2020-day13:oof modular arithmetic that I do not know...

2020/day13/main.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"strings"
 
+	"github.com/alexchao26/advent-of-code-go/mathutil"
 	"github.com/alexchao26/advent-of-code-go/util"
 )
 
@@ -16,36 +17,63 @@ func main() {
 
 	if part == 1 {
 		ans := part1(util.ReadFile("./input.txt"))
-		util.CopyToClipboard(fmt.Sprintf("%v", ans))
 		fmt.Println("Output:", ans)
 	} else {
 		ans := part2(util.ReadFile("./input.txt"))
-		util.CopyToClipboard(fmt.Sprintf("%v", ans))
 		fmt.Println("Output:", ans)
 	}
 }
 
 func part1(input string) int {
-	parsed := parseInput(input)
-	_ = parsed
+	estimate, busses := parseInput(input)
 
-	return 0
-}
-
-func part2(input string) int {
-	parsed := parseInput(input)
-	_ = parsed
-
-	return 0
-}
-
-func parseInput(input string) []int {
-	var ans []int
-
-	lines := strings.Split(input, "\n")
-	for _, l := range lines {
-		ans = append(ans, mathutil.StrToInt(l))
+	var busID, waited int
+	for timeValue := estimate; busID == 0; timeValue++ {
+		for _, inter := range busses {
+			if timeValue%inter[1] == 0 {
+				busID = inter[1]
+				waited = timeValue - estimate
+				break
+			}
+		}
 	}
 
-	return ans
+	return busID * waited
+}
+
+// i didn't come up with this myself... generally speaking i understand it...
+func part2(input string) int {
+	_, busses := parseInput(input)
+
+	var timeValue int
+	runningProduct := 1
+	for _, bus := range busses {
+		index, busID := bus[0], bus[1]
+		// this for loop adjusts the time until the constaint for this bus is met
+		// i.e. ensure (time + index) is divisible by the busID to ensure the bus arrives
+		for (timeValue+index)%busID != 0 {
+			// running product is used to increment because it will not affect
+			// the modulo of any of the previously scheduled busses, we've found
+			// the frequency to match them.
+			// e.g. if busID: 5 & index: 2, min timeValue is 3 b/c (3+2)%5 == 0
+			//      if the running product were 5, adding 5 means (8+2)%5 == 0
+			//      and (3 + 5x + 2) % 5 == 0 for any x
+			timeValue += runningProduct
+		}
+		runningProduct *= busID
+	}
+
+	return timeValue
+}
+
+// busses are [2]int{index, busID}, not the best way to parse stuff but it works
+func parseInput(input string) (estimate int, busses [][2]int) {
+	lines := strings.Split(input, "\n")
+	estimate = mathutil.StrToInt(lines[0])
+	for index, busID := range strings.Split(lines[1], ",") {
+		if busID != "x" {
+			busses = append(busses, [2]int{index, mathutil.StrToInt(busID)})
+		}
+	}
+	return estimate, busses
 }

2020/day13/main_test.go

@@ -1,18 +1,24 @@
 package main
 
-import "testing"
+import (
+	"testing"
 
-var tests1 = []struct {
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+var example = `939
+7,13,x,x,59,x,31,19`
+
+func Test_part1(t *testing.T) {
+	tests := []struct {
 		name  string
-	want  int
 		input string
-	// add extra args if needed
-}{
-	// {"actual", ACTUAL_ANSWER, util.ReadFile("input.txt")},
-}
-
-func TestPart1(t *testing.T) {
-	for _, tt := range tests1 {
+		want  int
+	}{
+		{"example", example, 295},
+		{"actual", util.ReadFile("input.txt"), 2092},
+	}
+	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			if got := part1(tt.input); got != tt.want {
 				t.Errorf("part1() = %v, want %v", got, tt.want)
@@ -21,17 +27,16 @@ func TestPart1(t *testing.T) {
 	}
 }
 
-var tests2 = []struct {
+func Test_part2(t *testing.T) {
+	tests := []struct {
 		name  string
-	want  int
 		input string
-	// add extra args if needed
-}{
-	// {"actual", ACTUAL_ANSWER, util.ReadFile("input.txt")},
-}
-
-func TestPart2(t *testing.T) {
-	for _, tt := range tests2 {
+		want  int
+	}{
+		{"example", example, 1068781},
+		{"actual", util.ReadFile("input.txt"), 702970661767766},
+	}
+	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			if got := part2(tt.input); got != tt.want {
 				t.Errorf("part2() = %v, want %v", got, tt.want)