Day 8 part 2

2022/src/days/day_8.gleam

@@ -1,9 +1,13 @@
+import gleam/io.{debug}
 import gleam/int
 import gleam/iterator.{zip}
 import gleam/string
 import gleam/list
 import gleam/option.{None, Some}
 
+type Point =
+  #(Int, Int)
+
 pub fn pt_1(input: String) -> Int {
   let trees_by_row =
     input
@@ -47,7 +51,16 @@ pub fn pt_1(input: String) -> Int {
 }
 
 pub fn pt_2(input: String) -> Int {
-  todo
+  let trees =
+    input
+    |> string.split("\n")
+    |> list.map(parse_row)
+
+  trees
+  |> list.index_map(fn(x, row) {
+    list.index_map(row, fn(y, tree) { scenic_score(trees, x, y, tree) })
+  })
+  |> best_score
 }
 
 fn parse_row(row: String) -> List(Int) {
@@ -113,3 +126,57 @@ fn count_if_visible(
     False -> count
   }
 }
+
+fn scenic_score(trees: List(List(Int)), x: Int, y: Int, tree: Int) {
+  let up = visible(trees, #(x, y), #(0, -1), tree, 0)
+  let down = visible(trees, #(x, y), #(0, 1), tree, 0)
+  let left = visible(trees, #(x, y), #(-1, 0), tree, 0)
+  let right = visible(trees, #(x, y), #(1, 0), tree, 0)
+  up * down * left * right
+}
+
+fn visible(
+  trees: List(List(Int)),
+  from: Point,
+  delta: Point,
+  tree: Int,
+  count: Int,
+) -> Int {
+  let last = list.length(trees) - 1
+  let coord = add(from, delta)
+  case coord {
+    #(-1, _) | #(_, -1) -> count
+    #(x, y) if x > last || y > last -> count
+    #(x, y) ->
+      case
+        trees
+        |> list.at(x)
+        |> unwrap
+        |> list.at(y)
+        |> unwrap
+      {
+        other if other < tree -> visible(trees, coord, delta, tree, count + 1)
+        otherwise -> count + 1
+      }
+  }
+}
+
+fn best_score(scores: List(List(Int))) -> Int {
+  scores
+  |> list.map(list_max)
+  |> list_max
+}
+
+fn add(from: Point, delta: Point) -> Point {
+  #(from.0 + delta.0, from.1 + delta.1)
+}
+
+fn unwrap(x: Result(a, Nil)) -> a {
+  assert Ok(ok) = x
+  ok
+}
+
+fn list_max(l: List(Int)) -> Int {
+  list.reduce(l, int.max)
+  |> unwrap
+}

2022/test/day_8_test.gleam

@@ -20,3 +20,8 @@ pub fn pt1_test() {
   day_8.pt_1("30373\n25512\n65332\n33549\n35390")
   |> should.equal(21)
 }
+
+pub fn pt2_test() {
+  day_8.pt_2("30373\n25512\n65332\n33549\n35390")
+  |> should.equal(8)
+}