day 18

aoc2020.test/DayTests.cs

@@ -24,8 +24,8 @@ namespace aoc2020.test
         [DataRow(typeof(Day14), "17481577045893", "4160009892257")]
         [DataRow(typeof(Day15), "257", "8546398")]
         [DataRow(typeof(Day16), "19093", "5311123569883")]
-        [DataRow(typeof(Day18), "", "")]
         // [DataRow(typeof(Day17), "293", "1816")]
+        [DataRow(typeof(Day18), "12918250417632", "171259538712010")]
         [DataRow(typeof(Day19), "", "")]
         [DataRow(typeof(Day20), "", "")]
         [DataRow(typeof(Day21), "", "")]

aoc2020/Day18.cs

@@ -1,5 +1,7 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
+using System.Text;
 
 namespace aoc2020
 {
@@ -8,18 +10,72 @@ namespace aoc2020
     /// </summary>
     public sealed class Day18 : Day
     {
+        private readonly List<string> _expressions;
+
         public Day18() : base(18, "Operation Order")
         {
+            _expressions = Input.Select(line => line.Replace(" ", "")).ToList();
+        }
+
+        private static long Calculate(string expr, Func<char, int> precedence)
+        {
+            var postfixNotation = new StringBuilder();
+            var postfixStack = new Stack<char>();
+
+            foreach (var c in expr)
+                if (char.IsDigit(c))
+                {
+                    postfixNotation.Append(c);
+                }
+                else if (c == '(')
+                {
+                    postfixStack.Push(c);
+                }
+                else if (c == ')')
+                {
+                    while (postfixStack.Count > 0 && postfixStack.Peek() != '(')
+                        postfixNotation.Append(postfixStack.Pop());
+
+                    postfixStack.TryPop(out _);
+                }
+                else
+                {
+                    while (postfixStack.Count > 0 && precedence(c) <= precedence(postfixStack.Peek()))
+                        postfixNotation.Append(postfixStack.Pop());
+
+                    postfixStack.Push(c);
+                }
+
+            while (postfixStack.Count > 0)
+                postfixNotation.Append(postfixStack.Pop());
+
+            var expressionStack = new Stack<long>();
+
+            foreach (var c in postfixNotation.ToString())
+                if (char.IsDigit(c))
+                {
+                    expressionStack.Push((long) char.GetNumericValue(c));
+                }
+                else
+                {
+                    var a = expressionStack.Pop();
+                    var b = expressionStack.Pop();
+
+                    if (c == '+') expressionStack.Push(a + b);
+                    if (c == '*') expressionStack.Push(a * b);
+                }
+
+            return expressionStack.Pop();
         }
 
         public override string Part1()
         {
-            return "";
+            return $"{_expressions.Sum(expr => Calculate(expr, c => c == '+' || c == '*' ? 1 : 0))}";
         }
 
         public override string Part2()
         {
-            return "";
+            return $"{_expressions.Sum(expr => Calculate(expr, c => c switch {'+' => 2, '*' => 1, _ => 0}))}";
         }
     }
 }