feat(angch/2024-20): Part1

Author: angch

angch/2024-20/main.go

@@ -0,0 +1,183 @@
+package main
+
+import (
+	"bufio"
+	"container/heap"
+	"fmt"
+	"log"
+	"os"
+	"time"
+)
+
+var dirs = [][2]int{{0, 1}, {0, -1}, {1, 0}, {-1, 0}}
+
+type boardstate struct {
+	index int
+	mm    map[[2]int]byte
+	moves [][2]int
+}
+
+// A PriorityQueue implements heap.Interface and holds Items.
+type PriorityQueue []*boardstate
+
+func (pq PriorityQueue) Len() int { return len(pq) }
+func (pq PriorityQueue) Less(i, j int) bool {
+	return len(pq[i].moves) < len(pq[j].moves)
+}
+func (pq PriorityQueue) Swap(i, j int) {
+	pq[i], pq[j] = pq[j], pq[i]
+	pq[i].index = i
+	pq[j].index = j
+}
+
+func (pq *PriorityQueue) Push(x any) {
+	n := len(*pq)
+	item := x.(*boardstate)
+	item.index = n
+	*pq = append(*pq, item)
+}
+func (pq *PriorityQueue) Pop() any {
+	old := *pq
+	n := len(old)
+	item := old[n-1]
+	old[n-1] = nil  // don't stop the GC from reclaiming the item eventually
+	item.index = -1 // for safety
+	*pq = old[0 : n-1]
+	return item
+}
+
+func solve(mm map[[2]int]byte, start, end [2]int, maxx, maxy int) []boardstate {
+	cur := start
+	move := 0
+	_ = move
+
+	pq := make(PriorityQueue, 1)
+	pq[0] = &boardstate{0, mm, [][2]int{cur}}
+	heap.Init(&pq)
+
+	for pq.Len() > 0 {
+		e := heap.Pop(&pq).(*boardstate)
+
+	a:
+		for _, d := range dirs {
+			newpos := [2]int{e.moves[len(e.moves)-1][0] + d[0], e.moves[len(e.moves)-1][1] + d[1]}
+			if newpos == end {
+				return []boardstate{{0, e.mm, append(e.moves, newpos)}}
+			}
+			c := mm[newpos]
+			if c == '#' {
+				// it's a wall
+				continue
+			}
+			if c == '.' {
+				for _, v := range e.moves {
+					if newpos == v {
+						continue a
+					}
+				}
+				heap.Push(&pq, &boardstate{0, e.mm, append(e.moves, newpos)})
+			}
+		}
+	}
+	return nil
+}
+
+func day20(file string) (part1, part2 int) {
+	f, err := os.Open(file)
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer f.Close()
+	scanner := bufio.NewScanner(f)
+
+	mm := make(map[[2]int]byte)
+	start := [2]int{}
+	end := [2]int{}
+	maxx := 0
+	y := 0
+	for scanner.Scan() {
+		t := scanner.Text()
+		_ = t
+		maxx = len(t)
+		for x, c := range t {
+			if c == 'S' {
+				start = [2]int{x, y}
+				mm[[2]int{x, y}] = '.'
+				continue
+			} else if c == 'E' {
+				end = [2]int{x, y}
+				mm[[2]int{x, y}] = '.'
+				continue
+			} else if c == '#' {
+				mm[[2]int{x, y}] = '#'
+				continue
+			}
+			mm[[2]int{x, y}] = '.'
+		}
+		y++
+	}
+	_ = start
+	_ = end
+	out := solve(mm, start, end, maxx, y)
+	// log.Printf("%+v\n", out[0].moves)
+	nocheat := len(out[0].moves) - 1
+	part1 = nocheat
+
+	moves := out[0].moves
+
+	path := map[[2]int]int{}
+	for k, move := range moves {
+		path[move] = k
+	}
+
+	counts := make(map[int]int)
+	shortcuts := make(map[[4]int]int)
+	for k, move := range moves {
+		for _, d1 := range dirs {
+			newpos1 := [2]int{move[0] + d1[0], move[1] + d1[1]}
+			if mm[newpos1] == '#' {
+				for _, d2 := range dirs {
+					newpos2 := [2]int{newpos1[0] + d2[0], newpos1[1] + d2[1]}
+					s, ok := path[newpos2]
+					if s > k+2 && ok {
+
+						startpos := move
+						endpos := newpos2
+						saved := s - k - 2
+
+						first, ok := shortcuts[[4]int{startpos[0], startpos[1], endpos[0], endpos[1]}]
+						if ok {
+							if first <= saved {
+								continue
+							}
+						}
+						// fmt.Println("Found shortcut", startpos, endpos, saved)
+
+						shortcuts[[4]int{startpos[0], startpos[1], endpos[0], endpos[1]}] = saved
+
+					}
+				}
+			}
+		}
+	}
+	for _, v := range shortcuts {
+		counts[v]++
+		if v >= 100 {
+			part2++
+		}
+	}
+	// fmt.Println(counts)
+	return
+}
+
+func main() {
+	log.SetFlags(log.Lshortfile | log.LstdFlags)
+	t1 := time.Now()
+	part1, part2 := day20("test.txt")
+	fmt.Println(part1, part2)
+	if part1 != 84 || part2 != 0 {
+		log.Fatal("Test failed ", part1, part2)
+	}
+	fmt.Println(day20("input.txt"))
+	fmt.Println("Elapsed time:", time.Since(t1))
+}

angch/2024-20/test.txt

@@ -0,0 +1,15 @@
+###############
+#...#...#.....#
+#.#.#.#.#.###.#
+#S#...#.#.#...#
+#######.#.#.###
+#######.#.#...#
+#######.#.###.#
+###..E#...#...#
+###.#######.###
+#...###...#...#
+#.#####.#.###.#
+#.#...#.#.#...#
+#.#.#.#.#.#.###
+#...#...#...###
+###############