From 81adaec4b945aa0bf966ceb17a3bc262b34b3472 Mon Sep 17 00:00:00 2001
From: alexchao26 <alexchaoo@gmail.com>
Date: Tue, 15 Dec 2020 23:58:43 -0500
Subject: [PATCH] 2017-day13: frequency calculation

---
 2017/day13/main.go      | 85 +++++++++++++++++++++++++++++++++++++++++
 2017/day13/main_test.go | 51 +++++++++++++++++++++++++
 2 files changed, 136 insertions(+)
 create mode 100644 2017/day13/main.go
 create mode 100644 2017/day13/main_test.go

diff --git a/2017/day13/main.go b/2017/day13/main.go
new file mode 100644
index 0000000..6ae9e8a
--- /dev/null
+++ b/2017/day13/main.go
@@ -0,0 +1,85 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"math"
+	"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)
+
+	var ans int
+	if part == 1 {
+		ans = part1(util.ReadFile("./input.txt"))
+	} else {
+		ans = part2(util.ReadFile("./input.txt"))
+	}
+	fmt.Println("Output:", ans)
+}
+
+func part1(input string) int {
+	freqs := parseFrequencies(input)
+
+	var severity int
+	for _, freq := range freqs {
+		// depthIndex is also equivalent to the number of picoseconds that have
+		// elapsed when we reach a particular depth of the firewall
+		depthIndex := freq[0]
+
+		// frequency to zero index is how many picoseconds it takes the scanner
+		// to return to the zero index position
+		frequencyToZeroIndex := (freq[1] - 1) * 2
+
+		// if frequency is evenly divisible by the time elapsed, then we're
+		// "caught" by this scanner, add to severity
+		if depthIndex%frequencyToZeroIndex == 0 {
+			severity += depthIndex * freq[1]
+		}
+	}
+
+	return severity
+}
+
+func part2(input string) int {
+	freqs := parseFrequencies(input)
+
+	// same logic to part 1, but add a delay to the time elapsed and return which
+	// delay leads to not getting caught by a scanner
+	for delay := 0; delay < math.MaxInt32; delay++ {
+		var gotCaught bool
+		for _, freq := range freqs {
+			depthIndex := freq[0]
+			frequencyToZeroIndex := (freq[1] - 1) * 2
+			// add delay to the time depthIndex, as it takes longer to get there
+			if (depthIndex+delay)%frequencyToZeroIndex == 0 {
+				gotCaught = true
+			}
+		}
+		if !gotCaught {
+			return delay
+		}
+	}
+
+	panic("loop ended, increase limit?")
+}
+
+func parseFrequencies(input string) [][2]int {
+	lines := strings.Split(input, "\n")
+	var freqs [][2]int // depth, range
+	for _, l := range lines {
+		// depth is equivalent to the index in the firewall
+		// range can be used to calculate the frequency in which a scanner
+		// returns to index zero
+		var depth, rng int
+		fmt.Sscanf(l, "%d: %d", &depth, &rng)
+		freqs = append(freqs, [2]int{depth, rng})
+	}
+	return freqs
+}
diff --git a/2017/day13/main_test.go b/2017/day13/main_test.go
new file mode 100644
index 0000000..87886de
--- /dev/null
+++ b/2017/day13/main_test.go
@@ -0,0 +1,51 @@
+package main
+
+import (
+	"testing"
+	"time"
+
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+var example = `0: 3
+1: 2
+4: 4
+6: 4`
+
+func Test_part1(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  int
+	}{
+		{"example", example, 24},
+		{"actual", util.ReadFile("input.txt"), 2508},
+	}
+	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)
+			}
+		})
+	}
+}
+
+func Test_part2(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  int
+	}{
+		{"example", example, 10},
+		{"actual", util.ReadFile("input.txt"), 3913186},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			startTime := time.Now()
+			if got := part2(tt.input); got != tt.want {
+				t.Errorf("part2() = %v, want %v", got, tt.want)
+			}
+			t.Logf("Runtime for %s: %v", tt.name, time.Since(startTime))
+		})
+	}
+}