2020 day 15

2020/15/day15.lua

@@ -0,0 +1,35 @@
+-- Maps numbers to their spoken turns
+local numbers = {}
+local turn, last_number = 1, nil
+
+-- Starting numbers
+for number in io.read("l"):gmatch("(%d+)") do
+    last_number = tonumber(number)
+    numbers[last_number] = {turn}
+    turn = turn + 1
+end
+
+local function do_turn()
+    if numbers[last_number] and #numbers[last_number] > 1 then
+        last_number = numbers[last_number][#numbers[last_number]] - numbers[last_number][#numbers[last_number] - 1]
+    else
+        last_number = 0
+    end
+    if numbers[last_number] then
+        table.insert(numbers[last_number], turn)
+    else
+        numbers[last_number] = {turn}
+    end
+    turn = turn + 1
+end
+
+while turn <= 2020 do
+    do_turn()
+end
+print(last_number)
+
+-- Part 2 takes a few seconds, which is not slow enough to be worth optimizing.
+while turn <= 30000000 do
+    do_turn()
+end
+print(last_number)

README.md

@@ -25,7 +25,7 @@ is acceptable; anything goes as long as I solve it myself!
 12 ██    ██       ██
 13       ██       █
 14 ██    ██       ██
-15       ██
+15       ██       ██
 16 ██    ██
 17       ██
 18       ██