Implement 2023 day 5 part 1

2023/src/day05.rs

@@ -1,7 +1,138 @@
-pub fn part1(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+use anyhow::Context;
+use nom::bytes::complete::tag;
+use nom::bytes::complete::take_until;
+use nom::character::complete::newline;
+use nom::combinator::map;
+use nom::multi::many1;
+use nom::sequence::delimited;
+use nom::sequence::preceded;
+use nom::sequence::terminated;
+use nom::sequence::tuple;
+use nom::IResult;
+
+use crate::common::parse_input;
+
+#[derive(PartialEq, Eq, PartialOrd, Ord)]
+struct Mapping {
+    source_start: u64,
+    dest_start: u64,
+    len: u64,
+}
+
+fn parse_mapping(i: &[u8]) -> IResult<&[u8], Vec<Mapping>> {
+    use nom::character::complete::u64;
+
+    let mut mapping = many1(map(
+        tuple((
+            terminated(u64, tag(" ")),
+            terminated(u64, tag(" ")),
+            terminated(u64, newline),
+        )),
+        |(dest_start, source_start, len)| Mapping {
+            source_start,
+            dest_start,
+            len,
+        },
+    ))(i)?;
+
+    // Sort mappings for O(log n) lookup of appropriate mapping later
+    mapping.1.sort_unstable();
+    Ok(mapping)
+}
+
+struct Almanac {
+    seeds: Vec<u64>,
+    // There's a lot of mappings but their names don't matter, they all work the same.
+    mappings: [Vec<Mapping>; 7],
+}
+
+fn parse_almanac(i: &[u8]) -> IResult<&[u8], Almanac> {
+    let parse_seeds = delimited(
+        tag("seeds:"),
+        many1(preceded(tag(" "), nom::character::complete::u64)),
+        newline,
+    );
+
+    let mapping_parser = |header| preceded(tag(header), parse_mapping);
+
+    map(
+        tuple((
+            parse_seeds,
+            mapping_parser("\nseed-to-soil map:\n"),
+            mapping_parser("\nsoil-to-fertilizer map:\n"),
+            mapping_parser("\nfertilizer-to-water map:\n"),
+            mapping_parser("\nwater-to-light map:\n"),
+            mapping_parser("\nlight-to-temperature map:\n"),
+            mapping_parser("\ntemperature-to-humidity map:\n"),
+            mapping_parser("\nhumidity-to-location map:\n"),
+        )),
+        |(seeds, soil, fertilizer, water, light, temperature, humidity, location)| Almanac {
+            seeds,
+            mappings: [
+                soil,
+                fertilizer,
+                water,
+                light,
+                temperature,
+                humidity,
+                location,
+            ],
+        },
+    )(i)
+}
+
+fn follow_mapping(node: u64, mappings: &[Mapping]) -> u64 {
+    let point = mappings.partition_point(|mapping| mapping.source_start <= node);
+    if point == 0 {
+        // There are no mappings that are smaller than the node, so it maps to itself
+        node
+    } else {
+        // `mapping`` is the last mapping that starts smaller or equal to our node, so it is the one
+        // that might contain it.
+        let mapping = &mappings[point - 1];
+
+        // Check if the node is in range of this mapping
+        if node - mapping.source_start < mapping.len {
+            // It is, note the order of operations to avoid underflow
+            node + mapping.dest_start - mapping.source_start
+        } else {
+            // It's not, return itself
+            node
+        }
+    }
+}
+
+pub fn part1(input: &[u8]) -> anyhow::Result<String> {
+    let almanac = parse_input(input, parse_almanac)?;
+
+    let min = almanac
+        .seeds
+        .iter()
+        .map(|node| {
+            let mut node = *node;
+            for mappings in &almanac.mappings {
+                node = follow_mapping(node, mappings)
+            }
+            node
+        })
+        .min()
+        .context("Unreachable, no seeds but parser ensures seeds")?;
+
+    Ok(min.to_string())
 }
 
 pub fn part2(_input: &[u8]) -> anyhow::Result<String> {
     anyhow::bail!("Not implemented")
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/05.txt");
+
+    #[test]
+    fn sample_part1() {
+        assert_eq!(part1(SAMPLE).unwrap(), "35");
+    }
+}