From baf854ce39df267121064ff97ef4ce6e257f3772 Mon Sep 17 00:00:00 2001
From: Dreaded_X <tim.huizinga@gmail.com>
Date: Fri, 3 Dec 2021 18:17:34 +0100
Subject: [PATCH] 2021 - Day 3

---
 2021/3/go.mod  |   9 +++
 2021/3/go.sum  |   2 +
 2021/3/main.go | 150 +++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 161 insertions(+)
 create mode 100644 2021/3/go.mod
 create mode 100644 2021/3/go.sum
 create mode 100644 2021/3/main.go

diff --git a/2021/3/go.mod b/2021/3/go.mod
new file mode 100644
index 0000000..4c58b8d
--- /dev/null
+++ b/2021/3/go.mod
@@ -0,0 +1,9 @@
+module AoC/2021/3
+
+require AoC/2021/common v0.0.0
+
+require github.com/joho/godotenv v1.4.0 // indirect
+
+replace AoC/2021/common v0.0.0 => ../common
+
+go 1.17
diff --git a/2021/3/go.sum b/2021/3/go.sum
new file mode 100644
index 0000000..8c9f290
--- /dev/null
+++ b/2021/3/go.sum
@@ -0,0 +1,2 @@
+github.com/joho/godotenv v1.4.0 h1:3l4+N6zfMWnkbPEXKng2o2/MR5mSwTrBih4ZEkkz1lg=
+github.com/joho/godotenv v1.4.0/go.mod h1:f4LDr5Voq0i2e/R5DDNOoa2zzDfwtkZa6DnEwAbqwq4=
diff --git a/2021/3/main.go b/2021/3/main.go
new file mode 100644
index 0000000..32a7c00
--- /dev/null
+++ b/2021/3/main.go
@@ -0,0 +1,150 @@
+package main
+
+import (
+	"AoC/2021/common"
+	"bufio"
+	"strconv"
+)
+
+func main() {
+	part1()
+	part2()
+}
+
+func part1() {
+	aoc := aoc.New(2021, 3)
+
+	aoc.Test(`
+00100
+11110
+10110
+10111
+10101
+01111
+00111
+11100
+10000
+11001
+00010
+01010
+	`, 198)
+
+	aoc.Solution(func (input *bufio.Scanner) int {
+		length := 0
+
+		var entries []int
+
+		for input.Scan() {
+			line := input.Text()
+			if len(line) > length {
+				length = len(line)
+			}
+
+			num, err := strconv.ParseInt(line, 2, 64)
+			if err != nil {
+				panic(err)
+			}
+
+			entries = append(entries, int(num))
+		}
+
+		gamma := 0
+		epsilon := 0
+		for j := 0; j < length; j++ {
+			var count int
+			for _, num := range entries {
+				count += num >> j & 1
+			}
+
+			if float64(count)/float64(len(entries)) >= 0.5 {
+				gamma += 1 << j
+			} else {
+				epsilon += 1 << j
+			}
+		}
+
+		return gamma*epsilon
+	})
+}
+
+func filter(entries []int, length int, findCommon bool) int {
+	e := make([]int, len(entries))
+	copy(e, entries)
+	for j := length-1; j >= 0; j-- {
+		var count int
+		for _, num := range e {
+			count += num >> j & 1
+		}
+
+		var common int
+		if !findCommon {
+			common = 1
+		}
+		if float64(count)/float64(len(e)) >= 0.5 {
+			if findCommon {
+				common = 1
+			} else {
+				common = 0
+			}
+		}
+
+		n := 0
+		for _, num := range e {
+			if num >> j & 1 == common {
+				e[n] = num
+				n++
+			}
+		}
+		e = e[:n]
+
+		if len(e) == 1 {
+			break
+		}
+	}
+
+	return e[0]
+}
+
+func part2() {
+	aoc := aoc.New(2021, 3)
+
+	aoc.Test(`
+00100
+11110
+10110
+10111
+10101
+01111
+00111
+11100
+10000
+11001
+00010
+01010
+	`, 230)
+
+	aoc.Solution(func (input *bufio.Scanner) int {
+		length := 0
+
+		var entries []int
+
+		for input.Scan() {
+			line := input.Text()
+			if len(line) > length {
+				length = len(line)
+			}
+
+			num, err := strconv.ParseInt(line, 2, 64)
+			if err != nil {
+				panic(err)
+			}
+
+			entries = append(entries, int(num))
+		}
+
+		oxygen := filter(entries, length, true)
+		co2 := filter(entries, length, false)
+
+		return int(oxygen * co2)
+	})
+}