2020 day 7

2020/7/day7.lua

@@ -0,0 +1,69 @@
+#!/usr/bin/env lua
+--[[
+    Representation in both ways:
+    The input mentions what bags a bag can contain,
+    we store both what bags a bag can contain and
+    what bags can contain a bag and in which amount.
+    Part 2 requires the former and part 1 requires the latter.
+--]]
+local containers = {}
+local contains = {}
+
+for line in io.lines() do
+    local bag_end, contain_start = line:find(" bags contain ")
+    local bag_color = line:sub(1, bag_end - 1)
+    contains[bag_color] = {}
+
+    for count, color in line:sub(contain_start + 1):gmatch("(%d+) (%w+ %w+) bag") do
+        if not containers[color] then
+            containers[color] = {}
+        end
+        containers[color][bag_color] = count
+        contains[bag_color][color] = count
+    end
+end
+
+--[[
+    For part 1, we build a set of bags that can contain the shiny gold bag.
+    We will iterate over and over on this set until we no longer find any new
+    bag colors that can contain others bags that can contain the shiny gold bag.
+--]]
+local part1set, found = {}, 0
+for color, _ in pairs(containers["shiny gold"] or {}) do
+    part1set[color] = true
+    found = found + 1
+end
+
+local found_in_iteration = 0
+repeat
+    found_in_iteration = 0
+    for known_color, _ in pairs(part1set) do
+        for color, _ in pairs(containers[known_color] or {}) do
+            if not part1set[color] then
+                found_in_iteration = found_in_iteration + 1
+                found = found + 1
+                part1set[color] = true
+            end
+        end
+    end
+until found_in_iteration < 1
+
+print(found)
+
+--[[
+    For part 2, we use a recursive function to find how many bags a single bag can hold.
+    We use a table to provide memoization, gotta go fast.
+--]]
+local part2memo = {}
+local function count_bags(color)
+    if part2memo[color] then return part2memo[color] end
+    local total = 0
+    for sub_color, count in pairs(contains[color]) do
+        -- Count the bag itself and all the bags it contains
+        total = total + count * (count_bags(sub_color) + 1)
+    end
+    part2memo[color] = total
+    return total
+end
+
+print(count_bags("shiny gold"))

README.md

@@ -12,7 +12,7 @@ My solutions to the Advent of Code puzzles.
  4 ██    ██ ██    ██
  5 ██    ██ ██    ██
  6 ██    ██       ██
- 7 ██    ██
+ 7 ██    ██       ██
  8 ██    ██
  9       ██
 10 ██    ██