Day6 solution

Advent24/Day6.cs

@@ -0,0 +1,257 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection.Emit;
+using System.Text;
+using System.Threading.Tasks;
+using System.Text.RegularExpressions;
+using System.IO.Compression;
+using System.Xml.Serialization;
+
+namespace Advent24
+{
+    internal class Day6
+    {
+        private static Boolean simulateGuardPath(String map, ref HashSet<Int32> visited, Int32 newObstacleLinearPosition = -1)
+        {
+            // get width of line to be able to navigate the text block in
+            // both dimensions
+            Int32 mapWidth = map.IndexOf('\n');
+
+            Int32 guardLinearPosition = 0;
+            Int32 guardDirection = 0; //1 = up, 2 = right, 3 = down, 4 = left
+
+            // counts the number of times we go forward in time without
+            Int32 numberOfIterationsWithoutNewVisits = 0;
+
+            Int32 previousNumberOfVisitedPositions = 0;
+
+            Int32 numberOfDirectionChanges = 0;
+
+            Boolean guardIsInMapBoundary = true;
+
+            Boolean wasInfiniteLoop = false;
+
+            // Find the initial position and direction of the guard
+            guardLinearPosition = map.IndexOf('^');
+
+            if (guardLinearPosition != -1) // in up direction
+            {
+                guardDirection = 1;
+                // invalid new obstacle position, so return prematurely with
+                // the assumption that it didn't lead to an infinite loop
+                if (newObstacleLinearPosition == guardLinearPosition)
+                    return false; // assumed value of wasInfiniteLoop
+            }
+            else // not in up direction
+            {
+                guardLinearPosition = map.IndexOf('>');
+                if (guardLinearPosition != -1) // in right direction
+                    guardDirection = 2;
+                else // not in up or right direction
+                {
+                    guardLinearPosition = map.IndexOf('v');
+                    if (guardLinearPosition != -1) // in down direction
+                        guardDirection = 3;
+                    else // not in up, right or down direction
+                    {
+                        guardLinearPosition = map.IndexOf('<');
+                        if (guardLinearPosition != -1) // in left direction
+                            guardDirection = 4;
+                        else // guard not found
+                        {
+                            guardDirection = -1;
+                            guardIsInMapBoundary = false;
+                        }
+                    }
+                }
+            }
+
+            // add intiial guard linear position
+            if (guardIsInMapBoundary) visited.Add(guardLinearPosition);
+
+            while (guardIsInMapBoundary)
+            {
+                // check next position of guard
+
+                // top
+                if (guardDirection == 1)
+                {
+                    // if obstacle detected
+                    if (guardLinearPosition - (mapWidth + 1) >= 0 &&
+                        (
+                            map[guardLinearPosition - (mapWidth + 1)] == '#' ||
+                            guardLinearPosition - (mapWidth + 1) == newObstacleLinearPosition
+                        ))
+                    {
+                        // turn 90 deg clockwise
+                        guardDirection = 2;
+                        numberOfDirectionChanges++;
+                    }
+                    // no obstacle detected
+                    else
+                        // go up one position
+                        guardLinearPosition -= mapWidth + 1;
+                }
+                // right
+                else if (guardDirection == 2)
+                {
+                    // if obstacle detected
+                    if (guardLinearPosition + 1 < map.Length &&
+                        (
+                            map[guardLinearPosition + 1] == '#' ||
+                            guardLinearPosition + 1 == newObstacleLinearPosition
+                        ))
+                    {
+                        // turn 90 deg clockwise
+                        guardDirection = 3;
+                        numberOfDirectionChanges++;
+                    }
+                    // no obstacle detected
+                    else
+                        // go up one position
+                        guardLinearPosition += 1;
+                }
+                // down
+                else if (guardDirection == 3)
+                {
+                    // if obstacle detected
+                    if (guardLinearPosition + (mapWidth + 1) < map.Length &&
+                        (
+                            map[guardLinearPosition + (mapWidth + 1)] == '#' ||
+                            guardLinearPosition + (mapWidth + 1) == newObstacleLinearPosition
+                        ))
+                    {
+                        // turn 90 deg clockwise
+                        guardDirection = 4;
+                        numberOfDirectionChanges++;
+                    }
+                    // no obstacle detected
+                    else
+                        // go up one position
+                        guardLinearPosition += (mapWidth + 1);
+                }
+                // left
+                else if (guardDirection == 4)
+                {
+                    // if obstacle detected
+                    if (guardLinearPosition - 1 >= 0 &&
+                        (
+                            map[guardLinearPosition - 1] == '#' ||
+                            guardLinearPosition - 1 == newObstacleLinearPosition
+                        ))
+                    {    
+                        // turn 90 deg clockwise
+                        guardDirection = 1;
+                        numberOfDirectionChanges++;
+                    }
+                    // no obstacle detected
+                    else
+                        // go up one position
+                        guardLinearPosition -= 1;
+                }
+
+                // right and left boundary check
+                if (guardLinearPosition > map.Length ||
+                    guardLinearPosition < 0 ||
+                    map[guardLinearPosition] == '\n')
+                    guardIsInMapBoundary = false;
+
+                // top and bottom boundary check
+                if (guardLinearPosition < 0 ||
+                    guardLinearPosition >= map.Length)
+                    guardIsInMapBoundary = false;
+
+                // if boundary check passes, add position to visited
+                if (guardIsInMapBoundary)
+                {
+                    previousNumberOfVisitedPositions = visited.Count;
+                    visited.Add(guardLinearPosition);
+                }
+
+                // if there is no change in the number of visited positions
+                if (previousNumberOfVisitedPositions == visited.Count)
+                {
+                    numberOfIterationsWithoutNewVisits++;
+
+                    // we've followed the whole path again, of which a good approximation is
+                    // that the number of unique visited positions plus the number of times
+                    // the direction has changes is equal to the number of iterations without new
+                    // visits - this will undercount intersections
+                    if (numberOfIterationsWithoutNewVisits > (visited.Count + numberOfDirectionChanges))
+                    {
+                        // Console.WriteLine("  numberOfIterationsWithoutNewVisits = {0:d}", numberOfIterationsWithoutNewVisits);
+                        wasInfiniteLoop = true;
+                        break;
+                    }
+                }
+            }
+
+            return wasInfiniteLoop;
+        }
+        public Day6()
+        {
+            String fileData = System.IO.File.ReadAllText(@"..\..\..\inputd6.txt");
+
+            
+            // set of visited linearPositions
+            HashSet<Int32> visited = [];
+
+            simulateGuardPath(fileData, ref visited);
+
+            Int32 numberOfDistinctPositions = visited.Count;
+
+            Console.WriteLine("numberOfDistinctPositions = {0:d}", numberOfDistinctPositions);
+
+            Int32 numberOfViableNewObstaclePositions = 0;
+
+            Boolean wasInfiniteLoop;
+
+            // a set of unchecked linearPositions
+            HashSet<Int32> uncheckedLinearPositions = [.. visited];
+
+            // a set of alreaedy-checked linearPositions
+            HashSet<Int32> checkedLinearPositions = [];
+
+            Int32 previousUncheckedLinearPositionsCount = uncheckedLinearPositions.Count;
+
+            // while there are unchecked visited positions
+            while (uncheckedLinearPositions.Count > 0)
+            {
+                // clear visited hashset for a fresh start per iteration
+                visited.Clear();
+
+                // get first non-checked linear position
+                var firstNonCheckedLinearPosition = uncheckedLinearPositions.First();
+
+                wasInfiniteLoop = simulateGuardPath(fileData, ref visited, firstNonCheckedLinearPosition);
+
+                // if the guard is in a loop, we have a viable obstacle position
+                if (wasInfiniteLoop)
+                    numberOfViableNewObstaclePositions++;
+
+                // update checkedLinearPositions
+                checkedLinearPositions.Add(firstNonCheckedLinearPosition);
+
+                // remove already-checked linear positions from visited
+                // so it only has new positions to add to our uncheckedLinearPositions
+                visited.ExceptWith(checkedLinearPositions);
+
+                previousUncheckedLinearPositionsCount = uncheckedLinearPositions.Count;
+
+                // update uncheckedLinearPositions to remove firstNonCheckedLinearPosition
+                // and add newly visisted linear positions
+                uncheckedLinearPositions.Remove(firstNonCheckedLinearPosition);
+                uncheckedLinearPositions.UnionWith(visited);
+
+             // if (uncheckedLinearPositions.Count > previousUncheckedLinearPositionsCount)
+             //     Console.WriteLine("(change) uncheckedLinearPositions.Count = {0:d}", uncheckedLinearPositions.Count - previousUncheckedLinearPositionsCount);
+            }
+
+            Console.WriteLine("numberOfViableNewObstaclePositions = {0:d}", numberOfViableNewObstaclePositions);
+
+            // to keep the console window from closing
+            Console.ReadKey();
+        }
+    }
+}

Advent24/Program.cs

@@ -11,7 +11,8 @@ namespace Advent24
             //_ = new Day2();
             //_ = new Day3();
             //_ = new Day4();
-            _ = new Day5();
+            // _ = new Day5();
+            _ = new Day6();
         }
     }
 }

Advent24/inputd6.txt

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+..........................#.........#......#.................................#...........................#........................
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