1 files changed, 138 insertions, 0 deletions
diff --git a/day16/main.go b/day16/main.go
new file mode 100644
index 0000000..fd8b346
--- /dev/null
+++ b/day16/main.go
@@ -0,0 +1,138 @@
+package main
+
+import (
+	"bufio"
+	"container/heap"
+	"fmt"
+	"math"
+	"os"
+	"strings"
+)
+
+func main() {
+	fmt.Println(7036, 45)
+	fmt.Println(part1And2(readData("example.txt")))
+	fmt.Println(11048, 64)
+	fmt.Println(part1And2(readData("example2.txt")))
+	fmt.Println(89460, 504)
+	fmt.Println(part1And2(readData("data.txt")))
+}
+
+type Position struct {
+	x, y int
+}
+
+type PositionDir struct {
+	x, y int
+	dir  int
+}
+
+type Candidate struct {
+	x, y    int
+	dir     int
+	cost    int
+	visited []PositionDir
+}
+
+type CandidateQueue struct {
+	elements []Candidate
+}
+
+func (queue *CandidateQueue) Len() int {
+	return len(queue.elements)
+}
+
+func (queue *CandidateQueue) Less(i, j int) bool {
+	return queue.elements[i].cost < queue.elements[j].cost
+}
+
+func (queue *CandidateQueue) Swap(i, j int) {
+	queue.elements[i], queue.elements[j] = queue.elements[j], queue.elements[i]
+}
+
+func (queue *CandidateQueue) Push(x any) {
+	queue.elements = append(queue.elements, x.(Candidate))
+}
+
+func (queue *CandidateQueue) AddElement(candidate Candidate) {
+	heap.Push(queue, candidate)
+}
+
+func (queue *CandidateQueue) Pop() any {
+	if len(queue.elements) == 0 {
+		return nil
+	} else {
+		elmt := queue.elements[len(queue.elements)-1]
+		queue.elements = queue.elements[:len(queue.elements)-1]
+		return elmt
+	}
+}
+
+func cloneVisited(v []PositionDir) []PositionDir {
+	clone := make([]PositionDir, len(v))
+	for i, p := range v {
+		clone[i] = p
+	}
+	return clone
+}
+
+var directions = [4][2]int{{1, 0}, {0, 1}, {-1, 0}, {0, -1}}
+
+// Originally I had implemented A* for part1. But because of part2 I modified it to be
+// dumber. I wish we had full requirements from the start so I wouldn't waste time...
+
+func part1And2(maze [][]byte) (scorePart1, scorePart2 int) {
+	endX, endY := len(maze[0])-2, 1
+	startX, startY := 1, len(maze)-2
+
+	queue := new(CandidateQueue)
+	heap.Push(queue, Candidate{startX, startY, 0, 0, []PositionDir{{startX, startY, 0}}})
+	minCost := math.MaxInt
+	visitedCost := make(map[PositionDir]int)
+	onOptimalPath := make(map[Position]bool)
+
+	for queue.Len() > 0 {
+		candidate := heap.Pop(queue).(Candidate)
+		posDir := PositionDir{candidate.x, candidate.y, candidate.dir}
+
+		alreadyVisitedCost, alreadyVisited := visitedCost[posDir]
+		if alreadyVisited && alreadyVisitedCost < candidate.cost {
+			continue
+		}
+		visitedCost[posDir] = candidate.cost
+
+		tryDirection := func(dir int, cost int) {
+			fwdX := candidate.x + directions[dir][0]
+			fwdY := candidate.y + directions[dir][1]
+			realCostFwd := candidate.cost + cost
+			if maze[fwdY][fwdX] != '#' && realCostFwd <= minCost {
+				if fwdX == endX && fwdY == endY && realCostFwd <= minCost {
+					for _, p := range candidate.visited {
+						onOptimalPath[Position{p.x, p.y}] = true
+					}
+					minCost = realCostFwd
+				} else {
+					visited := append(cloneVisited(candidate.visited), PositionDir{fwdX, fwdY, dir})
+					c := Candidate{fwdX, fwdY, dir, realCostFwd, visited}
+					heap.Push(queue, c)
+				}
+			}
+		}
+
+		tryDirection(candidate.dir, 1)
+		tryDirection((candidate.dir+1)%4, 1001)
+		tryDirection((candidate.dir+3)%4, 1001)
+	}
+
+	return minCost, len(onOptimalPath) + 1
+}
+
+func readData(fileName string) (maze [][]byte) {
+	fp, _ := os.Open(fileName)
+	scanner := bufio.NewScanner(fp)
+
+	for scanner.Scan() {
+		maze = append(maze, []byte(strings.TrimSpace(scanner.Text())))
+	}
+	return
+}