day(19): Part 2

adamrodger · Dec 19, 2023 · fcbe385 · fcbe385
1 parent c570c9b
commit fcbe385
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Showing 2 changed files with 37 additions and 18 deletions.
diff --git a/src/AdventOfCode/Day19.cs b/src/AdventOfCode/Day19.cs
@@ -62,6 +62,8 @@ public long Part2(string[] input)
             Dictionary<string, PartRule> rules = input.TakeWhile(line => !string.IsNullOrEmpty(line))
                                                       .Select(PartRule.Parse)
                                                       .ToDictionary(r => r.Id);
+            rules["A"] = new PartRule("A", Array.Empty<PartPredicate>());
+            rules["R"] = new PartRule("R", Array.Empty<PartPredicate>());
 
             // 171210436954933152 -- too high
 
@@ -70,24 +72,25 @@ public long Part2(string[] input)
 
         private long PossibleCombinations(PartRule current, PartBounds bounds, Dictionary<string, PartRule> rules)
         {
-            long total = 0;
+            if (current.Id == "R")
+            {
+                return 0;
+            }
 
-            foreach (PartPredicate predicate in current.Predicates)
+            if (current.Id == "A")
             {
-                if (predicate.Destination == "R")
-                {
-                    return 0;
-                }
+                return bounds.Possibilities;
+            }
 
-                if (predicate.Destination == "A")
-                {
-                    return bounds.Possibilities;
-                }
+            long total = 0;
 
-                PartRule destination = rules[predicate.Destination];
+            foreach (PartPredicate predicate in current.Predicates)
+            {
+                // reduce the valid bounds according to the current predicate
+                total += PossibleCombinations(rules[predicate.Destination], bounds.MatchedBounds(predicate), rules);
 
-                total += PossibleCombinations(destination, bounds.MatchedBounds(predicate), rules);
-                total += PossibleCombinations(destination, bounds.NotMatchedBounds(predicate), rules);
+                // next predicate didn't match this one, so reduce the bounds accordingly in the opposite direction
+                bounds = bounds.NotMatchedBounds(predicate);
             }
 
             return total;
@@ -196,7 +199,7 @@ private record PartBounds(int MinX, int MaxX, int MinM, int MaxM, int MinA, int
         {
             public static readonly PartBounds Default = new(1, 4000, 1, 4000, 1, 4000, 1, 4000);
 
-            public long Possibilities => (long)(MaxX - MinX) * (MaxM - MinM) * (MaxA - MinA) * (MaxS - MinS);
+            public long Possibilities => (long)(MaxX - MinX + 1) * (MaxM - MinM + 1) * (MaxA - MinA + 1) * (MaxS - MinS + 1);
 
             public PartBounds MatchedBounds(PartPredicate predicate) => (predicate.Property, predicate.Operation) switch
             {

diff --git a/tests/AdventOfCode.Tests/Day19Tests.cs b/tests/AdventOfCode.Tests/Day19Tests.cs
@@ -26,14 +26,30 @@ private static string[] GetSampleInput()
         {
             return new string[]
             {
-
+                "px{a<2006:qkq,m>2090:A,rfg}",
+                "pv{a>1716:R,A}",
+                "lnx{m>1548:A,A}",
+                "rfg{s<537:gd,x>2440:R,A}",
+                "qs{s>3448:A,lnx}",
+                "qkq{x<1416:A,crn}",
+                "crn{x>2662:A,R}",
+                "in{s<1351:px,qqz}",
+                "qqz{s>2770:qs,m<1801:hdj,R}",
+                "gd{a>3333:R,R}",
+                "hdj{m>838:A,pv}",
+                "",
+                "{x=787,m=2655,a=1222,s=2876}",
+                "{x=1679,m=44,a=2067,s=496}",
+                "{x=2036,m=264,a=79,s=2244}",
+                "{x=2461,m=1339,a=466,s=291}",
+                "{x=2127,m=1623,a=2188,s=1013}",
             };
         }
 
         [Fact]
         public void Part1_SampleInput_ProducesCorrectResponse()
         {
-            var expected = -1;
+            var expected = 19114;
 
             var result = solver.Part1(GetSampleInput());
 
@@ -54,7 +70,7 @@ public void Part1_RealInput_ProducesCorrectResponse()
         [Fact]
         public void Part2_SampleInput_ProducesCorrectResponse()
         {
-            var expected = -1;
+            var expected = 167_409_079_868_000;
 
             var result = solver.Part2(GetSampleInput());
 
@@ -64,7 +80,7 @@ public void Part2_SampleInput_ProducesCorrectResponse()
         [Fact]
         public void Part2_RealInput_ProducesCorrectResponse()
         {
-            var expected = -1;
+            var expected = 125_455_345_557_345;
 
             var result = solver.Part2(GetRealInput());
             output.WriteLine($"Day 19 - Part 2 - {result}");