Add solution for "Day 3: Lobby", part 1

src/solvers/lobby.rs

@@ -0,0 +1,35 @@
+use crate::solvers::Solver;
+use std::io::BufRead;
+
+pub struct Lobby {}
+
+impl Lobby {
+    fn largest_joltage(bank: &str) -> u64 {
+        let mut best = (0, 0);
+        for d in bank.bytes().take(bank.len() - 1) {
+            if best.0 < d {
+                best = (d, 0)
+            } else if best.1 < d {
+                best.1 = d;
+            }
+        }
+        let last = bank.bytes().last().unwrap();
+        if best.1 < last {
+            best.1 = last;
+        }
+        u64::from((best.0 - '0' as u8) * 10 + (best.1 - '0' as u8))
+    }
+}
+
+impl Solver for Lobby {
+    const PUZZLE_INDEX: u8 = 3;
+    const PUZZLE_NAME: &'static str = "Lobby";
+
+    fn process_data<R: BufRead>(&self, reader: R) -> (u64, u64) {
+        let mut joltage = 0;
+        for line in reader.lines().map_while(Result::ok) {
+            joltage += Self::largest_joltage(&line);
+        }
+        (joltage, 0)
+    }
+}