Remove most allocations from the code path

2023/src/day09.rs

@@ -1,7 +1,88 @@
-pub fn part1(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+use std::mem;
+use std::ops::Range;
+
+use nom::IResult;
+
+use crate::common::parse_input;
+
+fn parse_reports(mut i: &[u8]) -> IResult<&[u8], (Vec<Range<usize>>, Vec<i32>)> {
+    let mut begin = 0;
+    let mut numbers = Vec::new();
+    let mut ranges = Vec::new();
+    while !i.is_empty() {
+        let (rem, num) = nom::character::complete::i32(i)?;
+        numbers.push(num);
+
+        if rem[0] == b'\n' {
+            let end = numbers.len();
+            ranges.push(begin..end);
+            begin = end;
+        }
+
+        i = &rem[1..];
+    }
+
+    Ok((i, (ranges, numbers)))
 }
 
-pub fn part2(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+fn compute_next<'a>(report: impl IntoIterator<Item = &'a i32>, deltas: &mut Vec<i32>) -> i32 {
+    deltas.clear();
+
+    for &entry in report {
+        let mut delta = entry;
+        for prev_delta in &mut *deltas {
+            let prev = mem::replace(prev_delta, delta);
+            delta = delta - prev;
+        }
+
+        if delta != 0 {
+            deltas.push(delta);
+        }
+    }
+
+    deltas.iter().rev().fold(0, |c, d| c + d)
+}
+
+pub fn part1(input: &[u8]) -> anyhow::Result<String> {
+    let mut deltas = Vec::new();
+    let (ranges, numbers) = parse_input(input, parse_reports)?;
+    let result: i32 = ranges
+        .into_iter()
+        .map(|range| compute_next(&numbers[range], &mut deltas))
+        .sum();
+    Ok(result.to_string())
+}
+
+pub fn part2(input: &[u8]) -> anyhow::Result<String> {
+    let mut deltas = Vec::new();
+    let (ranges, numbers) = parse_input(input, parse_reports)?;
+    let result: i32 = ranges
+        .into_iter()
+        .map(|range| compute_next(numbers[range].iter().rev(), &mut deltas))
+        .sum();
+    Ok(result.to_string())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/09.txt");
+
+    #[test]
+    fn samples_separate() {
+        assert_eq!(18, compute_next(&[0, 3, 6, 9, 12, 15], &mut Vec::new()));
+        assert_eq!(28, compute_next(&[1, 3, 6, 10, 15, 21], &mut Vec::new()));
+        assert_eq!(68, compute_next(&[10, 13, 16, 21, 30, 45], &mut Vec::new()));
+    }
+
+    #[test]
+    fn sample_part1() {
+        assert_eq!("114", part1(SAMPLE).unwrap());
+    }
+
+    #[test]
+    fn sample_part2() {
+        assert_eq!("2", part2(SAMPLE).unwrap());
+    }
 }

2023/src/samples/09.txt

@@ -0,0 +1,3 @@
+0 3 6 9 12 15
+1 3 6 10 15 21
+10 13 16 21 30 45