Day 4

day4/main.roc

@@ -0,0 +1,67 @@
+app [main] {
+    pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.17.0/lZFLstMUCUvd5bjnnpYromZJXkQUrdhbva4xdBInicE.tar.br",
+    unicode: "https://github.com/roc-lang/unicode/releases/download/0.1.2/vH5iqn04ShmqP-pNemgF773f86COePSqMWHzVGrAKNo.tar.br"
+}
+
+import pf.Stdout
+import unicode.Grapheme
+import "data.txt" as data : Str
+
+main =
+    lines = Str.splitOn data "\n" |> List.dropLast 1
+    grid = List.map lines \l -> Result.withDefault (Grapheme.split l) [""]
+
+    part1 = List.walkWithIndex grid 0 \total, line, row ->
+        total + List.walkWithIndex line 0 \rowTotal, _, col ->
+            rowTotal + visit1 grid row col
+
+    Stdout.line! "Part 1: $(Num.toStr part1)"
+
+    part2 = List.walkWithIndex grid 0 \total, line, row ->
+        total + List.walkWithIndex line 0 \rowTotal, _, col ->
+            rowTotal + visit2 grid row col
+
+    Stdout.line! "Part 2: $(Num.toStr part2)"
+
+dirs1 = [
+    (-1, -1), (-1, 0), (-1, 1),
+    ( 0, -1),          ( 0, 1),
+    ( 1, -1), ( 1, 0), ( 1, 1),
+]
+
+visit1 = \grid, row, col ->
+    # for each direction
+    List.walk dirs1 0 \state, dir ->
+        # walk 4 steps
+        state + List.walkWithIndexUntil ["X","M","A","S"] 0 \_, targetChar, i ->
+            newRow = (Num.intCast row) + dir.0 * (Num.intCast i)
+            newCol = (Num.intCast col) + dir.1 * (Num.intCast i)
+            char = gridChar grid newRow newCol
+            when char is
+                Ok c if c == targetChar -> Continue 1
+                _ -> Break 0
+
+visit2 = \grid, row, col ->
+    char = gridChar grid row col
+
+    # if this cell isn't an "A", get out
+    if char != Ok "A" then
+        0
+    else
+        # grab the 4 corners
+        nw = gridChar grid (Num.intCast row - 1) (Num.intCast col - 1)
+        ne = gridChar grid (Num.intCast row - 1) (Num.intCast col + 1)
+        sw = gridChar grid (Num.intCast row + 1) (Num.intCast col - 1)
+        se = gridChar grid (Num.intCast row + 1) (Num.intCast col + 1)
+
+        if ((nw == Ok "M" && se == Ok "S") ||
+            (nw == Ok "S" && se == Ok "M")) &&
+           ((ne == Ok "M" && sw == Ok "S") ||
+            (ne == Ok "S" && sw == Ok "M"))
+            then 1
+            else 0
+
+# get the Result char at a row and col in the grid
+gridChar = \grid, row, col ->
+    r = List.get? grid (Num.intCast row)
+    List.get? r (Num.intCast col)