day 9

src/day9_p1.gleam

@@ -22,25 +22,29 @@ fn calculate_checksum(filemap: Dict(Int, Int)) -> Int {
 }
 
 fn compact(filemap: Dict(Int, Int)) -> Dict(Int, Int) {
-  filemap
-  |> dict.keys
+  let keys =
+    filemap
+    |> dict.keys
+    |> list.sort(int.compare)
+
+  let to_move = keys |> list.reverse
+
+  keys
   |> find_gaps
-  |> list.fold_until(filemap, fn(acc, gap) {
-    case
-      acc
-      |> dict.keys
-      |> list.reverse
-      |> list.first
-    {
-      Ok(x) if x <= gap -> Stop(acc)
-      Ok(x) ->
-        case dict.get(acc, x) {
-          Ok(val) -> Continue(acc |> dict.delete(x) |> dict.insert(gap, val))
+  |> list.fold_until(#(filemap, to_move), fn(acc, gap) {
+    let #(fmap, fdata) = acc
+    case fdata {
+      [x, ..] if x <= gap -> Stop(acc)
+      [x, ..rest] ->
+        case dict.get(fmap, x) {
+          Ok(val) ->
+            Continue(#(fmap |> dict.delete(x) |> dict.insert(gap, val), rest))
           _ -> Stop(acc)
         }
       _ -> Stop(acc)
     }
   })
+  |> pair.first
 }
 
 fn find_gaps(nums: List(Int)) -> List(Int) {

src/day9_p2.gleam

@@ -0,0 +1,115 @@
+import gleam/int
+import gleam/io
+import gleam/iterator
+import gleam/list.{fold, map}
+import gleam/result
+import gleam/string
+import stdin.{stdin}
+
+pub fn main() {
+  stdin()
+  |> read_input
+  |> compact
+  |> calculate_checksum
+  |> io.debug
+}
+
+fn calculate_checksum(filemap: List(Segment)) -> Int {
+  filemap
+  |> fold(0, fn(acc, next) {
+    case next {
+      File(pos, index, size) -> {
+        list.range(pos, pos + size - 1)
+        |> fold(acc, fn(total, x) { total + x * index })
+      }
+      _ -> acc
+    }
+  })
+}
+
+fn compact(filemap: List(Segment)) -> List(Segment) {
+  filemap
+  |> list.reverse
+  |> fold(filemap, fn(acc, next) {
+    case next {
+      File(pos, index, size) -> {
+        let next_gap =
+          list.find(acc, fn(x) {
+            case x {
+              Gap(gap_pos, gap_size) if gap_pos < pos && gap_size >= size ->
+                True
+              _ -> False
+            }
+          })
+
+        case next_gap {
+          Ok(Gap(gap_pos, _)) -> fill_gap(acc, gap_pos, index, size)
+          _ -> acc
+        }
+      }
+      _ -> acc
+    }
+  })
+}
+
+fn fill_gap(
+  segments: List(Segment),
+  gap_pos: Int,
+  file_index: Int,
+  file_size: Int,
+) -> List(Segment) {
+  segments
+  |> fold([], fn(acc, s) {
+    case s {
+      Gap(pos, size) if pos == gap_pos && size == file_size -> [
+        File(pos, file_index, file_size),
+        ..acc
+      ]
+      Gap(pos, size) if pos == gap_pos -> [
+        Gap(pos + file_size, size - file_size),
+        File(pos, file_index, file_size),
+        ..acc
+      ]
+      File(pos, index, size) if file_index == index -> [Gap(pos, size), ..acc]
+      _ -> [s, ..acc]
+    }
+  })
+  |> list.reverse
+}
+
+pub type Segment {
+  File(position: Int, index: Int, size: Int)
+  Gap(position: Int, size: Int)
+}
+
+fn read_input(it: iterator.Iterator(String)) -> List(Segment) {
+  let assert Ok(numbers) =
+    it
+    |> iterator.to_list
+    |> map(fn(line) {
+      line
+      |> string.trim
+      |> string.to_graphemes
+      |> map(int.parse)
+    })
+    |> list.flatten
+    |> result.all
+
+  numbers
+  |> list.index_fold(#([], 0, 0), fn(acc, size, i) {
+    let #(segments, index, position) = acc
+    case i % 2 == 0 {
+      True -> #(
+        [File(position, index, size), ..segments],
+        index + 1,
+        position + size,
+      )
+      False -> #([Gap(position, size), ..segments], index, position + size)
+    }
+  })
+  |> fn(x) {
+    let #(segments, _, _) = x
+    segments
+  }
+  |> list.reverse
+}