2024/21

2024/Day21/Solution.cs

@@ -4,81 +4,89 @@ namespace AdventOfCode.Y2024.Day21;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
-using System.Numerics;
-using AngleSharp.Common;
-using Cache = System.Collections.Concurrent.ConcurrentDictionary<(char, char, int),  long>;
-using Keypad = System.Collections.Generic.Dictionary<System.Numerics.Complex, char>;
-record struct Vec2(long x, long y);
 
+using Cache = System.Collections.Concurrent.ConcurrentDictionary<(char currentKey, char nextKey, int depth), long>;
+using Keypad = System.Collections.Generic.Dictionary<Vec2, char>;
+record struct Vec2(int x, int y);
 
 [ProblemName("Keypad Conundrum")]
 class Solution : Solver {
 
-    public object PartOne(string input) {
-        return input.Split("\n").Sum(line => Solve2(line, 2));
-    }
-    public object PartTwo(string input) {
-        return input.Split("\n").Sum(line => Solve2(line, 25));
-    }
-
-    static readonly Complex Right = 1;
-    static readonly Complex Up = Complex.ImaginaryOne;
-    static readonly Complex Down = -Complex.ImaginaryOne;
-
-    long Solve2(string line, int depth) {
+    public object PartOne(string input) => Solve(input, 2);
+    public object PartTwo(string input) => Solve(input, 25);
+    long Solve(string input, int depth) {
         var keypad1 = ParseKeypad("789\n456\n123\n 0A");
         var keypad2 = ParseKeypad(" ^A\n<v>");
         var keypads = Enumerable.Repeat(keypad2, depth).Prepend(keypad1).ToArray();
-        var res = EncodeString(line, keypads,  new Cache());
-        return res * int.Parse(line.Substring(0, line.Length - 1));
+
+        var cache = new Cache();
+        var res = 0L;
+
+        foreach (var line in input.Split("\n")) {
+            var num = int.Parse(line[..^1]);
+            res += num * EncodeKeys(line, keypads, cache);
+        }
+        return res;
     }
 
-    long EncodeString(string st, Keypad[] keypads, Cache cache) {
+    // Determines the length of the shortest sequence that is needed to enter the given 
+    // keys. An empty keypad array means that the sequence is simply entered by a human 
+    // and no further encoding is needed. Otherwise the sequence is entered by a robot
+    // which needs to be programmed. In practice this means that the keys are encoded 
+    // using the robots keypad (the first keypad), generating an other sequence of keys.
+    // This other sequence is then recursively encoded using the rest of the keypads.
+    long EncodeKeys(string keys, Keypad[] keypads, Cache cache) {
         if (keypads.Length == 0) {
-            return st.Length;
+            return keys.Length;
         } else {
-
-            // the robot starts and finishes by pointing to 'A' key
+            // invariant: the robot starts and finishes by pointing at the 'A' key
             var currentKey = 'A';
             var length = 0L;
-            foreach (var nextKey in st) {
+
+            foreach (var nextKey in keys) {
                 length += EncodeKey(currentKey, nextKey, keypads, cache);
+                // while the sequence is entered the current key changes accordingly
                 currentKey = nextKey;
             }
-            Debug.Assert(st.Last() == 'A', "The robot should point to the 'A' key");
+
+            // at the end the current key should be reset to 'A'
+            Debug.Assert(currentKey == 'A', "The robot should point at the 'A' key");
             return length;
         }
     }
-    long EncodeKey(char currentKey, char nextKey, Keypad[] keypads,  Cache cache) {
-        return cache.GetOrAdd((currentKey, nextKey, keypads.Length), _ => {
-            var currentPos = keypads[0].Single(kvp => kvp.Value == currentKey).Key;
-            var nextPos = keypads[0].Single(kvp => kvp.Value == nextKey).Key;
+    long EncodeKey(char currentKey, char nextKey, Keypad[] keypads, Cache cache) =>
+       cache.GetOrAdd((currentKey, nextKey, keypads.Length), _ => {
+           var keypad = keypads[0];
 
-            var dy = (int)(nextPos.Imaginary - currentPos.Imaginary);
-            var dx = (int)(nextPos.Real - currentPos.Real);
+           var currentPos = keypad.Single(kvp => kvp.Value == currentKey).Key;
+           var nextPos = keypad.Single(kvp => kvp.Value == nextKey).Key;
 
-            var vert = new string(dy < 0 ? 'v' : '^', Math.Abs(dy));
-            var horiz = new string(dx < 0 ? '<' : '>', Math.Abs(dx));
+           var dy = nextPos.y - currentPos.y;
+           var vert = new string(dy < 0 ? 'v' : '^', Math.Abs(dy));
 
-            var cost = long.MaxValue;
+           var dx = nextPos.x - currentPos.x;
+           var horiz = new string(dx < 0 ? '<' : '>', Math.Abs(dx));
 
-            if (keypads[0][currentPos + dy * Up] != ' ') {
-                cost = Math.Min(cost, EncodeString($"{vert}{horiz}A", keypads[1..], cache));
-            }
-
-            if (keypads[0][currentPos + dx * Right] != ' ') {
-                cost = Math.Min(cost, EncodeString($"{horiz}{vert}A", keypads[1..], cache));
-            }
-            return cost;
-        });
-    }
+           var cost = long.MaxValue;
+           // we can usually go vertical first then horizontal or vica versa,
+           // but we should check for the extra condition and don't position
+           // the robot over the ' ' key:
+           if (keypad[new Vec2(currentPos.x, nextPos.y)] != ' ') {
+               cost = Math.Min(cost, EncodeKeys($"{vert}{horiz}A", keypads[1..], cache));
+           }
+
+           if (keypad[new Vec2(nextPos.x, currentPos.y)] != ' ') {
+               cost = Math.Min(cost, EncodeKeys($"{horiz}{vert}A", keypads[1..], cache));
+           }
+           return cost;
+       });
 
     Keypad ParseKeypad(string keypad) {
         var lines = keypad.Split("\n");
         return (
             from y in Enumerable.Range(0, lines.Length)
             from x in Enumerable.Range(0, lines[0].Length)
-            select new KeyValuePair<Complex, char>(x + y * Down, lines[y][x])
+            select new KeyValuePair<Vec2, char>(new Vec2(x, -y), lines[y][x])
         ).ToDictionary();
     }
 }