1 files changed, 50 insertions, 0 deletions
diff --git a/21/a.lua b/21/a.lua
new file mode 100644
index 0000000..96ad83c
--- /dev/null
+++ b/21/a.lua
@@ -0,0 +1,50 @@
+candidates = {}
+appearances = {}
+
+function intersect(t1, t2)
+    t = {}
+    for k, _ in pairs(t1) do
+        if t2[k] then
+            t[k] = true
+        end
+    end
+
+    return t
+end
+
+for line in io.lines('input.txt') do
+    allergens_index = line:find('(', 1, true)
+    ingredients = {}
+    for ingredient in line:sub(1, allergens_index-2):gmatch("%a+") do
+        ingredients[ingredient] = true
+        if not appearances[ingredient] then
+            appearances[ingredient] = 0
+        end
+        appearances[ingredient] = appearances[ingredient] + 1
+    end
+
+    for allergen in line:sub(allergens_index+10, -2):gmatch("%a+") do
+        if not candidates[allergen] then
+            candidates[allergen] = ingredients
+        else
+            candidates[allergen] = intersect(candidates[allergen], ingredients)
+        end
+    end
+end
+
+unsafe = {}
+for _, ingredients in pairs(candidates) do
+    for ingredient in pairs(ingredients) do
+        unsafe[ingredient] = true
+    end
+end
+
+count = 0
+
+for ingredient, appearances in pairs(appearances) do
+    if not unsafe[ingredient] then
+        count = count + appearances
+    end
+end
+
+print(count)