1 files changed, 86 insertions, 0 deletions
diff --git a/d02/src/main.rs b/d02/src/main.rs
new file mode 100644
index 0000000..33c7ec0
--- /dev/null
+++ b/d02/src/main.rs
@@ -0,0 +1,86 @@
+use std::fs;
+
+fn solve_part1(lines: &Vec<&str>) -> String {
+    let mut result = String::new();
+
+    let mut num = 5;
+    for line in lines {
+        for c in line.chars() {
+            num = match c {
+                'U' => if num - 3 >  0 { num - 3 } else { num },
+                'D' => if num + 3 < 10 { num + 3 } else { num },
+                'L' => if num % 3 != 1 { num - 1 } else { num },
+                'R' => if num % 3 != 0 { num + 1 } else { num },
+                _   => num, // shouldn't happen
+            }
+        }
+        result.push_str(&num.to_string());
+    }
+
+    result
+}
+
+fn solve_part2(lines: &Vec<&str>) -> String {
+    let mut result = String::new();
+
+    // Part 2's keypad is a bit more exotic
+    let keypad = [
+        [0,  0,  1,  0,  0],
+        [0,  2,  3,  4,  0],
+        [5,  6,  7,  8,  9],
+        [0, 10, 11, 12,  0],
+        [0,  0, 13,  0,  0],
+    ];
+    let mut pos = (2, 0); // (row, col) index
+
+    for line in lines {
+        for c in line.chars() {
+            let mut new = pos;
+            match c {
+                'U' => new.0 = if pos.0 > 0 { pos.0 - 1 } else { pos.0 },
+                'D' => new.0 = if pos.0 < 4 { pos.0 + 1 } else { pos.0 },
+                'L' => new.1 = if pos.1 > 0 { pos.1 - 1 } else { pos.1 },
+                'R' => new.1 = if pos.1 < 4 { pos.1 + 1 } else { pos.1 },
+                _   => {}, // shouldn't happen
+            };
+            // Ignore this `c' if it would take us off the diamond's edge
+            if keypad[new.0][new.1] != 0 {
+                pos = new;
+            }
+        }
+
+        // Print as hexadecimal to follow puzzle description
+        result.push_str(&format!("{:X}", keypad[pos.0][pos.1]));
+    }
+
+    result
+}
+
+fn main() {
+    // Read directions from input text file
+    let input = fs::read_to_string("input.txt").unwrap();
+    let lines = input.lines().collect();
+
+    // Part 1 gives 74921 for me
+    println!("Part 1 solution: {}", solve_part1(&lines));
+
+    // Part 2 gives A6B35 for me
+    println!("Part 2 solution: {}", solve_part2(&lines));
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn part1_example1() {
+        let lines = vec!["ULL", "RRDDD", "LURDL", "UUUUD"];
+        assert_eq!(solve_part1(&lines), "1985");
+    }
+
+    #[test]
+    fn part2_example1() {
+        let lines = vec!["ULL", "RRDDD", "LURDL", "UUUUD"];
+        assert_eq!(solve_part2(&lines), "5DB3");
+    }
+}