4 files changed, 1119 insertions, 0 deletions

diff --git a/2021/day3/Cargo.toml b/2021/day3/Cargo.toml
new file mode 100644
index 0000000..898e70d
--- /dev/null
+++ b/2021/day3/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "day3"
+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/input b/2021/day3/input
new file mode 100644
index 0000000..929c455
--- /dev/null
+++ b/2021/day3/input
@@ -0,0 +1,1000 @@
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diff --git a/2021/day3/input.example b/2021/day3/input.example
new file mode 100644
index 0000000..a6366a8
--- /dev/null
+++ b/2021/day3/input.example
@@ -0,0 +1,12 @@
+00100
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diff --git a/2021/day3/src/main.rs b/2021/day3/src/main.rs
new file mode 100644
index 0000000..2440a39
--- /dev/null
+++ b/2021/day3/src/main.rs
@@ -0,0 +1,99 @@
+use std::env;
+use std::fs::File;
+use std::io::{BufRead, BufReader};
+
+fn main() {
+    let args: Vec<String> = env::args().collect();
+
+    if args.len() != 2 {
+        eprintln!("Usage: {} filename", args[0]);
+        std::process::exit(1);
+    }
+
+    let filename = &args[1];
+
+    let file = File::open(filename).unwrap_or_else(|_| panic!("No such file: {}", filename));
+    let reader = BufReader::new(file);
+
+    let mut numbers: Vec<u32> = Vec::new();
+
+    for line in reader.lines() {
+        let line = line.unwrap();
+        numbers.push(isize::from_str_radix(&line, 2).unwrap().try_into().unwrap());
+    }
+
+    let mut co2_numbers: Vec<u32> = numbers.clone();
+    let mut oxygen: u32 = 0;
+    let mut co2: u32 = 0;
+
+    for bit in (0..12).rev() {
+        let keep;
+        let mut one_bits = 0;
+        let mut zero_bits = 0;
+        for num in &numbers {
+            one_bits += (num >> bit) & 1;
+            zero_bits += ((num >> bit) & 1) ^ 1;
+        }
+
+        println!("for bit: {}", bit);
+        println!("one_bits: {}", one_bits);
+        println!("zero_bits: {}", zero_bits);
+
+        if zero_bits > one_bits {
+            keep = 0;
+        } else if zero_bits < one_bits {
+            keep = 1;
+        } else {
+            keep = 1;
+        }
+
+        println!("keeping {}", keep);
+
+        numbers.retain(|num| ((num >> bit) & 1) == keep);
+
+        if numbers.len() == 1 {
+            oxygen = numbers.pop().unwrap();
+            break;
+        }
+
+        println!("{:?}", numbers);
+    }
+
+    println!("====co2 now====");
+
+    for bit in (0..12).rev() {
+        let keep;
+        let mut one_bits = 0;
+        let mut zero_bits = 0;
+        for num in &co2_numbers {
+            one_bits += (num >> bit) & 1;
+            zero_bits += ((num >> bit) & 1) ^ 1;
+        }
+
+        println!("for bit: {}", bit);
+        println!("one_bits: {}", one_bits);
+        println!("zero_bits: {}", zero_bits);
+
+        if zero_bits > one_bits {
+            keep = 1;
+        } else if zero_bits < one_bits {
+            keep = 0;
+        } else {
+            keep = 0;
+        }
+
+        println!("keeping {}", keep);
+
+        co2_numbers.retain(|num| (num >> bit) & 1 == keep);
+
+        if co2_numbers.len() == 1 {
+            co2 = co2_numbers.pop().unwrap();
+            break;
+        }
+
+        println!("{:?}", co2_numbers);
+    }
+
+    println!("oxygen: {}\nco2: {}", oxygen, co2);
+    println!("answer: {}", oxygen * co2);
+}