make obtaining the register value *during* a cycle easier

day10/common.rs

@@ -65,7 +65,7 @@ impl Processor {
         self.cycle_count = instruction.duration();
     }
 
-    pub fn step(&mut self) -> Result<i64, &'static str> {
+    pub fn step(&mut self) -> i64 {
         match self.instruction {
             Instruction::Noop => self.cycle_count -= 1,
             Instruction::Addx { arg } => {
@@ -75,7 +75,7 @@ impl Processor {
                 }
             }
         }
-        Ok(self.x)
+        self.x
     }
 
     pub fn cycles(&mut self) -> Cycles {
@@ -83,12 +83,17 @@ impl Processor {
     }
 }
 
+pub struct Cycle {
+    pub x_during: i64,
+    pub x_after: i64,
+}
+
 pub struct Cycles<'a> {
     processor: &'a mut Processor,
 }
 
 impl Iterator for Cycles<'_> {
-    type Item = Result<i64, &'static str>;
+    type Item = Result<Cycle, &'static str>;
 
     fn next(&mut self) -> Option<Self::Item> {
         if self.processor.cycle_count == 0 {
@@ -102,6 +107,9 @@ impl Iterator for Cycles<'_> {
                 Err(_) => assert!(false),
             }
         }
-        Some(self.processor.step())
+        Some(Ok(Cycle {
+            x_during: self.processor.x,
+            x_after: self.processor.step(),
+        }))
     }
 }

day10/part1.rs

@@ -22,9 +22,9 @@ use common::Processor;
 fn main() -> Result<(), &'static str> {
     let mut processor = Processor::new();
     let mut result = 0;
-    for (count, register_x) in processor.cycles().enumerate() {
+    for (count, cycle) in processor.cycles().enumerate() {
-        if [20, 60, 100, 140, 180, 220].contains(&(count + 2)) {
+        if [20, 60, 100, 140, 180, 220].contains(&(count + 1)) {
-            result += (count + 2) as i64 * register_x?;
+            result += (count + 1) as i64 * cycle?.x_during;
         }
     }
     println!("{result}");

day10/part2.rs

@@ -21,10 +21,11 @@ use common::Processor;
 
 fn main() -> Result<(), &'static str> {
     let mut processor = Processor::new();
-    let mut line = String::from("#");
+    let mut line = String::with_capacity(40);
-    for (count, register_x) in processor.cycles().enumerate() {
+    for (count, cycle) in processor.cycles().enumerate() {
-        let sprite = register_x? - 1..=register_x? + 1;
+        let x_during = cycle?.x_during;
-        match (count + 1) % 40 {
+        let sprite = x_during - 1..=x_during + 1;
+        match count % 40 {
             pos if { sprite.contains(&(pos as i64)) } => line.push('#'),
             _ => line.push('.'),
         }