day24, example

day24/example

@@ -0,0 +1,5 @@
+19, 13, 30 @ -2,  1, -2
+18, 19, 22 @ -1, -1, -2
+20, 25, 34 @ -2, -2, -4
+12, 31, 28 @ -1, -2, -1
+20, 19, 15 @  1, -5, -3

day24/lines.go

@@ -0,0 +1,147 @@
+package day24
+
+import (
+	"log"
+	"os"
+	"strconv"
+	"strings"
+)
+
+const (
+	CoordMin int = 7
+	CoordMax int = 27
+)
+
+type Point struct {
+	x, y, Z int
+}
+
+type HailParam struct {
+	p0, p1 Point
+	Dx, Dy, Dz int
+	line string
+}
+func (h HailParam)String() string {
+	return "(" + h.line + ")"
+}
+
+func CheckPairwiseIntersections(hails []HailParam) (totalIntersections int) {
+	for i, hail := range hails {
+		for j := i+1; j < len(hails); j++ {
+			otherHail := hails[j]
+			intersect := CheckTaskIntersection(hail, otherHail)
+			if intersect {
+				totalIntersections += 1
+			}
+		}
+	}
+	return
+}
+
+func CheckTaskIntersection(h1, h2 HailParam) (doIntersect bool) {
+	log.Printf("intersecting %+v and %+v\n", h1, h2)
+	x, y, intersectAtAll := IntersectByTwoPoints(h1, h2)
+	if !intersectAtAll {
+		log.Printf("no intersection at all\n")
+		return false
+	}
+	if x < float64(CoordMin) || x > float64(CoordMax) ||
+		y < float64(CoordMin) || y > float64(CoordMax) {
+		log.Printf("intersect at %f %f but outside of area\n", x, y)
+		return false // outside of area
+	}
+	isH1Future := h1.FloatPointInFuture(x,y)
+	isH2Future := h2.FloatPointInFuture(x,y)
+
+	if !isH1Future {
+		log.Printf("in the past for h1\n")
+	}
+	if !isH2Future {
+		log.Printf("in the past for h2\n")
+	}
+
+	if !isH1Future || !isH2Future {
+		return false
+	}
+
+	log.Println("intersect inside of the area")
+	return true
+}
+
+// in 2d only
+func IntersectByTwoPoints(h1, h2 HailParam) (intersectionX, intersectoinY float64, isIntersecting bool) {
+	p1 := h1.p0
+	p2 := h1.p1
+	p3 := h2.p0
+	p4 := h2.p1
+
+	denominator := (p1.x - p2.x)*(p3.y - p4.y) - (p1.y - p2.y)*(p3.x - p4.x)
+	if denominator == 0 {
+		return 0, 0, false
+	}
+
+	divisibleX := (p1.x * p2.y - p1.y * p2.x)*(p3.x - p4.x) - (p1.x - p2.x)*(p3.x * p4.y - p3.y * p4.x)
+	divisibleY := (p1.x * p2.y - p1.y * p2.x)*(p3.y - p4.y) - (p1.y - p2.y)*(p3.x * p4.y - p3.y * p4.x)
+
+	x := float64(divisibleX) / float64(denominator)
+	y := float64(divisibleY) / float64(denominator)
+
+	return x, y, true
+}
+
+func (h HailParam)PointInFuture(p Point) bool {
+	xPositiveSteps := (p.x - h.p0.x) * h.Dx >= 0
+	yPositiveSteps := (p.y - h.p0.y) * h.Dy >= 0
+	zPositiveSteps := (p.Z - h.p0.Z) * h.Dz >= 0
+	return xPositiveSteps && yPositiveSteps && zPositiveSteps
+}
+func (h HailParam)FloatPointInFuture(x, y float64) bool {
+	xPositiveSteps := (x - float64(h.p0.x)) * float64(h.Dx) >= 0
+	yPositiveSteps := (y - float64(h.p0.y)) * float64(h.Dy) >= 0
+	return xPositiveSteps && yPositiveSteps
+}
+
+// 19, 13, 30 @ -2,  1, -2
+func ReadHailLine(line string) (h HailParam) {
+	h.line = line
+	line = strings.ReplaceAll(line, "@", "")
+	line = strings.ReplaceAll(line, ",", "")
+	fields := strings.Fields(line)
+
+	h.p0.x = AtoIOrPanic(fields[0])
+	h.p0.y = AtoIOrPanic(fields[1])
+	h.p0.Z = AtoIOrPanic(fields[2])
+	h.Dx = AtoIOrPanic(fields[3])
+	h.Dy = AtoIOrPanic(fields[4])
+	h.Dz = AtoIOrPanic(fields[5])
+
+	h.p1.x = h.p0.x + h.Dx
+	h.p1.y = h.p0.y + h.Dy
+	h.p1.Z = h.p0.Z + h.Dz
+
+	return
+}
+
+func ReadHailFile(filename string) []HailParam {
+	bytes, err := os.ReadFile(filename)
+	if err != nil {
+		panic(err)
+	}
+	text := strings.TrimSpace(string(bytes))
+	lines := strings.Split(text, "\n")
+	result := make([]HailParam, len(lines))
+
+	for i, line := range lines {
+		result[i] = ReadHailLine(line)
+	}
+
+	return result
+}
+
+func AtoIOrPanic(str string) (num int) {
+	num, err := strconv.Atoi(str)
+	if err != nil {
+		panic(err)
+	}
+	return
+}

day24/lines_test.go

@@ -0,0 +1,71 @@
+package day24
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestReadLine(t *testing.T) {
+	lines := `19, 13, 30 @ -2,  1, -2
+18, 19, 22 @ -1, -1, -2
+20, 25, 34 @ -2, -2, -4
+12, 31, 28 @ -1, -2, -1
+20, 19, 15 @  1, -5, -3`
+
+	for _, line := range strings.Split(lines, "\n") {
+		hail := ReadHailLine(line)
+		t.Log(hail)
+	}
+
+}
+
+func TestSecondPointIsInFuture(t *testing.T) {
+	lines := `19, 13, 30 @ -2,  1, -2
+18, 19, 22 @ -1, -1, -2
+20, 25, 34 @ -2, -2, -4
+12, 31, 28 @ -1, -2, -1
+20, 19, 15 @  1, -5, -3`
+
+	for _, line := range strings.Split(lines, "\n") {
+		hail := ReadHailLine(line)
+		t.Log(hail)
+		t.Logf("calced seconds point %+v is in future %t\n", hail.p1, hail.PointInFuture(hail.p1))
+	}
+
+}
+
+func TestIntersectExampleOne(t *testing.T) {
+	// Hailstone A: 19, 13, 30 @ -2, 1, -2
+	// Hailstone B: 18, 19, 22 @ -1, -1, -2
+	// Hailstones' paths will cross inside the test area (at x=14.333, y=15.333).
+
+	hA := ReadHailLine("19, 13, 30 @ -2, 1, -2")
+	hB := ReadHailLine("18, 19, 22 @ -1, -1, -2")
+
+	x, y, check := IntersectByTwoPoints(hA, hB)
+	if !check {
+		panic("should intersect")
+	}
+	t.Logf("got intersection at %f %f", x, y)
+}
+
+func TestIntersectExampleTwo(t *testing.T) {
+	// Hailstone A: 18, 19, 22 @ -1, -1, -2
+	// Hailstone B: 20, 25, 34 @ -2, -2, -4
+	hA := ReadHailLine("18, 19, 22 @ -1, -1, -2")
+	hB := ReadHailLine("20, 25, 34 @ -2, -2, -4")
+
+	x, y, check := IntersectByTwoPoints(hA, hB)
+	if check {
+		panic("should not intersect")
+	}
+	t.Logf("got intersection at %f %f", x, y)
+}
+
+func TestExamplePairwiseChecks(t *testing.T) {
+	filename := "example"
+	hails := ReadHailFile(filename)
+
+	intersections := CheckPairwiseIntersections(hails)
+	t.Log("counted intersections ", intersections)
+}

day24/neverTellMeTheOdds.go

@@ -0,0 +1,12 @@
+package day24
+
+import (
+	"fmt"
+)
+
+func Run() int {
+	fmt.Println("hello day 24, i'm getting tired")
+	filenae := "day24/example"
+	hails := ReadHailFile(filenae)
+	return CheckPairwiseIntersections(hails)
+}

day24/notes.org

@@ -0,0 +1,29 @@
+#+title: Notes
+* i want help from math
+https://math.stackexchange.com/questions/28503/how-to-find-intersection-of-two-lines-in-3d
+
+'vector parametric form' is exactly what we're getting in the input?
+* huh and only 'looking forward in time' so solutions with negative t are not important.
+cooool
+** i see that speeds are integers, so updated points are integers.
+maybe i do calculation of every line on every time point?
+
+and if i do that is there a way to get intersections efficietly?
+** i'll also need the ends for lines? ends to the line segments.
+with limits on the x & y by the task
+
+for example both 7 <= <= 27
+for input 200000000000000 <= <= 400000000000000
+
+also. can't we move the coords? maybe not? maybe only for one
+so, what do i do? to get the ends of the lines?
+i try to calcluate with both x & y in 2 min\max. then if the other is ok, than that's the ends?
+wait, what happens when i do x = 7, and x = 27 and y is outside? it means no intesections, i guess
+or it could be outside from different sides, so not all x are ok, but there's still line there
+* Using homogeneous coordinates
+https://en.wikipedia.org/wiki/Line%E2%80%93line_intersection
+no, i don't understant that
+* https://en.wikipedia.org/wiki/Line%E2%80%93line_intersection#Given_two_points_on_each_line
+with 2 points. i guess
+but also - check if the point in future of the hail, by comparing with speeds?
+should be easy

main.go

@@ -4,15 +4,15 @@ import (
 	"log"
 	"time"
 
-	"sunshine.industries/aoc2023/day23"
+	"sunshine.industries/aoc2023/day24"
 )
 
 func main() {
 	startTime := time.Now()
 	log.Print("> starting run:")
 
-	result := day23.Run()
-	log.Printf("\n\nday23 result: %d\n****\n", result)
+	result := day24.Run()
+	log.Printf("\n\nday24 result: %d\n****\n", result)
 	endTime := time.Now()
 	diff := endTime.Sub(startTime)
 	log.Printf("execution took %s", diff.String())