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Proper implementation for 2023 day 5 part 2

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Bert Peters
2023-12-05 19:15:41 +01:00
parent 63c1024d22
commit 018d41e753
1 changed files with 59 additions and 4 deletions
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63
2023/src/day05.rs
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@@ -1,3 +1,5 @@
use std::mem;
use anyhow::Context;
use nom::bytes::complete::tag;
use nom::character::complete::newline;
@@ -124,13 +126,66 @@ pub fn part1(input: &[u8]) -> anyhow::Result<String> {
pub fn part2(input: &[u8]) -> anyhow::Result<String> {
let almanac = parse_input(input, parse_almanac)?;
let min = almanac
let mut ranges: Vec<(u64, u64)> = almanac
.seeds
.chunks_exact(2)
.flat_map(|c| c[0]..c[0] + c[1])
.map(|node| follow_all_mappings(node, &almanac.mappings))
.map(|c| (c[0], c[1]))
.collect();
let mut target = Vec::new();
for mappings in &almanac.mappings {
for (mut start, mut len) in ranges.drain(..) {
debug_assert_ne!(len, 0);
let mut point = mappings.partition_point(|mapping| mapping.source_start <= start);
if point > 0 && start < mappings[point - 1].source_start + mappings[point - 1].len {
let mapping = &mappings[point - 1];
let overlapping_len = mapping.len - (start - mapping.source_start);
let use_len = Ord::min(len, overlapping_len);
debug_assert!(use_len > 0);
target.push((start - mapping.source_start + mapping.dest_start, use_len));
start += use_len;
len -= use_len;
}
// Loop invariant: start is not in a range and the next range is mappings[point]
while len > 0 && point < mappings.len() {
let mapping = &mappings[point];
let before_len = Ord::min(len, mapping.source_start - start);
if before_len > 0 {
len -= before_len;
target.push((start, before_len));
start += before_len;
}
if len > 0 {
let inside_len = Ord::min(len, mapping.len);
debug_assert!(inside_len > 0);
target.push((mapping.dest_start, inside_len));
start += inside_len;
len -= inside_len;
point += 1;
}
}
if len > 0 {
target.push((start, len));
}
}
mem::swap(&mut ranges, &mut target);
}
let min = ranges
.iter()
.map(|&(start, _)| start)
.min()
.context("Unreachable, no seeds but parser ensures seeds")?;
.context("Somehow lost all ranges")?;
Ok(min.to_string())
}