From 338f4ef241bd82d0d5c09e0a26befb86ea858d21 Mon Sep 17 00:00:00 2001
From: Xiretza <xiretza@xiretza.xyz>
Date: Sat, 4 Dec 2021 12:17:44 +0100
Subject: [PATCH] 2021 day3/rust: add solution

---
 2021/day3/day3_rs/Cargo.toml  |  8 +++++
 2021/day3/day3_rs/src/main.rs | 58 +++++++++++++++++++++++++++++++++++
 2 files changed, 66 insertions(+)
 create mode 100644 2021/day3/day3_rs/Cargo.toml
 create mode 100644 2021/day3/day3_rs/src/main.rs

diff --git a/2021/day3/day3_rs/Cargo.toml b/2021/day3/day3_rs/Cargo.toml
new file mode 100644
index 0000000..bc126b8
--- /dev/null
+++ b/2021/day3/day3_rs/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "day3_rs"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/2021/day3/day3_rs/src/main.rs b/2021/day3/day3_rs/src/main.rs
new file mode 100644
index 0000000..76496a8
--- /dev/null
+++ b/2021/day3/day3_rs/src/main.rs
@@ -0,0 +1,58 @@
+#![warn(clippy::pedantic)]
+use std::io::{stdin, BufRead};
+
+fn find_rating(num_bits: usize, mut range: &[usize], keep_most_common_bit: bool) -> usize {
+    for idx in (0..num_bits).rev() {
+        if range.len() == 1 {
+            break;
+        }
+
+        let nth_bit = |x| x >> idx & 1;
+        let partition_point = range.iter().position(|&n| nth_bit(n) == 1).unwrap();
+        let center_bit = nth_bit(range[range.len() / 2]);
+
+        let keep_first_half = if keep_most_common_bit {
+            center_bit == 0
+        } else {
+            center_bit == 1
+        };
+
+        if keep_first_half {
+            range = &range[..partition_point];
+        } else {
+            range = &range[partition_point..];
+        }
+    }
+
+    range[0]
+}
+
+fn main() {
+    let lines: Vec<_> = stdin().lock().lines().map(Result::unwrap).collect();
+
+    let num_bits = lines[0].len();
+
+    let gamma: usize = (0..num_bits)
+        .map(|i| {
+            lines
+                .iter()
+                .filter(|line| line.chars().nth(i).unwrap() == '1')
+                .count()
+        })
+        .map(|n| if n > lines.len() / 2 { 1 } else { 0 })
+        .reduce(|accum, bit| accum << 1 | bit)
+        .unwrap();
+    let sigma = !gamma & ((1 << num_bits) - 1);
+
+    println!("{}", gamma * sigma);
+
+    let mut numbers: Vec<_> = lines
+        .iter()
+        .map(|n| usize::from_str_radix(n, 2).unwrap())
+        .collect();
+    numbers.sort_unstable();
+
+    let oxygen = find_rating(num_bits, &numbers, true);
+    let co2 = find_rating(num_bits, &numbers, false);
+    println!("{}", oxygen * co2);
+}