From 5717adcf1cf46a2e3051c859f6d38d7562784faf Mon Sep 17 00:00:00 2001
From: "Timothy J. Warren" <tim@timshomepage.net>
Date: Tue, 13 Dec 2022 16:19:26 -0500
Subject: [PATCH] Complete day 9 part 1

---
 day9/src/main.rs | 110 ++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 94 insertions(+), 16 deletions(-)

diff --git a/day9/src/main.rs b/day9/src/main.rs
index 0cebe63..7e9eca2 100644
--- a/day9/src/main.rs
+++ b/day9/src/main.rs
@@ -11,7 +11,7 @@ use Direction::*;
 
 struct Move {
     dir: Direction,
-    amount: usize,
+    amount: isize,
 }
 
 impl Move {
@@ -25,7 +25,7 @@ impl Move {
             "R" => Right,
             _ => panic!("Invalid direction!"),
         };
-        let amount = parts[1].parse::<usize>().unwrap();
+        let amount = parts[1].parse::<isize>().unwrap();
 
         Move { dir, amount }
     }
@@ -33,17 +33,23 @@ impl Move {
 
 #[derive(Debug, Default, Copy, Clone, Eq, Hash, PartialEq)]
 struct Location {
-    x: usize,
-    y: usize,
+    x: isize,
+    y: isize,
 }
 
 impl Location {
-    fn new(x: usize, y: usize) -> Self {
+    fn new(x: isize, y: isize) -> Self {
         Location { x, y }
     }
+
+    fn get_distance(self, other: Self) -> f64 {
+        let squares = (other.x - self.x).pow(2) + (other.y - self.y).pow(2);
+
+        (squares as f64).sqrt()
+    }
 }
 
-// ----------------------------------------------------------------------------
+// ---------------------------------------------------------------------------
 
 #[derive(Debug, Default)]
 struct Rope {
@@ -55,21 +61,31 @@ struct Rope {
 
 impl Rope {
     pub fn new() -> Self {
-        Rope { ..Rope::default() }
+        let mut rope = Self::default();
+        rope.head_visited.insert(Location::default());
+        rope.tail_visited.insert(Location::default());
+
+        rope
     }
 
     pub fn move_head(&mut self, moves: Move) {
-        let from = self.head;
-
         for _ in 0..moves.amount {
-            let mut x = from.x;
-            let mut y = from.y;
+            let mut x = self.head.x;
+            let mut y = self.head.y;
 
             match moves.dir {
-                Up => y += 1,
-                Down => y -= 1,
-                Left => x -= 1,
-                Right => x += 1,
+                Up => {
+                    y += 1;
+                }
+                Down => {
+                    y -= 1;
+                }
+                Left => {
+                    x -= 1;
+                }
+                Right => {
+                    x += 1;
+                }
             }
 
             let to = Location::new(x, y);
@@ -80,7 +96,42 @@ impl Rope {
         }
     }
 
-    fn move_tail(&mut self, head: Location) {}
+    fn must_tail_move(&mut self, head: Location) -> bool {
+        let distance = self.tail.get_distance(head);
+
+        distance >= 2.0
+    }
+
+    fn move_tail(&mut self, head: Location) {
+        if !self.must_tail_move(head) {
+            return;
+        }
+
+        let mut tail = self.tail.clone();
+
+        if tail.y != head.y {
+            if head.y - tail.y < 0 {
+                tail.y -= 1;
+            } else {
+                tail.y += 1;
+            }
+        }
+
+        if tail.x != head.x {
+            if head.x - tail.x < 0 {
+                tail.x -= 1;
+            } else {
+                tail.x += 1;
+            }
+        }
+
+        self.tail = tail;
+        self.tail_visited.insert(tail);
+    }
+
+    fn get_tail_pos_count(&self) -> usize {
+        self.tail_visited.len()
+    }
 }
 
 // ----------------------------------------------------------------------------
@@ -93,6 +144,10 @@ fn main() {
         .lines()
         .map(Move::from_line)
         .for_each(|m| rope.move_head(m));
+
+    let tail_positions = rope.get_tail_pos_count();
+
+    println!("Part 1: Number of tail movements: {}", tail_positions);
 }
 
 #[cfg(test)]
@@ -102,4 +157,27 @@ mod tests {
     fn get_data() -> &'static str {
         include_str!("test-input.txt")
     }
+
+    #[test]
+    fn test_location_get_distance() {
+        let a = Location::new(0, 0);
+
+        assert_eq!(a.get_distance(Location::new(0, 0)), 0.0);
+        assert_eq!(a.get_distance(Location::new(1, 0)), 1.0);
+        assert_eq!(a.get_distance(Location::new(1, 1)), 2.0f64.sqrt());
+    }
+
+    #[test]
+    fn test_get_tail_position_count() {
+        let mut rope = Rope::new();
+
+        assert_eq!(rope.get_tail_pos_count(), 1);
+
+        get_data()
+            .lines()
+            .map(Move::from_line)
+            .for_each(|m| rope.move_head(m));
+
+        assert_eq!(rope.get_tail_pos_count(), 13);
+    }
 }