Add solution for Day 6

Signed-off-by: Knut Ahlers <knut@ahlers.me>

day06.go

@@ -0,0 +1,128 @@
+package aoc2019
+
+import (
+	"bufio"
+	"io"
+	"os"
+	"strings"
+
+	"github.com/pkg/errors"
+)
+
+type day06Planet struct {
+	Name   string
+	Orbits *day06Planet
+}
+
+func (d *day06Planet) getPathLengthToPlanet(tp *day06Planet) int {
+	if d == tp {
+		return 0
+	}
+
+	return 1 + d.Orbits.getPathLengthToPlanet(tp)
+}
+
+type day06OrbitMap map[string]*day06Planet
+
+func (d day06OrbitMap) getPathLengthToCOMByName(start string) int {
+	return d[start].getPathLengthToPlanet(d["COM"])
+}
+
+func (d day06OrbitMap) getCommonPlanet(a, b *day06Planet) *day06Planet {
+	var (
+		pathAToCOM, pathBToCOM []*day06Planet
+		ptr                    *day06Planet
+	)
+
+	// Get path A
+	ptr = a
+	for {
+		pathAToCOM = append(pathAToCOM, ptr)
+		if ptr.Orbits == nil {
+			break
+		}
+		ptr = ptr.Orbits
+	}
+
+	// Get path B
+	ptr = b
+	for {
+		pathBToCOM = append(pathBToCOM, ptr)
+		if ptr.Orbits == nil {
+			break
+		}
+		ptr = ptr.Orbits
+	}
+
+	// Find first common planet
+	for _, ptrA := range pathAToCOM {
+		for _, ptrB := range pathBToCOM {
+			if ptrA == ptrB {
+				return ptrA
+			}
+		}
+	}
+
+	return nil
+}
+
+func day06ParseOrbitMap(localMap io.Reader) (day06OrbitMap, error) {
+	m := day06OrbitMap{}
+
+	scanner := bufio.NewScanner(localMap)
+	for scanner.Scan() {
+		if err := scanner.Err(); err != nil {
+			return nil, errors.Wrap(err, "Scanner failed to scan")
+		}
+
+		parts := strings.Split(scanner.Text(), ")")
+		for _, p := range parts {
+			if _, ok := m[p]; !ok {
+				m[p] = &day06Planet{Name: p}
+			}
+		}
+
+		m[parts[1]].Orbits = m[parts[0]]
+	}
+
+	return m, nil
+}
+
+func solveDay6Part1(inFile string) (int, error) {
+	f, err := os.Open(inFile)
+	if err != nil {
+		return 0, errors.Wrap(err, "Unable to open input file")
+	}
+	defer f.Close()
+
+	oMap, err := day06ParseOrbitMap(f)
+	if err != nil {
+		return 0, errors.Wrap(err, "Unable to parse orbit map")
+	}
+
+	var c int
+	for _, p := range oMap {
+		c += p.getPathLengthToPlanet(oMap["COM"])
+	}
+
+	return c, nil
+}
+
+func solveDay6Part2(inFile string) (int, error) {
+	f, err := os.Open(inFile)
+	if err != nil {
+		return 0, errors.Wrap(err, "Unable to open input file")
+	}
+	defer f.Close()
+
+	oMap, err := day06ParseOrbitMap(f)
+	if err != nil {
+		return 0, errors.Wrap(err, "Unable to parse orbit map")
+	}
+
+	cp := oMap.getCommonPlanet(oMap["YOU"], oMap["SAN"])
+	dist := oMap["YOU"].getPathLengthToPlanet(cp) + oMap["SAN"].getPathLengthToPlanet(cp)
+
+	// Distance is 2 too high as we're not a planet but a ship cycling a planet: Close enough
+	return dist - 2, nil
+}

day06_test.go

@@ -0,0 +1,73 @@
+package aoc2019
+
+import (
+	"strings"
+	"testing"
+)
+
+var day06OrbitMapExample = strings.TrimSpace(`
+COM)B
+B)C
+C)D
+D)E
+E)F
+B)G
+G)H
+D)I
+E)J
+J)K
+K)L
+K)YOU
+I)SAN
+`)
+
+func TestDay6OrbitPathLengthToCOM(t *testing.T) {
+	oMap, err := day06ParseOrbitMap(strings.NewReader(day06OrbitMapExample))
+	if err != nil {
+		t.Fatalf("Orbit map parser failed: %s", err)
+	}
+
+	for start, expCount := range map[string]int{
+		"D":   3,
+		"L":   7,
+		"COM": 0,
+	} {
+		if c := oMap.getPathLengthToCOMByName(start); c != expCount {
+			t.Errorf("Number of recursive orbits to %q yield unexpected result: exp=%d got=%d", start, expCount, c)
+		}
+	}
+}
+
+func TestDay6OrbitMapCommonPlanet(t *testing.T) {
+	oMap, err := day06ParseOrbitMap(strings.NewReader(day06OrbitMapExample))
+	if err != nil {
+		t.Fatalf("Orbit map parser failed: %s", err)
+	}
+
+	p := oMap.getCommonPlanet(oMap["YOU"], oMap["SAN"])
+	if p == nil {
+		t.Fatalf("Found no common planet")
+	}
+
+	if p != oMap["D"] {
+		t.Errorf("Found wrong common planet: exp=D, got=%s", p.Name)
+	}
+}
+
+func TestCalculateDay6_Part1(t *testing.T) {
+	count, err := solveDay6Part1("day06_input.txt")
+	if err != nil {
+		t.Fatalf("Day 6 solver failed: %s", err)
+	}
+
+	t.Logf("Solution Day 6 Part 1: %d", count)
+}
+
+func TestCalculateDay6_Part2(t *testing.T) {
+	count, err := solveDay6Part2("day06_input.txt")
+	if err != nil {
+		t.Fatalf("Day 6 solver failed: %s", err)
+	}
+
+	t.Logf("Solution Day 6 Part 2: %d", count)
+}