Add solver constants

src/solvers/printing_department.rs

@@ -4,6 +4,10 @@ use std::io::BufRead;
 
 pub struct PrintingDepartment {}
 
+impl PrintingDepartment {
+    const PAPER_ROLL_CHAR: char = '@';
+}
+
 impl Solver for PrintingDepartment {
     const PUZZLE_INDEX: u8 = 4;
     const PUZZLE_NAME: &'static str = "Printing Department";
@@ -11,7 +15,7 @@ impl Solver for PrintingDepartment {
     fn process_data<R: BufRead>(&self, reader: R) -> (u64, u64) {
         let mut grid = Grid::new(0, 0);
         for line in reader.lines().map_while(Result::ok) {
-            grid.push_row(line.bytes().map(|c| c == b'@').collect());
+            grid.push_row(line.bytes().map(|c| c == Self::PAPER_ROLL_CHAR as u8).collect());
         }
 
         let mut part1 = 0;