Day 1: Part 2

day1/day1_part2.lua

@@ -0,0 +1,49 @@
+--Lua 5.4.2
+local File = io.open("input.txt", "r")
+
+if not File then
+    print("File not found")
+    return
+end
+
+local LeftList = {}
+local RightList = {}
+
+for Line in File:lines() do
+    local LeftId, RightId = Line:match("(%d+)%s+(%d+)")
+    table.insert(LeftList, tonumber(LeftId))
+    table.insert(RightList, tonumber(RightId))
+end
+
+File:close()
+
+if #LeftList ~= #RightList then
+    print("Lists are not the same size")
+    return
+end
+
+--[[
+    Format:
+    Appearances[Id] = AppearanceCount
+--]]
+local Appearances = {}
+
+for ListIndex = 1, #RightList do
+    local Id = RightList[ListIndex]
+    if not Appearances[Id] then
+        Appearances[Id] = 0
+    end
+    Appearances[Id] = Appearances[Id] + 1
+end
+
+local SimilarityScore = 0
+
+for ListIndex = 1, #LeftList do
+    local Id = LeftList[ListIndex]
+    local AppearanceCount = Appearances[Id]
+    if AppearanceCount then
+        SimilarityScore = SimilarityScore + Id * AppearanceCount
+    end
+end
+
+print("Similarity score:", SimilarityScore)