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

@@ -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)
 }