2022 day11/rust: clean up parsing

2022/day11/rust/src/main.rs

@@ -22,6 +22,17 @@ impl Operand {
     }
 }
 
+impl FromStr for Operand {
+    type Err = std::num::ParseIntError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s {
+            "old" => Ok(Operand::Old),
+            i => i.parse().map(Operand::Const),
+        }
+    }
+}
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 enum Operation {
     Add(Operand),
@@ -96,46 +107,39 @@ impl FromStr for Monkey {
 
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         let mut lines = s.lines();
-        lines.next().unwrap();
-        let items: Vec<_> = lines
-            .next()
-            .unwrap()
-            .split_once(':')
-            .unwrap()
-            .1
-            .trim()
-            .split(", ")
-            .map(|i| i.parse().unwrap())
-            .map(|value| Item { value })
-            .collect();
+        lines.next().ok_or(())?; // Monkey n:
 
-        let op: Vec<_> = lines.next().unwrap().split_whitespace().collect();
-        let (operand, op) = op.split_last().unwrap();
-        let (operator, _) = op.split_last().unwrap();
-        let operand = match *operand {
-            "old" => Operand::Old,
-            i => Operand::Const(i.parse().unwrap()),
-        };
-        let operation = match *operator {
+        let mut with_prefix = |prefix: &str| lines.next().ok_or(())?.strip_prefix(prefix).ok_or(());
+        let items: Vec<_> = with_prefix("  Starting items: ")?
+            .split(", ")
+            .map(|i| {
+                let value = i.parse().map_err(drop)?;
+                Ok(Item { value })
+            })
+            .collect::<Result<_, _>>()?;
+
+        let (operator, operand) = with_prefix("  Operation: new = old ")?
+            .split_once(' ')
+            .ok_or(())?;
+        let operand = operand.parse().map_err(drop)?;
+        let operation = match operator {
             "+" => Operation::Add(operand),
             "*" => Operation::Multiply(operand),
             _ => panic!("invalid operator {operator}"),
         };
 
-        let mut lastnum = || {
-            lines
-                .next()
-                .unwrap()
-                .split_whitespace()
-                .last()
-                .unwrap()
+        let test = Test::Divisible(
+            with_prefix("  Test: divisible by ")?
                 .parse()
-                .unwrap()
-        };
-        let test = Test::Divisible(lastnum());
+                .map_err(drop)?,
+        );
 
-        let target_pass = lastnum();
-        let target_fail = lastnum();
+        let target_pass = with_prefix("    If true: throw to monkey ")?
+            .parse()
+            .map_err(drop)?;
+        let target_fail = with_prefix("    If false: throw to monkey ")?
+            .parse()
+            .map_err(drop)?;
 
         Ok(Monkey {
             items,