block value: dpos value: ddist)
+                          ifTrue: [ count := count + 1 ]]]]].
+      ^count ]
+]
+
+AOC input: [ Racetrack new rows: stdin toLines asArray ];
+    part1: 2; part2: 20;
+    result: [ :track :time | track countCheatsWithTime: time ];
+    finish.

days/utils.st

@@ -156,6 +156,12 @@ Iterable extend [
     intoCollection: acc [ self do: [:x | acc add: x]. ^acc ]
 ]
 
+SequenceableCollection extend [
+    findFirstElt: block [ ^self at: (self findFirst: block) ]
+    findFirstElt: block ifAbsent: abs
+    [ ^self at: (self findFirst: block) ifAbsent: abs ]
+]
+
 Collection extend [
     transposed
     [ ^(self size <= 0) ifTrue: