2024 Day 7

Author: premun

src/2024/07/Program.cs

@@ -1,6 +1,43 @@
 using AdventOfCode.Common;
 
-var lines = Resources.GetInputFileLines();
+var equations = Resources.GetInputFileLines()
+    .Select(line => line.SplitBy(": "))
+    .Select(parts => new Equation(ulong.Parse(parts[0]), [.. parts[1].Split(' ').Select(ulong.Parse)]))
+    .ToArray();
 
-Console.WriteLine($"Part 1: {""}");
-Console.WriteLine($"Part 2: {""}");
+var validEquations = equations
+    .Where(equation => equation.IsValid(useConcatenation: false))
+    .Select(equation => equation.Target)
+    .ToList();
+
+var total = validEquations
+    .Sum();
+
+var totalWithConcatenation = equations
+    .Where(e => !validEquations.Contains(e.Target))
+    .Where(e => e.IsValid(useConcatenation: true))
+    .Select(equation => equation.Target)
+    .Sum();
+
+Console.WriteLine($"Part 1: {total}");
+Console.WriteLine($"Part 2: {total + totalWithConcatenation}");
+
+file record Equation(ulong Target, ulong[] Operands)
+{
+    public bool IsValid(bool useConcatenation)
+        => IsValid(useConcatenation, 0, 0);
+
+    private bool IsValid(bool useConcatenation, ulong acc, int index)
+    {
+        if (index == Operands.Length) return acc == Target;
+        if (acc > Target) return false;
+
+        return IsValid(useConcatenation, Math.Max(acc, 1) * Operands[index], index + 1)
+            || IsValid(useConcatenation, acc + Operands[index], index + 1)
+            || (useConcatenation && IsValid(true, Concatenate(acc, Operands[index]), index + 1));
+    }
+
+    // 1000 with as many zeros as a base-10 log (and account for '0')
+    private static ulong Concatenate(ulong first, ulong second)
+        => first * (ulong)Math.Max(10, Math.Pow(10, Math.Ceiling(Math.Log10(second)))) + second;
+}

src/2024/08/Program.cs

@@ -14,14 +14,14 @@
 int GetAntinodeCount(bool allInLine)
 {
     return antennasByFrequency
-    .SelectMany(group => group
-        .AllCombinations(includeIdentities: false)
-        .Where(pair => pair.Item1.Col < pair.Item2.Col) // We don't need both combinations (x,y) and (y,x)
-        .SelectMany(pair => allInLine
-            ? GetAntinodes2(pair.Item1, pair.Item2)
-            : GetAntinodes1(pair.Item1, pair.Item2)))
-    .Distinct()
-    .Count();
+        .SelectMany(group => group
+            .AllCombinations(includeIdentities: false)
+            .Where(pair => pair.Item1.Col < pair.Item2.Col) // We don't need both combinations (x,y) and (y,x)
+            .SelectMany(pair => allInLine
+                ? GetAntinodes2(pair.Item1, pair.Item2)
+                : GetAntinodes1(pair.Item1, pair.Item2)))
+        .Distinct()
+        .Count();
 }
 
 IEnumerable<Coor> GetAntinodes1(Coor a1, Coor a2)

src/Common/Resources.cs

@@ -162,4 +162,8 @@ public static long GetLeastCommonMultiple(long a, long b)
     {
         return a * b / GetGreatestCommonDivisor(a, b);
     }
+
+    public static ulong Sum(this IEnumerable<ulong> numbers)
+        => numbers.Aggregate(0UL, (i, acc) => i + acc);
+
 }