add day03 part1

Cargo.toml

@@ -18,3 +18,7 @@ path = "src/01.rs"
 [[bin]]
 name = "2"
 path = "src/02.rs"
+
+[[bin]]
+name = "3"
+path = "src/03.rs"

src/03.rs

@@ -0,0 +1,65 @@
+use std::iter::repeat;
+
+static INPUT: &str = include_str!("03.txt");
+
+fn main() {
+    part1(INPUT);
+}
+
+fn part1(input: &str) -> u32 {
+    let len = input
+        .lines()
+        .next()
+        .expect("must have at least one line")
+        .len();
+
+    let mut zeros: Vec<u32> = repeat(0).take(len).collect();
+    let mut ones: Vec<u32> = repeat(0).take(len).collect();
+    input.lines().for_each(|line| {
+        line.chars().enumerate().for_each(|(n, c)| match c {
+            '0' => zeros[n] += 1,
+            '1' => ones[n] += 1,
+            _ => unreachable!("invalid input"),
+        });
+    });
+
+    let gamma = ones.iter().zip(&zeros).fold(
+        0,
+        |acc, (a, b)| if a > b { (acc << 1) + 1 } else { acc << 1 },
+    );
+    let mask = (0..len).fold(0, |mask, _| (mask << 1) + 1);
+    let epsilon = !gamma & mask;
+
+    let result = gamma * epsilon;
+
+    println!("zeros: {:?}", zeros);
+    println!("ones: {:?}", ones);
+    println!("gamma: {}, epsilon: {:x}", gamma, epsilon);
+    println!("result: {}", result);
+    return result;
+}
+
+#[cfg(test)]
+mod test {
+    #[test]
+    fn part1() {
+        let result = super::part1(
+            "
+00100
+11110
+10110
+10111
+10101
+01111
+00111
+11100
+10000
+11001
+00010
+01010
+"
+            .trim(),
+        );
+        assert_eq!(result, 198);
+    }
+}