From 01699081b420b51cae36ead58e5177d982e736aa Mon Sep 17 00:00:00 2001
From: Jeremy Kaplan <jdkaplan@metagram.net>
Date: Sun, 5 Dec 2021 21:36:24 -0800
Subject: [PATCH] 2021 Day 6

---
 2021/day6/main.go | 73 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 73 insertions(+)
 create mode 100644 2021/day6/main.go

diff --git a/2021/day6/main.go b/2021/day6/main.go
new file mode 100644
index 0000000..0e35d9c
--- /dev/null
+++ b/2021/day6/main.go
@@ -0,0 +1,73 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/jdkaplan/advent-of-code/aoc"
+)
+
+func main() {
+	text := aoc.Input().ReadFile("day6.txt")
+	fmt.Println(part1(text))
+	fmt.Println(part2(text))
+}
+
+type Fish struct {
+	ticks int
+}
+
+func NewFish() *Fish {
+	return &Fish{8}
+}
+
+func (f *Fish) Tick() bool {
+	if f.ticks == 0 {
+		f.ticks = 6
+		return true
+	}
+	f.ticks--
+	return false
+}
+
+func part1(text string) int {
+	var fish []*Fish
+	for _, n := range strings.Split(text, ",") {
+		fish = append(fish, &Fish{aoc.MustInt(n)})
+	}
+	for i := 0; i < 80; i++ {
+		for _, f := range fish {
+			if f.Tick() {
+				fish = append(fish, NewFish())
+			}
+		}
+	}
+	return len(fish)
+}
+
+func part2(text string) int {
+	delays := make(map[int]int)
+	for _, n := range strings.Split(text, ",") {
+		delays[aoc.MustInt(n)]++
+	}
+	for i := 0; i < 256; i++ {
+		newDelays := make(map[int]int)
+
+		// Reproduce
+		newDelays[8] = delays[0]
+
+		// Tick
+		for d := 1; d <= 8; d++ {
+			newDelays[d-1] = delays[d]
+		}
+		newDelays[6] += delays[0]
+
+		delays = newDelays
+	}
+
+	var total int
+	for _, n := range delays {
+		total += n
+	}
+	return total
+}