Day 6

06/answers.txt

@@ -0,0 +1,2 @@
+227612
+454

06/main.go

@@ -0,0 +1,62 @@
+package main
+
+import (
+	"fmt"
+	"github.com/csmith/aoc-2019/common"
+)
+
+func countOrbits(satellites map[string][]string, start string, depth int) (res int) {
+	for _, satellite := range satellites[start] {
+		res += depth + 1 + countOrbits(satellites, satellite, depth+1)
+	}
+	return
+}
+
+func routeToCenter(orbits map[string]string, start string) (res []string) {
+	next := start
+	for next != "COM" {
+		next = orbits[next]
+		res = append(res, next)
+	}
+	return
+}
+
+func countToCenter(orbits map[string]string, start string) (res map[string]int) {
+	res = make(map[string]int)
+
+	steps := 0
+	next := start
+	for next != "COM" {
+		next = orbits[next]
+		res[next] = steps
+		steps++
+	}
+	return
+}
+
+func shortestPath(orbits map[string]string, from, to string) int {
+	route := routeToCenter(orbits, from)
+	steps := countToCenter(orbits, to)
+
+	for i, body := range route {
+		if count, ok := steps[body]; ok {
+			return count + i
+		}
+	}
+	panic(fmt.Sprintf("No path found between %s and %s.", from, to))
+}
+
+func main() {
+	lines := common.ReadFileAsStrings("06/input.txt")
+	satellites := make(map[string][]string, len(lines))
+	orbits := make(map[string]string, len(lines))
+	for _, line := range lines {
+		around := line[0:3]
+		body := line[4:7]
+		satellites[around] = append(satellites[around], body)
+		orbits[body] = around
+	}
+
+	println(countOrbits(satellites, "COM", 0))
+	println(shortestPath(orbits, "YOU", "SAN"))
+}