day 7 part 1

README.md

···
 | 04  |   ⭐   |   ⭐   | 
 | 05  |   ⭐   |   ⭐   | 
 | 06  |   ⭐   |   ⭐   | 
-| 07  |        |        |
+| 07  |   ⭐   |        | 
 | 08  |        |        |
 | 09  |        |        |
 | 10  |        |        |

src/aoc_2025/day_7.gleam

···
+import gleam/list
+import gleam/option.{None, Some}
+import gleam/pair
+import gleam/set
+import gleam/string
+
+pub type Input {
+  Input(start_index: Int, splitters: List(List(Int)))
+}
+
+pub fn parse(input: String) -> Input {
+  let assert [first, ..rest] =
+    input
+    |> string.split("\n")
+
+  let start_index = {
+    let assert Ok(s_codepoint) = string.utf_codepoint(83)
+
+    let assert Some(s) =
+      first
+      |> string.to_utf_codepoints
+      |> list.index_fold(None, fn(acc, c, idx) {
+        case c == s_codepoint {
+          True -> Some(idx)
+          _ -> acc
+        }
+      })
+
+    s
+  }
+
+  let splitters = {
+    let assert Ok(c_codepoint) = string.utf_codepoint(94)
+    rest
+    |> list.map(fn(line) {
+      line
+      |> string.to_utf_codepoints
+      |> list.index_fold([], fn(acc, c, idx) {
+        case c == c_codepoint {
+          True -> [idx, ..acc]
+          False -> acc
+        }
+      })
+    })
+  }
+
+  Input(start_index:, splitters:)
+}
+
+pub fn pt_1(input: Input) -> Int {
+  let Input(start_index:, splitters:) = input
+
+  splitters
+  |> list.fold(#(set.from_list([start_index]), 0), fn(acc, row) {
+    list.fold(row, acc, fn(acc, splitter) {
+      let #(rays, split_count) = acc
+
+      case set.contains(rays, splitter) {
+        True -> {
+          let new_rays =
+            rays
+            |> set.delete(splitter)
+            |> set.insert(splitter + 1)
+            |> set.insert(splitter - 1)
+
+          #(new_rays, split_count + 1)
+        }
+        False -> acc
+      }
+    })
+  })
+  |> pair.second
+}
+
+pub fn pt_2(input: Input) {
+  todo as "part 2 not implemented"
+}