Finish day 9

1 files changed, 10 insertions, 0 deletions

diff --git a/aoc-2020-gleam/src/ext/listx.gleam b/aoc-2020-gleam/src/ext/listx.gleam
index 2d4f4b6..8d20bd6 100644
--- a/aoc-2020-gleam/src/ext/listx.gleam
+++ b/aoc-2020-gleam/src/ext/listx.gleam
@@ -1,4 +1,6 @@
+import gleam/pair
 import gleam/iterator as iter
+import gleam/result as res
 import ext/iteratorx as iterx
 
 pub fn count(list: List(a), satisfying predicate: fn(a) -> Bool) -> Int {
@@ -18,3 +20,11 @@ fn set_helper(list: List(a), value: a, index: Int, counter: Int) -> List(a) {
 pub fn set(list: List(a), value: a, at index: Int) -> List(a) {
   set_helper(list, value, index, 0)
 }
+
+pub fn index_of(list: List(a), value: a) -> Result(Int, Nil) {
+  list
+  |> iter.from_list
+  |> iter.index
+  |> iter.find(one_that: fn(elem) { pair.second(elem) == value })
+  |> res.map(with: pair.first)
+}