diff --git a/sample/day9-sample.txt b/sample/day9-sample.txt
new file mode 100644
index 0000000..f96c390
--- /dev/null
+++ b/sample/day9-sample.txt
@@ -0,0 +1 @@
+2333133121414131402
diff --git a/sample/day9-sample2.txt b/sample/day9-sample2.txt
new file mode 100644
index 0000000..0235404
--- /dev/null
+++ b/sample/day9-sample2.txt
@@ -0,0 +1 @@
+3782
diff --git a/src/day3_p1.gleam b/src/day3_p1.gleam
index 983555d..0ceec43 100644
--- a/src/day3_p1.gleam
+++ b/src/day3_p1.gleam
@@ -91,7 +91,6 @@ fn read_num(str: String) -> Option(#(Int, String)) {
 
 fn read_input(it: iterator.Iterator(String)) -> String {
   it
-  |> iterator.fold([], fn(acc, line) { [line, ..acc] })
-  |> reverse
+  |> iterator.to_list
   |> join(with: "")
 }
diff --git a/src/day3_p2.gleam b/src/day3_p2.gleam
index 2aff4cb..6c5c8ba 100644
--- a/src/day3_p2.gleam
+++ b/src/day3_p2.gleam
@@ -90,7 +90,6 @@ fn read_num(str: String) -> Option(#(Int, String)) {
 
 fn read_input(it: iterator.Iterator(String)) -> String {
   it
-  |> iterator.fold([], fn(acc, line) { [line, ..acc] })
-  |> reverse
+  |> iterator.to_list
   |> join(with: "")
 }
diff --git a/src/day9_p1.gleam b/src/day9_p1.gleam
new file mode 100644
index 0000000..5d739c1
--- /dev/null
+++ b/src/day9_p1.gleam
@@ -0,0 +1,83 @@
+import gleam/bool
+import gleam/dict.{type Dict}
+import gleam/int
+import gleam/io
+import gleam/iterator
+import gleam/list.{Continue, Stop, filter, fold, map}
+import gleam/pair
+import gleam/result
+import gleam/string
+import stdin.{stdin}
+
+pub fn main() {
+  stdin()
+  |> read_input
+  |> compact
+  |> calculate_checksum
+  |> io.debug
+}
+
+fn calculate_checksum(filemap: Dict(Int, Int)) -> Int {
+  filemap |> dict.fold(0, fn(acc, k, v) { acc + k * v })
+}
+
+fn compact(filemap: Dict(Int, Int)) -> Dict(Int, Int) {
+  filemap
+  |> dict.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))
+          _ -> Stop(acc)
+        }
+      _ -> Stop(acc)
+    }
+  })
+}
+
+fn find_gaps(nums: List(Int)) -> List(Int) {
+  // Assume ordered list
+  let assert [max, ..] = nums |> list.reverse
+
+  list.range(0, max - 1)
+  |> filter(fn(n) { nums |> list.contains(n) |> bool.negate })
+}
+
+fn read_input(it: iterator.Iterator(String)) -> Dict(Int, Int) {
+  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, []), fn(acc, n, i) {
+    let #(position, files) = acc
+    case i % 2 == 0 {
+      True -> #(position + n, [#(position, n), ..files])
+      False -> #(position + n, files)
+    }
+  })
+  |> pair.second
+  |> list.reverse
+  |> list.index_fold(dict.new(), fn(acc, file, i) {
+    let #(position, size) = file
+
+    list.range(position, position + size - 1)
+    |> fold(acc, fn(dic, pos) { dict.insert(dic, pos, i) })
+  })
+}