1 files changed, 73 insertions, 0 deletions
diff --git a/21/b.lua b/21/b.lua
new file mode 100644
index 0000000..75c90e7
--- /dev/null
+++ b/21/b.lua
@@ -0,0 +1,73 @@
+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
+
+changed = true
+mapping = {}
+decided = {}
+
+while changed do
+    changed = false
+    for allergen, ingredients in pairs(candidates) do
+        if not mapping[allergen] then
+            size = 0
+            last_ingredient = nil
+            for ingredient, candidate in pairs(ingredients) do
+                if candidate and not decided[ingredient] then
+                    last_ingredient = ingredient
+                    size = size + 1
+                end
+            end
+
+            if size == 1 then
+                mapping[allergen] = last_ingredient
+                decided[last_ingredient] = true
+                changed = true
+            end
+        end
+    end
+end
+
+list = {}
+for allergen, ingredient in pairs(mapping) do
+    table.insert(list, { allergen, ingredient })
+end
+
+table.sort(list, function (a, b)
+    return a[1] < b[1]
+end)
+
+for i, pair in pairs(list) do
+    io.write(pair[2])
+    io.write(',')
+end