[wip] [2020-12-13] Add solutions for day13.02

day13/parse.go

@@ -9,7 +9,7 @@ import (
 
 var schedule struct {
 	Departure int
-	Lines     []int
+	Lines     map[int]int
 }
 
 func ParseFile(input string) {
@@ -21,14 +21,15 @@ func ParseFile(input string) {
 
 	// Store time
 	schedule.Departure = time
+	schedule.Lines = make(map[int]int)
 
-	for _, row := range strings.Split(rows[1], ",") {
+	for offset, row := range strings.Split(rows[1], ",") {
 		if row != "x" {
 			// Convert bus number
 			line, _ := strconv.Atoi(row)
 
 			// Add to schedule
-			schedule.Lines = append(schedule.Lines, line)
+			schedule.Lines[offset] = line
 		}
 	}
 }

day13/solve2.go

@@ -1,12 +1,51 @@
 package day13
 
 import (
-	"fmt"
 	"log"
 )
 
 func Solve2() {
-	fmt.Println(len(schedule.Lines))
+	// Storage if it's a valid run or not
+	isValidRun := false
 
-	log.Printf("2020-12-13.02: Answer: %d\n", len(schedule.Lines))
+	i := 1
+	for isValidRun == false {
+		// Calculate the departure time for this first run
+		departureTime := i * schedule.Lines[0]
+
+		// Storage if it's a valid start or not
+		isValidStart := true
+
+		// Go throgh the rest of the lines
+		for offset, line := range schedule.Lines {
+			// Ignore first column which has no offset which we base
+			// this brute force on anyways.
+			if offset == 0 {
+				continue
+			}
+
+			// Calculate next departure time for this line
+			nextDeparture := departureTime/line*line + line
+
+			// If this doesn't match the rules to follow the offsets
+			// to go a minute after previous bus, break the loop and
+			// set it as an invalid start.
+			if departureTime+offset != nextDeparture {
+				isValidStart = false
+
+				break
+			}
+		}
+
+		if isValidStart {
+			isValidRun = true
+		}
+
+		i++
+	}
+
+	validRunId := i * schedule.Lines[0]
+
+	log.Printf("2020-12-13.02: Answer: %d\n", validRunId)
+	log.Printf("2020-12-13.02: Answer: %d\n", validRunId-schedule.Lines[0])
 }