From cdb9021fa920744577f65679ef31e81d839b9f2a Mon Sep 17 00:00:00 2001
From: Marcin Chrzanowski <marcin.j.chrzanowski@gmail.com>
Date: Fri, 25 Dec 2020 15:35:08 +0100
Subject: Add day 18

---
 18/a.lua | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)
 create mode 100644 18/a.lua

(limited to '18/a.lua')

diff --git a/18/a.lua b/18/a.lua
new file mode 100644
index 0000000..b9dd2d1
--- /dev/null
+++ b/18/a.lua
@@ -0,0 +1,55 @@
+input = io.lines('input.txt')
+
+function is_op(token)
+    return token == '+' or token == '*'
+end
+
+function f_op(op)
+    if op == '+' then
+        return function (a, b) return a + b end
+    else
+        return function (a, b) return a * b end
+    end
+end
+
+function is_digit(token)
+    return token >= '1' and token <= '9'
+end
+
+function do_math(expression)
+    stack = {}
+    for i = 1, #expression do
+        token = expression:sub(i, i)
+        if is_op(token) then
+            table.insert(stack, token)
+        elseif is_digit(token) then
+            if is_op(stack[#stack]) then
+                op = table.remove(stack)
+                n = table.remove(stack)
+                table.insert(stack, f_op(op)(n, tonumber(token)))
+            else
+                table.insert(stack, tonumber(token))
+            end
+        elseif token == '(' then
+            table.insert(stack, token)
+        elseif token == ')' then
+            res = table.remove(stack)
+            table.remove(stack)
+            if is_op(stack[#stack]) then
+                op = table.remove(stack)
+                n = table.remove(stack)
+                table.insert(stack, f_op(op)(n, res))
+            else
+                table.insert(stack, res)
+            end
+        end
+    end
+    return stack[1]
+end
+
+sum = 0
+for line in input do
+    sum = sum + do_math(line)
+end
+
+print(sum)
-- 
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