Solve Part 2

src/main.rs

@@ -2,9 +2,11 @@
 
 const DATA: &'static str = include_str!("input.txt");
 
+// I can use unwrap()/expect() as much as I want here, since for AoC the input data is expected to make sense.
+
 use std::str::FromStr;
 
-#[derive(PartialEq, Eq)]
+#[derive(PartialEq, Eq, Clone, Copy)]
 enum Move
 {
     Rock,
@@ -13,12 +15,11 @@ enum Move
 }
 
 #[derive(PartialEq, Eq)]
-#[repr(i32)]
 enum Outcome
 {
-    Loss = 0,
-    Draw = 3,
-    Win = 6
+    Loss,
+    Draw,
+    Win
 }
 
 impl Outcome
@@ -44,7 +45,7 @@ impl Move
         }
     }
 
-    pub fn did_win(&self, other: &Move) -> Outcome 
+    pub fn did_win_against(&self, other: &Move) -> Outcome 
     {
         if self == other { Outcome::Draw }
         else {
@@ -66,6 +67,29 @@ impl Move
             }
         }
     }
+
+    pub fn move_for_outcome(&self, outcome: Outcome) -> Move
+    {
+        if outcome == Outcome::Draw { *self }
+        else {
+            match self {
+                Self::Rock => {
+                    if outcome == Outcome::Win { Self::Paper }
+                    else { Self::Scissors }
+                }
+
+                Self::Paper => {
+                    if outcome == Outcome::Win { Self::Scissors }
+                    else { Self::Rock }
+                }
+
+                Self::Scissors => {
+                    if outcome == Outcome::Win { Self::Rock }
+                    else { Self::Paper }
+                }
+            }
+        }
+    }
 }
 
 #[derive(Debug)]
@@ -89,18 +113,38 @@ impl FromStr for Move {
     }
 }
 
+#[derive(Debug)]
+enum OutcomeParseFailure
+{
+    InvalidOutcome,
+}
+
+impl FromStr for Outcome {
+    type Err = OutcomeParseFailure;
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s {
+            "X" => Ok(Self::Loss),
+            "Y" => Ok(Self::Draw),
+            "Z" => Ok(Self::Win),
+            _ => Err(OutcomeParseFailure::InvalidOutcome)
+        }
+    }
+}
+
 fn score_for_round(my_move: &Move, other_move: &Move) -> i32 
 {
     let mut score = my_move.as_score();
 
-    let outcome = my_move.did_win(other_move);
+    let outcome = my_move.did_win_against(other_move);
 
     score += outcome.as_score();
 
     score
 }
 
-fn parse_round_and_get_score(round_str: &str) -> Option<i32>
+// Calling nth() on an iterator consumes the returned element, that's why I'm calling nth(0) several times
+
+fn parse_round_and_get_score_part1(round_str: &str) -> Option<i32>
 {
     let mut moves = round_str.split(" ");
 
@@ -111,16 +155,42 @@ fn parse_round_and_get_score(round_str: &str) -> Option<i32>
         &first_move.parse().expect("Expected a valid move")))
 }
 
+fn parse_round_and_get_score_part2(round_str: &str) -> Option<i32>
+{
+    let mut moves = round_str.split(" ");
+
+    let opponent_move_str = moves.nth(0)?; // Opponent's move goes first
+    let expected_outcome_str = moves.nth(0)?; // Expected outcome goes second
+
+    let expected_outcome: Outcome = expected_outcome_str.parse().expect("Expected a valid outcome");
+    let opponent_move: Move = opponent_move_str.parse().expect("Expected a valid move");
+
+    Some(score_for_round(&opponent_move.move_for_outcome(expected_outcome), 
+        &opponent_move))
+}
+
+#[allow(dead_code)]
 fn part1()
 {
     let mut score = 0;
     for slice in DATA.split("\n")
     {
-        score += parse_round_and_get_score(slice).expect("Input data is not correctly structured");
+        score += parse_round_and_get_score_part1(slice).expect("Input data is not correctly structured");
+    }
+    println!("Your total score is {}", score);
+}
+
+fn part2()
+{
+    let mut score = 0;
+    for slice in DATA.split("\n")
+    {
+        score += parse_round_and_get_score_part2(slice).expect("Input data is not correctly structured");
     }
     println!("Your total score is {}", score);
 }
 
 fn main() {
-    part1();
+    //part1();
+    part2();
 }