2021 Day 17 Part 1

2021/day17/main.go

@@ -0,0 +1,107 @@
+package main
+
+import (
+	"fmt"
+	"regexp"
+
+	"github.com/jdkaplan/advent-of-code/aoc"
+)
+
+func main() {
+	text := aoc.Input().ReadFile("day17.txt")
+	re := regexp.MustCompile(`^target area: x=(-?\d+)\.\.(-?\d+), y=(-?\d+)..(-?\d+)$`)
+	m := re.FindStringSubmatch(text)
+	var (
+		x1 = aoc.MustInt(m[1])
+		x2 = aoc.MustInt(m[2])
+		y1 = aoc.MustInt(m[3])
+		y2 = aoc.MustInt(m[4])
+	)
+	r := Region{x1, x2, y1, y2}
+	fmt.Println(part1(r))
+}
+
+type Region struct{ x1, x2, y1, y2 int }
+
+type Position struct{ x, y int }
+
+func (p Position) Move(v Velocity) Position {
+	return Position{p.x + v.x, p.y + v.y}
+}
+
+func (p Position) InRegion(r Region) bool {
+	return r.x1 <= p.x && p.x <= r.x2 && r.y1 <= p.y && p.y <= r.y2
+}
+
+func (p Position) Overshot(r Region) bool {
+	return p.x > r.x2 || p.y < r.y1
+}
+
+type Velocity struct{ x, y int }
+
+func (v Velocity) Drag() Velocity {
+	x := v.x
+	if x < 0 {
+		x++
+	} else if x > 0 {
+		x--
+	}
+	return Velocity{x, v.y - 1}
+}
+
+func part1(r Region) int {
+	vxmin, vxmax := vxrange(r.x1, r.x2)
+	vymin, vymax := vyrange(r.y1, r.y2)
+	hmax := 0
+	for vx := vxmin; vx <= vxmax; vx++ {
+		for vy := vymin; vy <= vymax; vy++ {
+			hit := simulate(Velocity{vx, vy}, r)
+			if !hit {
+				continue
+			}
+			if h := tri(vy); h > hmax {
+				hmax = h
+			}
+		}
+	}
+	return hmax
+}
+
+func simulate(v Velocity, r Region) bool {
+	p := Position{0, 0}
+	for !p.InRegion(r) && !p.Overshot(r) {
+		p = p.Move(v)
+		v = v.Drag()
+	}
+	return p.InRegion(r)
+}
+
+func vxrange(x1, x2 int) (int, int) {
+	// Zoom directly to the far edge.
+	vxmax := x2
+	// Find the first value that crosses the near edge.
+	vxmin := 0
+	for vx := 0; vx < vxmax; vx++ {
+		xterm := tri(vx)
+		if xterm >= x1 {
+			vxmin = vx
+			break
+		}
+	}
+	return vxmin, vxmax
+}
+
+func vyrange(y1, y2 int) (int, int) {
+	if y1 >= 0 {
+		panic("This only works for negative-y regions!")
+	}
+	// Zoom directly to the close edge.
+	vymin := -y2
+	// Zoom directly to the far edge.
+	vymax := -y1
+	return vymin, vymax
+}
+
+func tri(x int) int {
+	return (x * (x + 1)) / 2
+}