Day 2

days/day02.st

@@ -0,0 +1,26 @@
+Collection extend [
+    checkSafeStep: a to: b
+    [ |delta| (a < 1) | (b > self size) ifTrue: [^true].
+      delta := (self at: b) - (self at: a).
+      ^(1 <= delta) & (delta <= 3) ]
+
+    safetyFwd
+    [ | dp | dp := Array new: self size + 1.
+      (self size to: 0 by: -1) do: [
+          :i | | v |
+          v := i + 1 to: self size + 1.
+          v := v select: [ :j | self checkSafeStep: i to: j ].
+          v := v collect: [ :j | (dp at: (j + 1) ifAbsent: [0]) + (j - i - 1) ].
+          v := v fold: [ :l :r | l min: r ].
+          dp at: (i + 1) put: v ].
+      ^dp at: 1 ]
+    safety [ ^self safetyFwd min: self reverse safetyFwd ]
+]
+
+AOC input: [ stdin toLines collect:
+                 [ :ln | ((ln tokenize: ' ')
+                              collect: [ :lvl | lvl scanf: '%d' with: [:e|e] ])
+                             safety ] ];
+    part1: 0; part2: 1;
+    result: [ :reports :damp | reports count: [ :it | it <= damp ] ];
+    finish