Binary search is best search

2023/benches/days.rs

@@ -9,7 +9,7 @@ use criterion::Criterion;
 use aoc_2023::get_implementation;
 
 /// Number of days we have an implementation to benchmark
-const DAYS_IMPLEMENTED: u8 = 5;
+const DAYS_IMPLEMENTED: u8 = 6;
 
 fn read_input(day: u8) -> std::io::Result<Vec<u8>> {
     let input_path = format!("inputs/{day:02}.txt");

2023/src/day06.rs

@@ -1,7 +1,106 @@
-pub fn part1(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+use nom::bytes::complete::tag;
+use nom::character::complete::digit1;
+use nom::character::complete::newline;
+use nom::character::complete::space1;
+use nom::multi::fold_many1;
+use nom::multi::many1;
+use nom::sequence::delimited;
+use nom::sequence::pair;
+use nom::sequence::preceded;
+use nom::IResult;
+
+use crate::common::parse_input;
+
+fn parse_race(i: &[u8]) -> IResult<&[u8], (Vec<u64>, Vec<u64>)> {
+    let line = |header| {
+        delimited(
+            tag(header),
+            many1(preceded(space1, nom::character::complete::u64)),
+            newline,
+        )
+    };
+
+    pair(line("Time:"), line("Distance:"))(i)
 }
 
-pub fn part2(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+fn parse_long_race(i: &[u8]) -> IResult<&[u8], (u64, u64)> {
+    let line = |header| {
+        delimited(
+            tag(header),
+            fold_many1(
+                preceded(space1, digit1),
+                || 0,
+                |mut cur, sequence| {
+                    for &c in sequence {
+                        cur *= 10;
+                        cur += u64::from(c - b'0');
+                    }
+
+                    cur
+                },
+            ),
+            newline,
+        )
+    };
+
+    pair(line("Time:"), line("Distance:"))(i)
+}
+
+fn ways(time: u64, distance: u64) -> u64 {
+    let half = time / 2;
+    let mut min = 1;
+    let mut max = half;
+
+    while min < max {
+        let mid = min + (max - min) / 2;
+
+        if mid * (time - mid) < distance {
+            min = mid + 1;
+        } else {
+            max = mid;
+        }
+    }
+
+    let make_it = half - min + 1;
+
+    if time % 2 == 0 {
+        2 * make_it - 1
+    } else {
+        2 * make_it
+    }
+}
+
+pub fn part1(input: &[u8]) -> anyhow::Result<String> {
+    let (time, distance) = parse_input(input, parse_race)?;
+
+    let total: u64 = time
+        .iter()
+        .zip(&distance)
+        .map(|(&time, &distance)| ways(time, distance))
+        .product();
+
+    Ok(total.to_string())
+}
+
+pub fn part2(input: &[u8]) -> anyhow::Result<String> {
+    let (time, distance) = parse_input(input, parse_long_race)?;
+
+    Ok(ways(time, distance).to_string())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/06.txt");
+
+    #[test]
+    fn sample_part1() {
+        assert_eq!(part1(SAMPLE).unwrap(), "288");
+    }
+
+    #[test]
+    fn sample_part2() {
+        assert_eq!(part2(SAMPLE).unwrap(), "71503");
+    }
 }

2023/src/samples/06.txt

@@ -0,0 +1,2 @@
+Time:      7  15   30
+Distance:  9  40  200