2020 day 9

2020/9/day9.lua

@@ -0,0 +1,67 @@
+local function sum(items)
+    local total = 0
+    for _, value in ipairs(items) do
+        total = total + value
+    end
+    return total
+end
+
+local function min(items)
+    local smol
+    for _, value in ipairs(items) do
+        if not smol or smol > value then
+            smol = value
+        end
+    end
+    return smol
+end
+
+local function max(items)
+    local lorge
+    for _, value in ipairs(items) do
+        if not lorge or lorge < value then
+            lorge = value
+        end
+    end
+    return lorge
+end
+
+PREAMBLE_SIZE = 25
+local numbers = {}
+
+for line in io.lines() do
+    table.insert(numbers, tonumber(line))
+end
+
+local function has_sum(preamble, number)
+    for _, i in pairs(preamble) do
+        for _, j in pairs(preamble) do
+            if i ~= j and i + j == number then
+                return true
+            end
+        end
+    end
+end
+
+local function find_invalid_number()
+    for i = PREAMBLE_SIZE + 1, #numbers do
+        if not has_sum(table.pack(table.unpack(numbers, i-PREAMBLE_SIZE, i-1)), numbers[i]) then
+            return numbers[i]
+        end
+    end
+end
+
+local function find_weakness(number)
+    for size = 2, #numbers do
+        for i = size, #numbers do
+            local range = table.pack(table.unpack(numbers, i-size+1, i))
+            if sum(range) == number then
+                return min(range) + max(range)
+            end
+        end
+    end
+end
+
+local part1 = assert(find_invalid_number())
+print(part1)
+print(assert(find_weakness(part1)))

README.md

@@ -19,7 +19,7 @@ is acceptable; anything goes as long as I solve it myself!
  6 ██    ██       ██
  7 ██    ██       ██
  8 ██    ██       ██
- 9       ██
+ 9       ██       ██
 10 ██    ██
 11 ██    ██
 12 ██    ██