Optimize 2017 day 5

Reduces runtime from ~37ms to ~25ms

Author: ictrobot

crates/year2017/src/day05.rs

@@ -6,6 +6,9 @@ pub struct Day05 {
     jumps: Vec<i32>,
 }
 
+type Compressed = usize;
+const BITS: usize = Compressed::BITS as usize;
+
 impl Day05 {
     pub fn new(input: &str, _: InputType) -> Result<Self, InputError> {
         Ok(Self {
@@ -35,11 +38,51 @@ impl Day05 {
         let mut steps = 0;
         let mut pc = 0;
 
+        // After each jump instruction is run enough times the offset oscillates back and forth
+        // between 2 and 3. Once this happens, represent the jump as a single bit in a compressed
+        // bit mask, which allows processing multiple jumps at once without each one requiring a
+        // random memory read.
+        let mut threes: Vec<Compressed> = vec![0; jumps.len().next_multiple_of(BITS) / BITS];
+        // boundary represents the point where all prior jumps have stabilized on oscillating
+        // between 2 and 3
+        let mut boundary = 0;
+
         while pc < jumps.len() {
             let offset = jumps[pc];
             jumps[pc] += if offset >= 3 { -1 } else { 1 };
+
+            if pc == boundary && (jumps[pc] == 2 || jumps[pc] == 3) {
+                // Next jump after the boundary stabilized on 2/3
+                boundary += 1;
+
+                let element_index = pc / BITS;
+                let bit_index = pc % BITS;
+                threes[element_index] |= ((jumps[pc] & 1) as Compressed) << bit_index;
+            }
+
             pc = pc.wrapping_add_signed(offset as isize);
             steps += 1;
+
+            while pc < boundary {
+                // While inside the boundary loop over each compressed element and handle the jumps
+                // in bulk
+                let element_index = pc / BITS;
+                let mut element = threes[element_index];
+
+                let bit_index = pc % BITS;
+                let mut bit = 1 << bit_index;
+
+                let max = boundary.min((element_index + 1) * BITS);
+                while pc < max {
+                    let offset = 2 + usize::from(element & bit != 0);
+                    element ^= bit;
+                    bit <<= offset;
+                    pc += offset;
+                    steps += 1;
+                }
+
+                threes[element_index] = element;
+            }
         }
 
         steps