Rewrite old code into 2018 style.

Nice test for my 2018 repo set up. Which turns out needs some tweaking.

2016/src/common.rs

@@ -0,0 +1,63 @@
+use std::io;
+
+/// Apply Erathostenes's sieve to the supplied array
+///
+/// # Arguments
+///
+/// * `dest` - the destination slice to fill with the sieve. This is
+///   assumed to be filled with "true" before being handed to this
+///   method.
+pub fn prime_sieve(dest: &mut[bool]) {
+    if dest.len() >= 1 {
+        dest[0] = false;
+    }
+
+    if dest.len() >= 2 {
+        dest[1] = false;
+    }
+
+    let limit = (dest.len() as f64).sqrt() as usize;
+
+    for i in 1..(limit + 1) {
+        if !dest[i] {
+            continue
+        }
+
+        for j in ((i * i)..(dest.len())).step_by(i) {
+            dest[j] = false;
+        }
+    }
+}
+
+/// Solution trait
+///
+/// Every day's solution should implement this function so that it can
+/// be easily run from the main program.
+pub trait Solution {
+    /// Solve the first part of the day
+    fn part1(&mut self, input: &mut io::Read) -> String;
+
+    /// Solve the second part of the day
+    fn part2(&mut self, input: &mut io::Read) -> String;
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_prime_sieve() {
+        let mut input = [true; 10];
+        prime_sieve(&mut input);
+
+        let output = [
+            false, false,
+            true, true,
+            false, true,
+            false, true,
+            false, false
+        ];
+
+        assert_eq!(output, input);
+    }
+}