From b519e0bad2f6b79d3e5bb0d705388a4dec0e3ce8 Mon Sep 17 00:00:00 2001
From: Joseph Montanaro <jfmontanaro@modg.org>
Date: Fri, 3 Dec 2021 14:08:03 -0800
Subject: [PATCH] day 3 part 1

---
 2021/src/day3.rs | 36 ++++++++++++++++++++++++++++++++++++
 2021/src/main.rs |  6 +++---
 2 files changed, 39 insertions(+), 3 deletions(-)
 create mode 100644 2021/src/day3.rs

diff --git a/2021/src/day3.rs b/2021/src/day3.rs
new file mode 100644
index 0000000..4902b93
--- /dev/null
+++ b/2021/src/day3.rs
@@ -0,0 +1,36 @@
+use color_eyre::eyre;
+
+
+pub fn part1(numbers: &Vec<u16>) -> u64 {
+    let mut bit_diffs: [isize; 12] = [0; 12];
+    for num in numbers {
+        for i in 0..11 {
+            if num & (1 << i) > 0 {
+                bit_diffs[i] += 1
+            }
+            else {
+                bit_diffs[i] -= 1
+            }
+        }
+    }
+
+    let mut gamma = 0u16;
+    for (i, sum) in bit_diffs.iter().enumerate() {
+        let bit = if *sum >= 0 {1} else {0};
+        gamma |= bit << i;
+    }
+
+    let epsilon = !gamma & (65535 >> 4); // since these are really only 12-bit numbers we have to clear the 4 spurious 1's that come from inverting epsilon
+    epsilon as u64 * gamma as u64
+}
+
+
+pub fn run(data: &str) -> eyre::Result<()> {
+    let mut nums = Vec::new();
+    for line in data.lines() {
+        nums.push(u16::from_str_radix(line, 2)?);
+    }
+
+    println!("One: {}", part1(&nums));
+    Ok(())
+}
\ No newline at end of file
diff --git a/2021/src/main.rs b/2021/src/main.rs
index 0405bb6..da91b81 100644
--- a/2021/src/main.rs
+++ b/2021/src/main.rs
@@ -4,13 +4,13 @@ use color_eyre::eyre;
 
 mod lib;
 use lib::load;
-mod day2;
+mod day3;
 
 
 fn main() -> eyre::Result<()> {
-    let data = load("data/02.txt")?;
+    let data = load("data/03.txt")?;
     let start = Instant::now();
-    day2::run(&data)?;
+    day3::run(&data)?;
     let (dur, unit) = format_ns(start.elapsed().as_nanos());
     println!("Completed in {}{}", dur, unit);
     Ok(())