2016-day15: simple linear equation for position alignments

2016/day15/main.go

@@ -0,0 +1,68 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"strings"
+
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+func main() {
+	var part int
+	flag.IntVar(&part, "part", 1, "part 1 or 2")
+	flag.Parse()
+	fmt.Println("Running part", part)
+
+	ans := timingIsEverything(util.ReadFile("./input.txt"), part)
+	fmt.Println("Output:", ans)
+}
+
+func timingIsEverything(input string, part int) int {
+	discs := parseInput(input)
+
+	if part == 2 {
+		discs = append(discs, &disc{
+			number:    len(discs) + 1,
+			positions: 11,
+			starting:  0,
+		})
+	}
+
+	t := 0
+	for {
+		var capsuleCollides bool
+		for _, d := range discs {
+			// some math equation for position must equal zero for the capsule to pass through each disc
+			timeSinceDrop := d.number
+			position := d.starting + t + timeSinceDrop
+			position %= d.positions
+			if position != 0 {
+				capsuleCollides = true
+			}
+		}
+		if !capsuleCollides {
+			break
+		}
+		t++
+	}
+
+	return t
+}
+
+type disc struct {
+	number    int
+	positions int
+	starting  int
+}
+
+func parseInput(input string) []*disc {
+	var discs []*disc
+	for _, l := range strings.Split(input, "\n") {
+		d := disc{}
+		fmt.Sscanf(l, "Disc #%d has %d positions; at time=0, it is at position %d.",
+			&d.number, &d.positions, &d.starting)
+		discs = append(discs, &d)
+	}
+	return discs
+}

2016/day15/main_test.go

@@ -0,0 +1,30 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+var example = `Disc #1 has 5 positions; at time=0, it is at position 4.
+Disc #2 has 2 positions; at time=0, it is at position 1.`
+
+func Test_timingIsEverything(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		part  int
+		want  int
+	}{
+		{"example", example, 1, 5},
+		{"actual", util.ReadFile("input.txt"), 1, 317371},
+		{"actual", util.ReadFile("input.txt"), 2, 2080951},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := timingIsEverything(tt.input, tt.part); got != tt.want {
+				t.Errorf("timingIsEverything() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}