Completed part 2 of Day 24 puzzle (with the approach proposed by tckmn on reddit (https://www.reddit.com/r/adventofcode/comments/18q40he/2023_day_24_part_2_a_straightforward_nonsolver/)

day24/day24.go

@@ -1,6 +1,7 @@
 package day24
 
 import (
+	"math"
 	"strings"
 
 	"gitea.paas.celticinfo.fr/oabrivard/aoc2023/utils"
@@ -69,3 +70,85 @@ func Part1(lines []string, min, max float64) int {
 
 	return result
 }
+
+// Adapted from Java code given here : https://github.com/ash42/adventofcode/blob/main/adventofcode2023/src/nl/michielgraat/adventofcode2023/day24/Day24.java
+// Explanations can be found here : https://www.cs.emory.edu/~cheung/Courses/170/Syllabus/09/gaussian-elim.html
+func gaussianElimination(coefficients [][]float64, rhs []float64) {
+	size := len(coefficients)
+	for i := 0; i < size; i++ {
+		// Select pivot
+		pivot := coefficients[i][i]
+		// Normalize row i
+		for j := 0; j < size; j++ {
+			coefficients[i][j] = coefficients[i][j] / pivot
+		}
+		rhs[i] = rhs[i] / pivot
+		// Sweep using row i
+		for k := 0; k < size; k++ {
+			if k != i {
+				factor := coefficients[k][i]
+				for j := 0; j < size; j++ {
+					coefficients[k][j] = coefficients[k][j] - factor*coefficients[i][j]
+				}
+				rhs[k] = rhs[k] - factor*rhs[i]
+			}
+		}
+	}
+}
+
+// Adapted from solution given here : https://www.reddit.com/r/adventofcode/comments/18q40he/2023_day_24_part_2_a_straightforward_nonsolver/
+// and from Java code given here : https://github.com/ash42/adventofcode/blob/main/adventofcode2023/src/nl/michielgraat/adventofcode2023/day24/Day24.java
+func getCoordinates(stones []Hailstone) Hailstone {
+	coefficients := [][]float64{{0.0, 0.0, 0.0, 0.0}, {0.0, 0.0, 0.0, 0.0}, {0.0, 0.0, 0.0, 0.0}, {0.0, 0.0, 0.0, 0.0}}
+	rhs := []float64{0.0, 0.0, 0.0, 0.0}
+
+	// Get X,Y,VX,VY
+	for i := 0; i < 4; i++ {
+		s1 := stones[i]
+		s2 := stones[i+1]
+		coefficients[i][0] = float64(s2.vy - s1.vy)
+		coefficients[i][1] = float64(s1.vx - s2.vx)
+		coefficients[i][2] = float64(s1.py - s2.py)
+		coefficients[i][3] = float64(s2.px - s1.px)
+		rhs[i] = float64(-s1.px*s1.vy + s1.py*s1.vx + s2.px*s2.vy - s2.py*s2.vx)
+	}
+
+	gaussianElimination(coefficients, rhs)
+
+	var x int64 = int64(math.Round(rhs[0]))
+	var y int64 = int64(math.Round(rhs[1]))
+	var vx int64 = int64(math.Round(rhs[2]))
+	var vy int64 = int64(math.Round(rhs[3]))
+
+	// Get X,VZ
+	coefficients = [][]float64{{0.0, 0.0}, {0.0, 0.0}}
+	rhs = []float64{0.0, 0.0}
+	for i := 0; i < 2; i++ {
+		s1 := stones[i]
+		s2 := stones[i+1]
+		coefficients[i][0] = float64(s1.vx - s2.vx)
+		coefficients[i][1] = float64(s2.px - s1.px)
+		rhs[i] = float64(-s1.px*s1.vz + s1.pz*s1.vx + s2.px*s2.vz - s2.pz*s2.vx - ((s2.vz - s1.vz) * x) - ((s1.pz - s2.pz) * vx))
+	}
+
+	gaussianElimination(coefficients, rhs)
+
+	var z int64 = int64(math.Round(rhs[0]))
+	var vz int64 = int64(math.Round(rhs[1]))
+
+	return Hailstone{x, y, z, vx, vy, vz}
+}
+
+func Part2(lines []string) int64 {
+	stones := []Hailstone{}
+
+	for _, line := range lines {
+		values := utils.ParseInt64Array(strings.Replace(line, " @", ",", -1), ", ")
+		stone := Hailstone{values[0], values[1], values[2], values[3], values[4], values[5]}
+		stones = append(stones, stone)
+	}
+
+	stone := getCoordinates(stones)
+
+	return stone.px + stone.py + stone.pz
+}

day24/day24_test.go

@@ -31,3 +31,29 @@ func TestPart1WithInput(t *testing.T) {
 		t.Fatalf("expected 21785, got %d", result)
 	}
 }
+
+func TestPart2(t *testing.T) {
+	lines := []string{
+		"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",
+	}
+
+	result := Part2(lines)
+
+	if result != 47 {
+		t.Fatalf("expected 47, got %d", result)
+	}
+}
+
+func TestPart2WithInput(t *testing.T) {
+	lines := utils.ReadLines("input.txt")
+
+	result := Part2(lines)
+
+	if result != 554668916217145 {
+		t.Fatalf("expected 554668916217145, got %d", result)
+	}
+}