76 lines
2.1 KiB
C#
76 lines
2.1 KiB
C#
namespace aoc2020;
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/// <summary>
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/// Day 18: <see href="https://adventofcode.com/2020/day/18" />
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/// </summary>
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public sealed class Day18 : Day
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{
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private readonly List<string> _expressions;
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public Day18() : base(18, "Operation Order")
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{
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_expressions = Input.Select(line => line.Replace(" ", "")).ToList();
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}
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private static long Calculate(string expr, Func<char, int> precedence)
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{
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var postfixNotation = new StringBuilder();
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var postfixStack = new Stack<char>();
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foreach (var c in expr)
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if (char.IsDigit(c))
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{
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postfixNotation.Append(c);
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}
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else if (c == '(')
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{
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postfixStack.Push(c);
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}
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else if (c == ')')
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{
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while (postfixStack.Count > 0 && postfixStack.Peek() != '(')
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postfixNotation.Append(postfixStack.Pop());
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postfixStack.TryPop(out _);
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}
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else
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{
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while (postfixStack.Count > 0 && precedence(c) <= precedence(postfixStack.Peek()))
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postfixNotation.Append(postfixStack.Pop());
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postfixStack.Push(c);
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}
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while (postfixStack.Count > 0)
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postfixNotation.Append(postfixStack.Pop());
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var expressionStack = new Stack<long>();
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foreach (var c in postfixNotation.ToString())
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if (char.IsDigit(c))
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{
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expressionStack.Push((long)char.GetNumericValue(c));
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}
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else
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{
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var a = expressionStack.Pop();
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var b = expressionStack.Pop();
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if (c == '+') expressionStack.Push(a + b);
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if (c == '*') expressionStack.Push(a * b);
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}
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return expressionStack.Pop();
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}
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public override string Part1()
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{
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return $"{_expressions.Sum(expr => Calculate(expr, c => c == '+' || c == '*' ? 1 : 0))}";
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}
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public override string Part2()
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{
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return $"{_expressions.Sum(expr => Calculate(expr, c => c switch { '+' => 2, '*' => 1, _ => 0 }))}";
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}
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}
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