optimize parsing

2023/day09/src/lib.rs

@@ -1,4 +1,5 @@
 mod less_alloc;
+mod number_parsing;
 
 use std::ops::Add;
 
@@ -15,10 +16,12 @@ helper::define_variants! {
     part1 {
         basic => crate::part1;
         less_alloc => crate::less_alloc::part1;
+        number_parsing => crate::number_parsing::part1;
     }
     part2 {
         basic => crate::part2;
         less_alloc => crate::less_alloc::part2;
+        number_parsing => crate::number_parsing::part2;
     }
 }
 

2023/day09/src/number_parsing.rs

@@ -0,0 +1,67 @@
+use std::ops::Add;
+
+fn parse(line: &str, into: &mut Vec<i64>) {
+    let mut neg = false;
+    let mut acc = 0;
+
+    for byte in line.bytes() {
+        match byte {
+            b' ' => {
+                if neg {
+                    acc = -acc
+                };
+                into.push(acc);
+                acc = 0;
+                neg = false;
+            }
+            b'-' => neg = true,
+            // must be b'0'..=b'9'
+            _ => {
+                let value = byte - b'0';
+                acc = (acc * 10) + (value as i64);
+            }
+        }
+    }
+}
+
+fn execute(
+    input: &str,
+    last_or_first: impl Fn(&[i64]) -> i64,
+    fold: impl Fn(i64, i64) -> i64 + Copy,
+) -> i64 {
+    let mut row1 = Vec::with_capacity(128);
+    let mut row2 = Vec::with_capacity(128);
+
+    let mut last_values = Vec::with_capacity(16);
+
+    input
+        .lines()
+        .map(|line| {
+            row1.clear();
+            parse(line, &mut row1);
+            row2.clone_from(&row1);
+
+            last_values.clear();
+            last_values.push(last_or_first(&row1));
+
+            while !row2.iter().all(|&n| n == 0) {
+                row1.clear();
+                row1.extend(row2.windows(2).map(|s| s[1] - s[0]));
+
+                last_values.push(last_or_first(&row1));
+
+                std::mem::swap(&mut row1, &mut row2);
+            }
+
+            last_values.iter().copied().rev().fold(0, fold)
+        })
+        .sum::<i64>()
+}
+
+pub fn part1(input: &str) -> u64 {
+    execute(input, |s| *s.last().unwrap(), Add::add) as u64
+}
+
+pub fn part2(input: &str) -> u64 {
+    execute(input, |s| *s.first().unwrap(), |a, b| b - a) as u64
+}