From ce6e63b5b6c50b65b419577999365aef1f9ad5eb Mon Sep 17 00:00:00 2001
From: Parnic <github@parnic.com>
Date: Mon, 13 Jun 2022 08:31:43 -0500
Subject: [PATCH] Day 15 Part 2

Well...that turned out easier than I thought. I suspected this solution would work, but wasn't completely confident. It can only work for the type of maze used by this problem (where there are no loops of open areas).
---
 days/15.go | 32 +++++++++++++++++++++++++++++++-
 1 file changed, 31 insertions(+), 1 deletion(-)

diff --git a/days/15.go b/days/15.go
index ef8f4d0..8c11a24 100644
--- a/days/15.go
+++ b/days/15.go
@@ -5,6 +5,7 @@ package days
 import (
 	"fmt"
 	"math"
+	"sort"
 
 	u "parnic.com/aoc2019/utilities"
 )
@@ -99,6 +100,17 @@ func (d Day15) getCellTypeInDirection(pos point, dir dirType) (cellStatus, point
 	return d.grid[target], target
 }
 
+func (d Day15) getAdjacentCellsOfType(pos point, cellType cellStatus) []point {
+	points := make([]point, 0, 4)
+	for i := dirFirst; i <= dirLast; i++ {
+		adjacentCell := d.getPointInDirection(pos, i)
+		if d.grid[adjacentCell] == cellType {
+			points = append(points, adjacentCell)
+		}
+	}
+	return points
+}
+
 func (d Day15) getDirToNextCellType(pos point, t cellStatus, maxNumVisited int, dirs []dirType) (dirType, point, error) {
 	for _, dir := range dirs {
 		cellInDirection, targetCell := d.getCellTypeInDirection(pos, dir)
@@ -284,6 +296,16 @@ func (d *Day15) exploreFullMap() map[point]*visitedStatus {
 	return goalVisited
 }
 
+func (d *Day15) tagDistanceRecursive(pos, last point, dist int, distances map[point]int) {
+	distances[pos] = dist
+	for _, cell := range d.getAdjacentCellsOfType(pos, cellStatusOpen) {
+		if cell == last {
+			continue
+		}
+		d.tagDistanceRecursive(cell, pos, dist+1, distances)
+	}
+}
+
 func (d *Day15) Part1() string {
 	d.visited = d.exploreFullMap()
 	d.markShortestPath()
@@ -298,5 +320,13 @@ func (d *Day15) Part1() string {
 }
 
 func (d *Day15) Part2() string {
-	return fmt.Sprintf("%s%d%s", u.TextBold, 0, u.TextReset)
+	startLoc := d.goalPos
+	distanceMap := map[point]int{startLoc: 0}
+
+	d.tagDistanceRecursive(startLoc, point{}, 0, distanceMap)
+
+	cellDistances := u.MapValues(distanceMap)
+	sort.Slice(cellDistances, func(i, j int) bool { return cellDistances[i] > cellDistances[j] })
+
+	return fmt.Sprintf("%s%d%s", u.TextBold, cellDistances[0], u.TextReset)
 }