Day 20 completed

The key feature for part 2 is changing up what the Dijkstra solver views as an "adjacent" node, injecting the appropriate depth as we traverse inner/outer portals. The main thing that tripped me up here from part 1 was that I needed to be carrying more data along with portals than just their locations, and I could no longer get away with storing the portals as pairs of their inner/outer locations, so a small refactor was needed. Once I made those corrections, it was mostly a matter of ironing out the "get neighbors" function to adhere to part 2's rules, which took me a lot of debugging to get just right.

days/14.go

@@ -104,26 +104,10 @@ func (d *Day14) Part1() string {
 func (d *Day14) Part2() string {
 	oreAvailable := int64(1000000000000)
 	estimate := oreAvailable / d.getOreRequiredForFuel(1)
-
-	high := estimate * 2
-	low := estimate
-
-	lastSuccess := low
-	lastFailure := high
-	fuelProduced := low
-
-	for math.Abs(float64(lastFailure-lastSuccess)) > 1 {
-		oreConsumed := d.getOreRequiredForFuel(fuelProduced)
-		adjustment := (lastFailure - lastSuccess) / 2
-		if oreConsumed < oreAvailable {
-			lastSuccess = fuelProduced
-		} else {
-			lastFailure = fuelProduced
-			adjustment = -adjustment
-		}
-
-		fuelProduced += adjustment
-	}
+	lastSuccess := u.Bisect(estimate, estimate*2, 1, func(val int64) bool {
+		oreConsumed := d.getOreRequiredForFuel(val)
+		return oreConsumed < oreAvailable
+	})
 
 	return fmt.Sprintf("Maximum fuel we can make from 1 trillion ore: %s%d%s", u.TextBold, lastSuccess, u.TextReset)
 }

days/18.go

@@ -0,0 +1,345 @@
+package days
+
+import (
+	"container/heap"
+	"fmt"
+	"math"
+	"strings"
+
+	u "parnic.com/aoc2019/utilities"
+)
+
+type day18Cell int
+type day18Vec u.Vec2[int]
+type day18Graph map[rune][]u.Pair[rune, int]
+
+const (
+	day18CellWall day18Cell = iota
+	day18CellOpen
+)
+
+var (
+	day18AdjacentOffsets = []day18Vec{
+		{X: -1, Y: 0},
+		{X: 1, Y: 0},
+		{X: 0, Y: -1},
+		{X: 0, Y: 1},
+	}
+)
+
+type reachableKeysMemo struct {
+	pos       rune
+	keysFound int
+}
+
+type minStepsMemo struct {
+	pos        string
+	keysToFind int
+	keysFound  int
+}
+
+type Day18 struct {
+	entrance           day18Vec
+	grid               [][]day18Cell
+	doors              map[day18Vec]int
+	keys               map[day18Vec]int
+	knownReachableKeys map[reachableKeysMemo][]u.Pair[rune, int]
+	knownMinimumSteps  map[minStepsMemo]int
+}
+
+func (d *Day18) Parse() {
+	d.doors = make(map[day18Vec]int)
+	d.keys = make(map[day18Vec]int)
+	d.knownReachableKeys = make(map[reachableKeysMemo][]u.Pair[rune, int])
+	d.knownMinimumSteps = make(map[minStepsMemo]int, 0)
+
+	lines := u.GetStringLines("18p")
+	d.grid = make([][]day18Cell, len(lines))
+	for i, line := range lines {
+		d.grid[i] = make([]day18Cell, len(line))
+		for j, char := range line {
+			if char == '#' {
+				d.grid[i][j] = day18CellWall
+			} else if char == '.' {
+				d.grid[i][j] = day18CellOpen
+			} else if char == '@' {
+				d.grid[i][j] = day18CellOpen
+				d.entrance = day18Vec{X: j, Y: i}
+			} else if char >= 'A' && char <= 'Z' {
+				d.grid[i][j] = day18CellOpen
+				d.doors[day18Vec{X: j, Y: i}] = int(char - 'A')
+			} else if char >= 'a' && char <= 'z' {
+				d.grid[i][j] = day18CellOpen
+				d.keys[day18Vec{X: j, Y: i}] = int(char - 'a')
+			}
+		}
+	}
+}
+
+func (d Day18) Num() int {
+	return 18
+}
+
+func (d Day18) Draw(grid [][]day18Cell, keys, doors map[day18Vec]int, entrances ...day18Vec) {
+	for y := range grid {
+		for x := range grid[y] {
+			switch grid[y][x] {
+			case day18CellWall:
+				fmt.Print("█")
+			case day18CellOpen:
+				posVec := day18Vec{X: x, Y: y}
+				if _, exists := doors[posVec]; exists {
+					fmt.Printf("%c", rune(doors[posVec]+'A'))
+				} else if _, exists := keys[posVec]; exists {
+					fmt.Printf("%c", rune(keys[posVec]+'a'))
+				} else if u.ArrayContains(entrances, posVec) {
+					fmt.Print("@")
+				} else {
+					fmt.Print(".")
+				}
+			}
+		}
+		fmt.Println()
+	}
+}
+
+func (d Day18) findAdjacentCells(inPos day18Vec, keys, doors map[day18Vec]int, grid [][]day18Cell) []u.Pair[rune, int] {
+	found := make([]u.Pair[rune, int], 0)
+
+	getAdjacent := func(pos day18Vec) []day18Vec {
+		retAdjacent := make([]day18Vec, 0, len(day18AdjacentOffsets))
+		for _, off := range day18AdjacentOffsets {
+			offVec := day18Vec{X: pos.X + off.X, Y: pos.Y + off.Y}
+			if grid[offVec.Y][offVec.X] == day18CellWall {
+				continue
+			}
+			retAdjacent = append(retAdjacent, offVec)
+		}
+
+		return retAdjacent
+	}
+
+	queue := make([]u.Pair[int, day18Vec], 0)
+	visited := make(map[day18Vec]bool)
+	for _, adjacent := range getAdjacent(inPos) {
+		queue = append(queue, u.Pair[int, day18Vec]{First: 1, Second: adjacent})
+	}
+
+	for len(queue) > 0 {
+		next := queue[0]
+		queue = queue[1:]
+
+		if _, exists := visited[next.Second]; !exists {
+			visited[next.Second] = true
+
+			key, adjacentIsKey := keys[next.Second]
+			door, adjacentIsDoor := doors[next.Second]
+			if adjacentIsKey || adjacentIsDoor {
+				var rVal rune
+				if adjacentIsKey {
+					rVal = rune('a' + key)
+				} else if adjacentIsDoor {
+					rVal = rune('A' + door)
+				}
+
+				alreadyFound := false
+				for _, p := range found {
+					if p.First == rVal {
+						alreadyFound = true
+						break
+					}
+				}
+				if !alreadyFound {
+					found = append(found, u.Pair[rune, int]{First: rVal, Second: next.First})
+					continue
+				}
+			}
+
+			for _, neighbor := range getAdjacent(next.Second) {
+				if _, exists := visited[neighbor]; !exists {
+					queue = append(queue, u.Pair[int, day18Vec]{First: next.First + 1, Second: neighbor})
+				}
+			}
+		}
+	}
+
+	return found
+}
+
+type day18PriorityQueue struct {
+	distance int
+	neighbor rune
+}
+type day18PriorityQueueHeap []day18PriorityQueue
+
+func (h day18PriorityQueueHeap) Len() int           { return len(h) }
+func (h day18PriorityQueueHeap) Less(i, j int) bool { return h[i].distance < h[j].distance }
+func (h day18PriorityQueueHeap) Swap(i, j int)      { h[i], h[j] = h[j], h[i] }
+
+func (h *day18PriorityQueueHeap) Push(x any) {
+	*h = append(*h, x.(day18PriorityQueue))
+}
+
+func (h *day18PriorityQueueHeap) Pop() any {
+	old := *h
+	n := len(old)
+	x := old[n-1]
+	*h = old[0 : n-1]
+	return x
+}
+
+func (d Day18) reachableKeys(inPos rune, keysFound int, graph day18Graph) []u.Pair[rune, int] {
+	memo := reachableKeysMemo{
+		pos:       inPos,
+		keysFound: keysFound,
+	}
+	if v, exists := d.knownReachableKeys[memo]; exists {
+		return v
+	}
+
+	ret := make([]u.Pair[rune, int], 0)
+	distance := make(map[rune]int)
+
+	ih := make(day18PriorityQueueHeap, 0)
+
+	for _, p := range graph[inPos] {
+		ih = append(ih, day18PriorityQueue{
+			distance: p.Second,
+			neighbor: p.First,
+		})
+	}
+
+	heap.Init(&ih)
+
+	for ih.Len() > 0 {
+		node := heap.Pop(&ih).(day18PriorityQueue)
+
+		// it's a key and we haven't picked it up yet...
+		if node.neighbor >= 'a' && node.neighbor <= 'z' && (1<<int(node.neighbor-'a')&keysFound) == 0 {
+			ret = append(ret, u.Pair[rune, int]{First: node.neighbor, Second: node.distance})
+			continue
+		}
+
+		// it's a door but we don't have the key yet...
+		if node.neighbor >= 'A' && node.neighbor <= 'Z' && ((1<<int(node.neighbor-'A'))&keysFound) == 0 {
+			continue
+		}
+
+		for _, p := range graph[node.neighbor] {
+			newDistance := node.distance + p.Second
+			if dist, exists := distance[p.First]; !exists || newDistance < dist {
+				distance[p.First] = newDistance
+				heap.Push(&ih, day18PriorityQueue{
+					distance: newDistance,
+					neighbor: p.First,
+				})
+			}
+		}
+	}
+
+	d.knownReachableKeys[memo] = ret
+	return ret
+}
+
+func (d Day18) minimumSteps(inPos string, keysToFind int, keysFound int, graph day18Graph) int {
+	memo := minStepsMemo{
+		pos:        inPos,
+		keysToFind: keysToFind,
+		keysFound:  keysFound,
+	}
+	if v, exists := d.knownMinimumSteps[memo]; exists {
+		return v
+	}
+
+	if keysToFind == 0 {
+		return 0
+	}
+
+	best := math.Inf(1)
+	for _, item := range inPos {
+		for _, p := range d.reachableKeys(item, keysFound, graph) {
+			sb := strings.Builder{}
+			oldIdx := strings.IndexRune(inPos, item)
+			for i := range inPos {
+				if i == oldIdx {
+					sb.WriteRune(p.First)
+				} else {
+					sb.WriteByte(inPos[i])
+				}
+			}
+			newKeys := keysFound + (1 << (p.First - 'a'))
+			dist := p.Second
+
+			dist += d.minimumSteps(sb.String(), keysToFind-1, newKeys, graph)
+
+			if float64(dist) < best {
+				best = float64(dist)
+			}
+		}
+	}
+
+	d.knownMinimumSteps[memo] = int(best)
+	return int(best)
+}
+
+func (d Day18) buildGraph(pos []day18Vec, keys map[day18Vec]int, doors map[day18Vec]int, grid [][]day18Cell) day18Graph {
+	graph := make(day18Graph)
+	for i, p := range pos {
+		adjacent := d.findAdjacentCells(p, keys, doors, grid)
+		graph[rune('1'+i)] = adjacent
+	}
+	for keyPos, keyType := range keys {
+		graph[rune('a'+keyType)] = d.findAdjacentCells(keyPos, keys, doors, grid)
+	}
+	for doorPos, doorType := range doors {
+		graph[rune('A'+doorType)] = d.findAdjacentCells(doorPos, keys, doors, grid)
+	}
+
+	return graph
+}
+
+func (d Day18) part2PatchMap(grid [][]day18Cell, entrance day18Vec) []day18Vec {
+	grid[entrance.Y-1][entrance.X] = day18CellWall
+	grid[entrance.Y][entrance.X-1] = day18CellWall
+	grid[entrance.Y][entrance.X] = day18CellWall
+	grid[entrance.Y][entrance.X+1] = day18CellWall
+	grid[entrance.Y+1][entrance.X] = day18CellWall
+
+	return []day18Vec{
+		{X: entrance.X - 1, Y: entrance.Y - 1},
+		{X: entrance.X + 1, Y: entrance.Y - 1},
+		{X: entrance.X - 1, Y: entrance.Y + 1},
+		{X: entrance.X + 1, Y: entrance.Y + 1},
+	}
+}
+
+func (d *Day18) Part1() string {
+	// fmt.Println("initial state:")
+	// d.Draw(d.grid, d.keys, d.doors, d.entrance)
+
+	graph := d.buildGraph([]day18Vec{d.entrance}, d.keys, d.doors, d.grid)
+	minSteps := d.minimumSteps("1", len(d.keys), 0, graph)
+
+	return fmt.Sprintf("Total distance traveled: %s%d%s", u.TextBold, minSteps, u.TextReset)
+}
+
+func (d *Day18) Part2() string {
+	// fmt.Println("initial state:")
+	grid := make([][]day18Cell, len(d.grid))
+	for i := range d.grid {
+		grid[i] = make([]day18Cell, len(d.grid[i]))
+		copy(grid[i], d.grid[i])
+	}
+
+	entrances := d.part2PatchMap(grid, d.entrance)
+	// d.Draw(grid, d.keys, d.doors, entrances...)
+
+	// clear memoized maps that (might have) came from part1
+	d.knownMinimumSteps = make(map[minStepsMemo]int)
+	d.knownReachableKeys = make(map[reachableKeysMemo][]u.Pair[rune, int])
+
+	graph := d.buildGraph(entrances, d.keys, d.doors, grid)
+	minSteps := d.minimumSteps("1234", len(d.keys), 0, graph)
+
+	return fmt.Sprintf("Total distance traveled: %s%d%s", u.TextBold, minSteps, u.TextReset)
+}

days/19.go

@@ -0,0 +1,146 @@
+package days
+
+import (
+	"fmt"
+
+	u "parnic.com/aoc2019/utilities"
+)
+
+type Day19 struct {
+	program u.IntcodeProgram
+}
+
+func (d *Day19) Parse() {
+	d.program = u.LoadIntcodeProgram("19p")
+}
+
+func (d Day19) Num() int {
+	return 19
+}
+
+func (d *Day19) Part1() string {
+	grid := make([][]bool, 50)
+	for y := 0; y < len(grid); y++ {
+		grid[y] = make([]bool, 50)
+	}
+
+	count := int64(0)
+
+	for y := 0; y < 50; y++ {
+		for x := 0; x < 50; x++ {
+			d.program.Reset()
+			d.program.RunIn(func(inputStep int) int64 {
+				if inputStep == 1 {
+					return int64(x)
+				}
+
+				return int64(y)
+			}, func(val int64, state u.IntcodeProgramState) {
+				res := val == 1
+				grid[y][x] = res
+				if res {
+					count++
+				}
+			})
+		}
+	}
+
+	// fmt.Println("50x50 tractor view:")
+	// for y := 0; y < len(grid); y++ {
+	// 	for x := 0; x < len(grid[y]); x++ {
+	// 		if grid[y][x] {
+	// 			fmt.Print("█")
+	// 		} else {
+	// 			fmt.Print(" ")
+	// 		}
+	// 	}
+	// 	fmt.Println()
+	// }
+
+	return fmt.Sprintf("Points affected in 50x50 area: %s%d%s", u.TextBold, count, u.TextReset)
+}
+
+func (d *Day19) Part2() string {
+	f := func(x, y int) bool {
+		ret := false
+		d.program.Reset()
+		d.program.RunIn(func(inputStep int) int64 {
+			if inputStep == 1 {
+				return int64(x)
+			}
+			return int64(y)
+		}, func(val int64, state u.IntcodeProgramState) {
+			ret = val == 1
+		})
+
+		return ret
+	}
+
+	// find lower bound
+	// this may not be necessary, but helps seed the bisect with a known-good lower bound
+	startY := 0
+	startX := 0
+	for y := 1; startY == 0; y++ {
+		for x := 0; x < 10*y; x++ {
+			if f(x, y) {
+				startY = y
+				startX = x
+				break
+			}
+		}
+	}
+
+	lastGoodX := 0
+	threshold := 1
+	// add 100 to start y since we know it has to be a 100x100 square.
+	// since we multiply x by 10,000 for the final result, that tells us y cannot be 10k+
+	y := u.Bisect(startY+100, 9999, threshold, func(y int) bool {
+		foundX := false
+		for x := startX; ; x++ {
+			// check top left
+			if !f(x, y) {
+				if !foundX {
+					continue
+				} else {
+					return true
+				}
+			}
+			if !foundX {
+				foundX = true
+			}
+
+			// check top right
+			if !f(x+99, y) {
+				return true
+			}
+			// check bottom left
+			if !f(x, y+99) {
+				continue
+			}
+
+			// we believe the corners work, so run final validations on the full borders.
+			// this may not be necessary, but i've seen some rows end up shorter than a
+			// previous row because of the angle of the beam and our integer fidelity.
+			// plus it's really not that much more expensive to do to be certain we're correct.
+			for y2 := y; y2 < y+100; y2++ {
+				// right wall
+				if !f(x+99, y2) {
+					return true
+				}
+				// left wall
+				if !f(x, y2) {
+					return true
+				}
+			}
+
+			lastGoodX = x
+			return false
+		}
+	})
+
+	// since we invert our bisect success returns, we need to increment y to
+	// tip back over into the "success" range
+	y += threshold
+	result := (lastGoodX * 10000) + y
+	return fmt.Sprintf("Closest 100x100 square for the ship starts at %d,%d = %s%d%s", lastGoodX, y, u.TextBold, result, u.TextReset)
+}

days/20.go

@@ -0,0 +1,398 @@
+package days
+
+import (
+	"container/heap"
+	"fmt"
+	"math"
+	"strings"
+
+	u "parnic.com/aoc2019/utilities"
+)
+
+type day20Cell int8
+type day20Graph map[day20Portal][]u.Pair[day20Portal, int]
+
+const (
+	day20CellWall day20Cell = iota
+	day20CellPath
+	day20CellDonutHole
+)
+
+var (
+	day20AdjacentOffsets = []u.Vec2i{
+		{X: -1, Y: 0},
+		{X: 1, Y: 0},
+		{X: 0, Y: -1},
+		{X: 0, Y: 1},
+	}
+
+	entrancePortal = day20Portal{name: "AA"}
+	exitPortal     = day20Portal{name: "ZZ"}
+)
+
+type day20Portal struct {
+	name  string
+	inner bool
+	depth int
+}
+
+type Day20 struct {
+	grid        [][]day20Cell
+	entrance    u.Vec2i
+	exit        u.Vec2i
+	portals     map[day20Portal]u.Vec2i
+	portalNames []string
+}
+
+func (d *Day20) Parse() {
+	d.portals = make(map[day20Portal]u.Vec2i)
+	d.portalNames = make([]string, 0)
+
+	lines := u.GetStringLines("20p")
+	d.grid = make([][]day20Cell, len(lines)-4)
+	currPortal := strings.Builder{}
+
+	for row, line := range lines {
+		y := row - 2
+		isGridRow := row >= 2 && row < len(lines)-2
+
+		if isGridRow {
+			d.grid[y] = make([]day20Cell, len(line)-4)
+		}
+
+		for col, ch := range lines[row] {
+			x := col - 2
+			isGridCol := col >= 2 && col < len(line)-2
+
+			if isGridRow && isGridCol {
+				if ch == '#' {
+					d.grid[y][x] = day20CellWall
+				} else if ch == '.' {
+					d.grid[y][x] = day20CellPath
+				} else {
+					d.grid[y][x] = day20CellDonutHole
+				}
+			}
+
+			if ch >= 'A' && ch <= 'Z' {
+				portalX := 0
+				portalY := 0
+				if len(line) > col+1 && line[col+1] >= 'A' && line[col+1] <= 'Z' {
+					currPortal.WriteRune(ch)
+					currPortal.WriteRune(rune(line[col+1]))
+
+					if len(line) > col+2 && line[col+2] == '.' {
+						portalY = y
+						portalX = x + 2
+					} else if col-1 >= 0 && line[col-1] == '.' {
+						portalY = y
+						portalX = x - 1
+					} else {
+						panic("!")
+					}
+				} else if len(lines) > row+1 && lines[row+1][col] >= 'A' && lines[row+1][col] <= 'Z' {
+					currPortal.WriteRune(ch)
+					currPortal.WriteRune(rune(lines[row+1][col]))
+
+					if len(lines) > row+2 && lines[row+2][col] == '.' {
+						portalY = y + 2
+						portalX = x
+					} else if row-1 >= 0 && lines[row-1][col] == '.' {
+						portalY = y - 1
+						portalX = x
+					} else {
+						panic("!")
+					}
+				}
+
+				if currPortal.Len() == 2 {
+					portalName := currPortal.String()
+					portalVec := u.Vec2i{X: portalX, Y: portalY}
+
+					if portalName == entrancePortal.name {
+						d.entrance = portalVec
+					} else if portalName == exitPortal.name {
+						d.exit = portalVec
+					} else {
+						portal := day20Portal{
+							name:  portalName,
+							inner: !d.isOuterPortal(portalVec),
+						}
+						d.portals[portal] = portalVec
+
+						u.AddToArray(&d.portalNames, portalName)
+					}
+
+					currPortal.Reset()
+				}
+			}
+		}
+	}
+}
+
+func (d Day20) Num() int {
+	return 20
+}
+
+func (d Day20) isPortal(vec u.Vec2i) (bool, int) {
+	if d.grid[vec.Y][vec.X] != day20CellPath {
+		return false, 0
+	}
+
+	for i, name := range d.portalNames {
+		p, exists := d.portals[day20Portal{name: name, inner: !d.isOuterPortal(vec)}]
+		if exists && vec == p {
+			return true, i
+		}
+	}
+
+	return false, 0
+}
+
+func (d Day20) Draw() {
+	for y := range d.grid {
+		for x := range d.grid[y] {
+			switch d.grid[y][x] {
+			case day20CellWall:
+				fmt.Print("█")
+			case day20CellDonutHole:
+				fmt.Print(" ")
+			case day20CellPath:
+				posVec := u.Vec2i{X: x, Y: y}
+				if posVec == d.entrance {
+					fmt.Print("@")
+				} else if posVec == d.exit {
+					fmt.Print("!")
+				} else {
+					isPortal, portalIdx := d.isPortal(posVec)
+					if isPortal {
+						ch := 'a' + portalIdx
+						if ch > 'z' {
+							ch = 'A' + (portalIdx - 26)
+						}
+						fmt.Printf("%c", ch)
+					} else {
+						fmt.Print(" ")
+					}
+				}
+			}
+		}
+
+		fmt.Println()
+	}
+}
+
+func (d Day20) isOuterPortal(pos u.Vec2i) bool {
+	return pos.X == 0 || pos.Y == 0 || pos.X == len(d.grid[0])-1 || pos.Y == len(d.grid)-1
+}
+
+func (d Day20) findAdjacentCells(inPos u.Vec2i) []u.Pair[day20Portal, int] {
+	found := make([]u.Pair[day20Portal, int], 0)
+
+	getAdjacent := func(pos u.Vec2i) []u.Vec2i {
+		retAdjacent := make([]u.Vec2i, 0, len(day20AdjacentOffsets))
+		for _, off := range day20AdjacentOffsets {
+			offVec := u.Vec2i{X: pos.X + off.X, Y: pos.Y + off.Y}
+			if offVec.Y < 0 || offVec.Y >= len(d.grid) || offVec.X < 0 || offVec.X >= len(d.grid[0]) {
+				continue
+			}
+			if d.grid[offVec.Y][offVec.X] != day20CellPath {
+				continue
+			}
+			retAdjacent = append(retAdjacent, offVec)
+		}
+
+		return retAdjacent
+	}
+
+	queue := make([]u.Pair[int, u.Vec2i], 0)
+	visited := map[u.Vec2i]bool{
+		inPos: true,
+	}
+	for _, adjacent := range getAdjacent(inPos) {
+		queue = append(queue, u.Pair[int, u.Vec2i]{First: 1, Second: adjacent})
+	}
+
+	for len(queue) > 0 {
+		next := queue[0]
+		queue = queue[1:]
+
+		if _, exists := visited[next.Second]; !exists {
+			visited[next.Second] = true
+
+			adjacentIsPortal, portalIdx := d.isPortal(next.Second)
+			if adjacentIsPortal || next.Second == d.entrance || next.Second == d.exit {
+				var portalName string
+				if next.Second == d.entrance {
+					portalName = entrancePortal.name
+				} else if next.Second == d.exit {
+					portalName = exitPortal.name
+				} else {
+					portalName = d.portalNames[portalIdx]
+				}
+
+				alreadyFound := false
+				for _, p := range found {
+					if p.First.name == portalName {
+						alreadyFound = true
+						break
+					}
+				}
+				if !alreadyFound {
+					found = append(found, u.Pair[day20Portal, int]{First: day20Portal{
+						name:  portalName,
+						inner: !d.isOuterPortal(next.Second),
+					}, Second: next.First})
+					continue
+				}
+			}
+
+			for _, neighbor := range getAdjacent(next.Second) {
+				if _, exists := visited[neighbor]; !exists {
+					queue = append(queue, u.Pair[int, u.Vec2i]{First: next.First + 1, Second: neighbor})
+				}
+			}
+		}
+	}
+
+	return found
+}
+
+type day20PriorityQueue struct {
+	distance int
+	neighbor day20Portal
+}
+type day20PriorityQueueHeap []day20PriorityQueue
+
+func (h day20PriorityQueueHeap) Len() int           { return len(h) }
+func (h day20PriorityQueueHeap) Less(i, j int) bool { return h[i].distance < h[j].distance }
+func (h day20PriorityQueueHeap) Swap(i, j int)      { h[i], h[j] = h[j], h[i] }
+
+func (h *day20PriorityQueueHeap) Push(x any) {
+	*h = append(*h, x.(day20PriorityQueue))
+}
+
+func (h *day20PriorityQueueHeap) Pop() any {
+	old := *h
+	n := len(old)
+	x := old[n-1]
+	*h = old[0 : n-1]
+	return x
+}
+
+func (d Day20) dijkstra(graph day20Graph, start, end day20Portal, neighborFunc func(inPortal day20Portal) []u.Pair[day20Portal, int]) int {
+	distance := make(map[day20Portal]int)
+
+	ih := day20PriorityQueueHeap{
+		day20PriorityQueue{
+			distance: 0,
+			neighbor: start,
+		},
+	}
+
+	heap.Init(&ih)
+	visited := make(map[day20Portal]bool)
+
+	for ih.Len() > 0 {
+		node := heap.Pop(&ih).(day20PriorityQueue)
+
+		if node.neighbor == end {
+			return node.distance
+		}
+
+		if _, exists := visited[node.neighbor]; exists {
+			continue
+		}
+
+		visited[node.neighbor] = true
+
+		for _, p := range neighborFunc(node.neighbor) {
+			if _, exists := visited[p.First]; exists {
+				continue
+			}
+
+			newDistance := node.distance + p.Second
+			if dist, exists := distance[p.First]; !exists || newDistance < dist {
+				distance[p.First] = newDistance
+				heap.Push(&ih, day20PriorityQueue{
+					distance: newDistance,
+					neighbor: p.First,
+				})
+			}
+		}
+	}
+
+	return math.MaxInt
+}
+
+func (d Day20) buildGraph() day20Graph {
+	graph := make(day20Graph, len(d.portals)+1)
+
+	adjacent := d.findAdjacentCells(d.entrance)
+	graph[entrancePortal] = adjacent
+
+	for portal, portalVec := range d.portals {
+		adjacent = d.findAdjacentCells(portalVec)
+		graph[portal] = adjacent
+	}
+
+	return graph
+}
+
+func (d Day20) getDepthNeighbors(graph day20Graph, portal day20Portal) []u.Pair[day20Portal, int] {
+	basePortal := portal
+	basePortal.depth = 0
+	baseNeighbors := graph[basePortal]
+
+	neighbors := make([]u.Pair[day20Portal, int], 0)
+
+	if portal.inner {
+		n := day20Portal{name: portal.name, inner: false, depth: portal.depth + 1}
+		neighbors = append(neighbors, u.Pair[day20Portal, int]{First: n, Second: 1})
+	}
+
+	if portal.depth == 0 {
+		for _, i := range baseNeighbors {
+			if i.First.inner || i.First.name == entrancePortal.name || i.First.name == exitPortal.name {
+				neighbors = append(neighbors, u.Pair[day20Portal, int]{First: i.First, Second: i.Second})
+			}
+		}
+	} else {
+		if !portal.inner {
+			n := day20Portal{name: portal.name, inner: true, depth: portal.depth - 1}
+			neighbors = append(neighbors, u.Pair[day20Portal, int]{First: n, Second: 1})
+		}
+
+		for _, i := range baseNeighbors {
+			if i.First.name != entrancePortal.name && i.First.name != exitPortal.name {
+				n := day20Portal{name: i.First.name, inner: i.First.inner, depth: portal.depth}
+				neighbors = append(neighbors, u.Pair[day20Portal, int]{First: n, Second: i.Second})
+			}
+		}
+	}
+
+	return neighbors
+}
+
+func (d *Day20) Part1() string {
+	// d.Draw()
+
+	graph := d.buildGraph()
+
+	for portal, adjacent := range graph {
+		if portal.name == entrancePortal.name {
+			continue
+		}
+
+		graph[portal] = append(adjacent, u.Pair[day20Portal, int]{First: day20Portal{name: portal.name, inner: !portal.inner}, Second: 1})
+	}
+
+	distance := d.dijkstra(graph, entrancePortal, exitPortal, func(inPortal day20Portal) []u.Pair[day20Portal, int] { return graph[inPortal] })
+	return fmt.Sprintf("Steps to traverse maze: %s%d%s", u.TextBold, distance, u.TextReset)
+}
+
+func (d *Day20) Part2() string {
+	graph := d.buildGraph()
+	distance := d.dijkstra(graph, entrancePortal, exitPortal, func(inPortal day20Portal) []u.Pair[day20Portal, int] { return d.getDepthNeighbors(graph, inPortal) })
+	return fmt.Sprintf("Steps to traverse recursive maze: %s%d%s", u.TextBold, distance, u.TextReset)
+}

inputs/18p.txt

@@ -0,0 +1,81 @@
+#################################################################################
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+#.#########.#.###.#####.#.#.#######.#####.#######.#.#######.#.###.###.#.#.#####.#
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+#......o..#...#...#.......#...#.A.....#.#.........#r......#.....#.....#.........#
+#################################################################################

inputs/18s1.txt

@@ -0,0 +1,3 @@
+#########
+#b.A.@.a#
+#########

inputs/18s2.txt

@@ -0,0 +1,5 @@
+########################
+#f.D.E.e.C.b.A.@.a.B.c.#
+######################.#
+#d.....................#
+########################

inputs/18s3.txt

@@ -0,0 +1,5 @@
+########################
+#...............b.C.D.f#
+#.######################
+#.....@.a.B.c.d.A.e.F.g#
+########################

inputs/18s4.txt

@@ -0,0 +1,9 @@
+#################
+#i.G..c...e..H.p#
+########.########
+#j.A..b...f..D.o#
+########@########
+#k.E..a...g..B.n#
+########.########
+#l.F..d...h..C.m#
+#################

inputs/18s5.txt

@@ -0,0 +1,6 @@
+########################
+#@..............ac.GI.b#
+###d#e#f################
+###A#B#C################
+###g#h#i################
+########################

inputs/18s6.txt

@@ -0,0 +1,7 @@
+#######
+#a.#Cd#
+##...##
+##.@.##
+##...##
+#cB#Ab#
+#######

inputs/18s7.txt

@@ -0,0 +1,7 @@
+###############
+#d.ABC.#.....a#
+######...######
+######.@.######
+######...######
+#b.....#.....c#
+###############

inputs/18s8.txt

@@ -0,0 +1,7 @@
+#############
+#DcBa.#.GhKl#
+#.###...#I###
+#e#d#.@.#j#k#
+###C#...###J#
+#fEbA.#.FgHi#
+#############

inputs/18s9.txt

@@ -0,0 +1,9 @@
+#############
+#g#f.D#..h#l#
+#F###e#E###.#
+#dCba...BcIJ#
+#####.@.#####
+#nK.L...G...#
+#M###N#H###.#
+#o#m..#i#jk.#
+#############

inputs/19p.txt

@@ -0,0 +1 @@
+109,424,203,1,21102,11,1,0,1105,1,282,21101,0,18,0,1105,1,259,2101,0,1,221,203,1,21102,1,31,0,1105,1,282,21102,1,38,0,1105,1,259,20102,1,23,2,21201,1,0,3,21102,1,1,1,21102,57,1,0,1106,0,303,2102,1,1,222,21002,221,1,3,20101,0,221,2,21101,0,259,1,21101,0,80,0,1105,1,225,21101,44,0,2,21102,91,1,0,1105,1,303,1202,1,1,223,21002,222,1,4,21102,259,1,3,21102,1,225,2,21102,225,1,1,21101,118,0,0,1106,0,225,21002,222,1,3,21101,163,0,2,21101,0,133,0,1106,0,303,21202,1,-1,1,22001,223,1,1,21102,148,1,0,1106,0,259,1202,1,1,223,20101,0,221,4,21001,222,0,3,21102,1,24,2,1001,132,-2,224,1002,224,2,224,1001,224,3,224,1002,132,-1,132,1,224,132,224,21001,224,1,1,21101,195,0,0,105,1,108,20207,1,223,2,21002,23,1,1,21102,-1,1,3,21102,1,214,0,1106,0,303,22101,1,1,1,204,1,99,0,0,0,0,109,5,2101,0,-4,249,22102,1,-3,1,22101,0,-2,2,22101,0,-1,3,21102,250,1,0,1106,0,225,21202,1,1,-4,109,-5,2105,1,0,109,3,22107,0,-2,-1,21202,-1,2,-1,21201,-1,-1,-1,22202,-1,-2,-2,109,-3,2106,0,0,109,3,21207,-2,0,-1,1206,-1,294,104,0,99,22102,1,-2,-2,109,-3,2105,1,0,109,5,22207,-3,-4,-1,1206,-1,346,22201,-4,-3,-4,21202,-3,-1,-1,22201,-4,-1,2,21202,2,-1,-1,22201,-4,-1,1,21202,-2,1,3,21101,0,343,0,1106,0,303,1106,0,415,22207,-2,-3,-1,1206,-1,387,22201,-3,-2,-3,21202,-2,-1,-1,22201,-3,-1,3,21202,3,-1,-1,22201,-3,-1,2,21201,-4,0,1,21101,384,0,0,1105,1,303,1105,1,415,21202,-4,-1,-4,22201,-4,-3,-4,22202,-3,-2,-2,22202,-2,-4,-4,22202,-3,-2,-3,21202,-4,-1,-2,22201,-3,-2,1,21202,1,1,-4,109,-5,2105,1,0

inputs/20p.txt

@@ -0,0 +1,125 @@
+                                         J       L   P           P   V T       X                                           
+                                         G       V   M           I   O R       C                                           
+  #######################################.#######.###.###########.###.#.#######.#########################################  
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+  #...#...#.....#...#.#...#.#.#...#.#.....#.#.......#...#.#.#...#.#.#.......#.#.#...............#.#.#.........#.#.#.....#  
+  ###.#.#.#########.#.###.#.#.#.###.#.#.#########.#####.#.#.#.#.#.#.###.#####.###.#.#.#.###.#.###.###.#######.#.#.#.#####  
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+                                       O     C       N       X   F         S     W                                         
+                                       G     K       B       L   Z         L     W                                         

inputs/20s1.txt

@@ -0,0 +1,19 @@
+         A           
+         A           
+  #######.#########  
+  #######.........#  
+  #######.#######.#  
+  #######.#######.#  
+  #######.#######.#  
+  #####  B    ###.#  
+BC...##  C    ###.#  
+  ##.##       ###.#  
+  ##...DE  F  ###.#  
+  #####    G  ###.#  
+  #########.#####.#  
+DE..#######...###.#  
+  #.#########.###.#  
+FG..#########.....#  
+  ###########.#####  
+             Z       
+             Z       

inputs/20s2.txt

@@ -0,0 +1,37 @@
+                   A               
+                   A               
+  #################.#############  
+  #.#...#...................#.#.#  
+  #.#.#.###.###.###.#########.#.#  
+  #.#.#.......#...#.....#.#.#...#  
+  #.#########.###.#####.#.#.###.#  
+  #.............#.#.....#.......#  
+  ###.###########.###.#####.#.#.#  
+  #.....#        A   C    #.#.#.#  
+  #######        S   P    #####.#  
+  #.#...#                 #......VT
+  #.#.#.#                 #.#####  
+  #...#.#               YN....#.#  
+  #.###.#                 #####.#  
+DI....#.#                 #.....#  
+  #####.#                 #.###.#  
+ZZ......#               QG....#..AS
+  ###.###                 #######  
+JO..#.#.#                 #.....#  
+  #.#.#.#                 ###.#.#  
+  #...#..DI             BU....#..LF
+  #####.#                 #.#####  
+YN......#               VT..#....QG
+  #.###.#                 #.###.#  
+  #.#...#                 #.....#  
+  ###.###    J L     J    #.#.###  
+  #.....#    O F     P    #.#...#  
+  #.###.#####.#.#####.#####.###.#  
+  #...#.#.#...#.....#.....#.#...#  
+  #.#####.###.###.#.#.#########.#  
+  #...#.#.....#...#.#.#.#.....#.#  
+  #.###.#####.###.###.#.#.#######  
+  #.#.........#...#.............#  
+  #########.###.###.#############  
+           B   J   C               
+           U   P   P               

inputs/20s3.txt

@@ -0,0 +1,37 @@
+             Z L X W       C                 
+             Z P Q B       K                 
+  ###########.#.#.#.#######.###############  
+  #...#.......#.#.......#.#.......#.#.#...#  
+  ###.#.#.#.#.#.#.#.###.#.#.#######.#.#.###  
+  #.#...#.#.#...#.#.#...#...#...#.#.......#  
+  #.###.#######.###.###.#.###.###.#.#######  
+  #...#.......#.#...#...#.............#...#  
+  #.#########.#######.#.#######.#######.###  
+  #...#.#    F       R I       Z    #.#.#.#  
+  #.###.#    D       E C       H    #.#.#.#  
+  #.#...#                           #...#.#  
+  #.###.#                           #.###.#  
+  #.#....OA                       WB..#.#..ZH
+  #.###.#                           #.#.#.#  
+CJ......#                           #.....#  
+  #######                           #######  
+  #.#....CK                         #......IC
+  #.###.#                           #.###.#  
+  #.....#                           #...#.#  
+  ###.###                           #.#.#.#  
+XF....#.#                         RF..#.#.#  
+  #####.#                           #######  
+  #......CJ                       NM..#...#  
+  ###.#.#                           #.###.#  
+RE....#.#                           #......RF
+  ###.###        X   X       L      #.#.#.#  
+  #.....#        F   Q       P      #.#.#.#  
+  ###.###########.###.#######.#########.###  
+  #.....#...#.....#.......#...#.....#.#...#  
+  #####.#.###.#######.#######.###.###.#.#.#  
+  #.......#.......#.#.#.#.#...#...#...#.#.#  
+  #####.###.#####.#.#.#.#.###.###.#.###.###  
+  #.......#.....#.#...#...............#...#  
+  #############.#.#.###.###################  
+               A O F   N                     
+               A A D   M                     

main.go

@@ -51,6 +51,9 @@ var dayMap = []day{
 	&days.Day15{},
 	&days.Day16{},
 	&days.Day17{},
+	&days.Day18{},
+	&days.Day19{},
+	&days.Day20{},
 }
 
 func main() {

utilities/bisect.go

@@ -0,0 +1,26 @@
+package utilities
+
+import (
+	"math"
+)
+
+// Bisect takes a known-good low and known-bad high value as the bounds
+// to bisect, and a function to test each value for success or failure.
+// If the function succeeds, the value is adjusted toward the maximum,
+// and if the function fails, the value is adjusted toward the minimum.
+// The final value is returned when the difference between the success
+// and the failure is less than or equal to the acceptance threshold
+// (usually 1, for integers).
+func Bisect[T Number](low, high, threshold T, tryFunc func(val T) bool) T {
+	for T(math.Abs(float64(high-low))) > threshold {
+		currVal := low + ((high - low) / 2)
+		success := tryFunc(currVal)
+		if success {
+			low = currVal
+		} else {
+			high = currVal
+		}
+	}
+
+	return low
+}

utilities/intcode.go

@@ -128,9 +128,15 @@ func (p *IntcodeProgram) ensureMemoryCapacity(address int) {
 }
 
 func (p *IntcodeProgram) Reset() {
-	p.memory = nil
+	wiped := false
+	if len(p.memory) != len(p.program) {
+		wiped = true
+		p.memory = nil
+	}
 	p.init()
-	copy(p.memory, p.program)
+	if !wiped {
+		copy(p.memory, p.program)
+	}
 	p.relativeBase = 0
 }
 

utilities/map.go

@@ -15,3 +15,13 @@ func MapValues[T comparable, U any](m map[T]U) []U {
 	}
 	return r
 }
+
+// CopyMap returns a copy of the passed-in map. Note: currently only works if [U]
+// is not a map or slice.
+func CopyMap[T comparable, U any](m map[T]U) map[T]U {
+	r := make(map[T]U)
+	for k, v := range m {
+		r[k] = v
+	}
+	return r
+}

utilities/vector.go

@@ -44,11 +44,16 @@ func (v Vec2[T]) Equals(other Vec2[T]) bool {
 		v.Y == other.Y
 }
 
+func (v Vec2[T]) ManhattanDistance(other Vec2[T]) T {
+	return T(math.Abs(float64(v.X-other.X)) + math.Abs(float64(v.Y-other.Y)))
+}
+
 func VecBetween[T Number](a, b Vec2[T]) Vec2[T] {
-	return Vec2[T]{
-		X: a.X - b.X,
-		Y: a.Y - b.Y,
-	}
+	return a.To(b)
+}
+
+func ManhattanDistance[T Number](a, b Vec2[T]) T {
+	return a.ManhattanDistance(b)
 }
 
 func (v Vec3[T]) Dot(other Vec3[T]) T {