Day 2. Easy peasy

day2/main.roc

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+app [main] {
+    pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.17.0/lZFLstMUCUvd5bjnnpYromZJXkQUrdhbva4xdBInicE.tar.br",
+}
+
+import pf.Stdout
+import "data.txt" as data : Str
+
+main =
+    lines = Str.splitOn data "\n" |> List.dropLast 1
+    levels = List.map lines \l -> Str.splitOn l " " |> List.keepOks Str.toU8
+
+    part1 = List.keepIf levels \l -> isSafe l Bool.false
+    Stdout.line! "Safe Reports, Part 1: $(Num.toStr (List.len part1))"
+
+    # my algorithm doesn't work great with tolerating the first or 2nd elements, so just run it again with those elements dropped and no dampening
+    part2 = List.keepIf levels \l -> (isSafe l Bool.true) || (isSafe (List.dropAt l 0) Bool.false) || (isSafe (List.dropAt l 1) Bool.false)
+    Stdout.line! "Safe Reports, Part 2: $(Num.toStr (List.len part2))"
+
+isSafe = \level, isDampening ->
+    List.walkWithIndexUntil level { previous: 0, direction: Unset, valid: Bool.true, skipped: 0 } \state, elem, idx ->
+        if idx == 0 then # on first iteration, can't fail, just return
+            Continue { state & previous: elem, direction: Unset }
+        else
+            diff = Num.absDiff elem state.previous
+
+            if diff < 1 || diff > 3 then # always check diff
+                if isDampening && state.skipped == 0 then
+                    Continue { state & skipped: 1 }
+                else
+                    Break { state & valid: Bool.false }
+            else
+                direction = if elem > state.previous then Ascending else Descending
+
+                if idx == 1 then # on 2nd iteration, we don't check for direction change
+                    Continue { state & previous: elem, direction: direction }
+                else if state.direction != direction then
+                    if isDampening && state.skipped == 0 then
+                        Continue { state & skipped: 1 }
+                    else
+                        Break { state & valid: Bool.false }
+                else
+                    Continue { state & previous: elem, direction: direction }
+
+    |> .valid

day2/test.txt

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+7 6 4 2 1
+1 2 7 8 9
+9 7 6 2 1
+1 3 2 4 5
+8 6 4 4 1
+1 3 6 7 9