Day 19 solution

I had some trouble with the logic of this puzzle in my head, and the visuals were too large to realistically render for me to spot-check my solution, so I kept getting part 2 wrong. I eventually realized what needed to be done to get the right answer, which was partially the realization that I needed to check +99 instead of +100, and partially that I probably didn't need to check the entire space, just the bounding points, at least for the initial filters.

And then I optimized it with a bisect instead of a linear search. Always a good idea when you need to narrow a large range to a single point.

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)
+}

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

main.go

@@ -52,6 +52,7 @@ var dayMap = []day{
 	&days.Day16{},
 	&days.Day17{},
 	&days.Day18{},
+	&days.Day19{},
 }
 
 func main() {