2020 day 10

.gitignore

@@ -11,3 +11,5 @@ ENV/
 # Advent of Code puzzle inputs
 input.txt
 input
+example.txt
+example

2020/10/day10.lua

@@ -0,0 +1,61 @@
+local adapters = {}
+
+for line in io.lines() do
+    table.insert(adapters, tonumber(line))
+end
+table.sort(adapters)
+
+local diff_1, diff_3 = 1, 1
+
+for i = 2, #adapters do
+    local diff = adapters[i] - adapters[i-1]
+    if diff == 1 then
+        diff_1 = diff_1 + 1
+    elseif diff == 3 then
+        diff_3 = diff_3 + 1
+    else
+        print('oh no')
+        return
+    end
+end
+print(diff_1 * diff_3)
+
+-- Maps an adapter to the list of other adapters it can connect with, to build a graph
+local accessible_adapters = {}
+
+for i, adapter in ipairs(adapters) do
+    accessible_adapters[adapter] = {}
+    local j = i + 1
+    repeat
+        table.insert(accessible_adapters[adapter], adapters[j])
+        j = j + 1
+    until j > #adapters or adapters[j] - adapter > 3
+end
+
+--[[
+    For a given adapter, find how many possible paths it has.
+    Uses memoization for faster output
+--]]
+local count = {}
+local function count_adapters(adapter)
+    if count[adapter] then return count[adapter] end
+
+    count[adapter] = #accessible_adapters[adapter]
+    if count[adapter] == 0 then
+        count[adapter] = 1
+    else
+        for _, other_adapter in ipairs(accessible_adapters[adapter]) do
+            count[adapter] = count[adapter] + count_adapters(other_adapter) - 1
+        end
+    end
+    return count[adapter]
+end
+
+-- Result of part 2 is the sum of paths of every adapter immediately accessible
+-- from the outlet (anything < 3).
+local total, i = 0, 1
+repeat
+    total = total + count_adapters(adapters[i])
+    i = i + 1
+until adapters[i] > 3
+print(total)

README.md

@@ -20,7 +20,7 @@ is acceptable; anything goes as long as I solve it myself!
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