From c13f4ca2361f342306d51cc54f7b36c8d820b43f Mon Sep 17 00:00:00 2001
From: Jeremy Kaplan <jdkaplan@metagram.net>
Date: Thu, 2 Dec 2021 21:48:37 -0800
Subject: [PATCH] 2021 Day 3

---
 2021/aoc/aoc.go   | 20 ++++++++++
 2021/day3/main.go | 93 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 113 insertions(+)
 create mode 100644 2021/aoc/aoc.go
 create mode 100644 2021/day3/main.go

diff --git a/2021/aoc/aoc.go b/2021/aoc/aoc.go
new file mode 100644
index 0000000..78b46b6
--- /dev/null
+++ b/2021/aoc/aoc.go
@@ -0,0 +1,20 @@
+package aoc
+
+import (
+	"bufio"
+	"os"
+)
+
+func ReadLines(path string) (lines []string) {
+	f, err := os.Open(path)
+	if err != nil {
+		panic(err)
+	}
+	defer f.Close()
+
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		lines = append(lines, scanner.Text())
+	}
+	return lines
+}
diff --git a/2021/day3/main.go b/2021/day3/main.go
new file mode 100644
index 0000000..3891928
--- /dev/null
+++ b/2021/day3/main.go
@@ -0,0 +1,93 @@
+package main
+
+import (
+	"fmt"
+	"strconv"
+
+	"github.com/jdkaplan/advent-of-code/aoc"
+)
+
+const bits = 12
+
+func part1(lines []string) uint {
+	var count [bits]int
+	for _, line := range lines {
+		for i, char := range line {
+			// Fix the index to be from the right
+			i := bits - i - 1
+			switch char {
+			case '0':
+				count[i]--
+			case '1':
+				count[i]++
+			default:
+				panic(char)
+			}
+		}
+	}
+
+	var gamma uint
+	var epsilon uint
+	for i, c := range count {
+		mask := uint(1 << i)
+		if c > 0 {
+			gamma |= mask
+			epsilon &= ^mask
+		} else {
+			gamma &= ^mask
+			epsilon |= mask
+		}
+	}
+
+	return gamma * epsilon
+}
+
+func partition(lines []string, idx int) (zeros []string, ones []string) {
+	for _, line := range lines {
+		char := line[idx]
+		switch char {
+		case '0':
+			zeros = append(zeros, line)
+		case '1':
+			ones = append(ones, line)
+		default:
+			panic(char)
+		}
+	}
+	return
+}
+
+func mustBin(s string) uint {
+	u, err := strconv.ParseUint(s, 2, 0)
+	if err != nil {
+		panic(err)
+	}
+	return uint(u)
+}
+
+func part2(lines []string) int {
+	oxy, co2 := lines, lines
+	for i := 0; i < bits && len(oxy) > 1; i++ {
+		zeros, ones := partition(oxy, i)
+		if len(ones) >= len(zeros) {
+			oxy = ones
+		} else {
+			oxy = zeros
+		}
+	}
+	for i := 0; i < bits && len(co2) > 1; i++ {
+		zeros, ones := partition(co2, i)
+		if len(ones) >= len(zeros) {
+			co2 = zeros
+		} else {
+			co2 = ones
+		}
+	}
+	return int(mustBin(oxy[0]) * mustBin(co2[0]))
+}
+
+func main() {
+	lines := aoc.ReadLines("./input/day3.txt")
+	fmt.Println(part1(lines))
+	fmt.Println(part2(lines))
+}