From 35899f02f9e38d459806a4020039c0f120a50e82 Mon Sep 17 00:00:00 2001 From: Antoine Martin Date: Wed, 9 Dec 2020 08:53:19 +0100 Subject: [PATCH] 2020: day09: optimize using cumulated sum --- aoc2020/src/day09.rs | 25 +++++++++++++++---------- 1 file changed, 15 insertions(+), 10 deletions(-) diff --git a/aoc2020/src/day09.rs b/aoc2020/src/day09.rs index 4a91a22..e203d42 100644 --- a/aoc2020/src/day09.rs +++ b/aoc2020/src/day09.rs @@ -54,15 +54,20 @@ fn part1(input: &str) -> aoc::Result { Ok(solution) } -fn find_contiguous_range(numbers: &[u64], total: u64, total_idx: usize) -> aoc::Result<(u64, u64)> { - for i in 0..total_idx { - for j in (i + 1)..total_idx { - let range = &numbers[i..=j]; +fn find_contiguous_range(numbers: &[u64], total: u64) -> aoc::Result<(u64, u64)> { + // compute cumulated sums for the whole range + let (sums, _) = numbers.iter().fold((vec![0], 0), |(mut vec, acc), n| { + let acc = acc + n; + vec.push(acc); - if range.iter().sum::() == total { - // it's safe to unwrap here because j > i so the range always has one number - let min = range.iter().min().unwrap(); - let max = range.iter().max().unwrap(); + (vec, acc) + }); + + for i in 0..sums.len() { + for j in (i + 1)..sums.len() { + if sums[j] - sums[i] == total { + let min = numbers[i..j].iter().min().unwrap(); + let max = numbers[i..j].iter().max().unwrap(); return Ok((*min, *max)); } @@ -81,7 +86,7 @@ fn part2(input: &str) -> aoc::Result { let (outlier, idx) = find_outlier(&numbers, 25)?; - let (min, max) = find_contiguous_range(&numbers, outlier, idx)?; + let (min, max) = find_contiguous_range(&numbers[..idx], outlier)?; Ok(min + max) } @@ -116,7 +121,7 @@ mod tests { let (outlier, idx) = find_outlier(&numbers, 5).unwrap(); - let (min, max) = find_contiguous_range(&numbers, outlier, idx).unwrap(); + let (min, max) = find_contiguous_range(&numbers[..idx], outlier).unwrap(); assert_eq!(min, 15); assert_eq!(max, 47);