2021: day07: QuickSelect for median computation
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parent
f7bbbf5cce
commit
f621e603f3
64
Cargo.lock
generated
64
Cargo.lock
generated
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@ -88,6 +88,7 @@ version = "0.1.0"
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dependencies = [
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"anyhow",
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"aoc",
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"rand",
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]
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[[package]]
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@ -348,6 +349,17 @@ dependencies = [
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"typenum",
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]
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[[package]]
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name = "getrandom"
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version = "0.2.3"
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source = "registry+https://github.com/rust-lang/crates.io-index"
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checksum = "7fcd999463524c52659517fe2cea98493cfe485d10565e7b0fb07dbba7ad2753"
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dependencies = [
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"cfg-if",
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"libc",
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"wasi",
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]
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[[package]]
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name = "half"
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version = "1.8.2"
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@ -528,6 +540,12 @@ dependencies = [
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"plotters-backend",
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]
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[[package]]
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name = "ppv-lite86"
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version = "0.2.15"
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source = "registry+https://github.com/rust-lang/crates.io-index"
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checksum = "ed0cfbc8191465bed66e1718596ee0b0b35d5ee1f41c5df2189d0fe8bde535ba"
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[[package]]
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name = "proc-macro2"
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version = "1.0.32"
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@ -552,6 +570,46 @@ version = "0.5.3"
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source = "registry+https://github.com/rust-lang/crates.io-index"
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checksum = "941ba9d78d8e2f7ce474c015eea4d9c6d25b6a3327f9832ee29a4de27f91bbb8"
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[[package]]
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name = "rand"
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version = "0.8.4"
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source = "registry+https://github.com/rust-lang/crates.io-index"
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checksum = "2e7573632e6454cf6b99d7aac4ccca54be06da05aca2ef7423d22d27d4d4bcd8"
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dependencies = [
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"libc",
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"rand_chacha",
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"rand_core",
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"rand_hc",
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]
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[[package]]
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name = "rand_chacha"
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version = "0.3.1"
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source = "registry+https://github.com/rust-lang/crates.io-index"
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checksum = "e6c10a63a0fa32252be49d21e7709d4d4baf8d231c2dbce1eaa8141b9b127d88"
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dependencies = [
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"ppv-lite86",
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"rand_core",
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]
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[[package]]
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name = "rand_core"
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version = "0.6.3"
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source = "registry+https://github.com/rust-lang/crates.io-index"
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checksum = "d34f1408f55294453790c48b2f1ebbb1c5b4b7563eb1f418bcfcfdbb06ebb4e7"
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dependencies = [
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"getrandom",
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]
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[[package]]
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name = "rand_hc"
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version = "0.3.1"
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source = "registry+https://github.com/rust-lang/crates.io-index"
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checksum = "d51e9f596de227fda2ea6c84607f5558e196eeaf43c986b724ba4fb8fdf497e7"
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dependencies = [
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"rand_core",
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]
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[[package]]
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name = "rayon"
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version = "1.5.1"
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@ -749,6 +807,12 @@ dependencies = [
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"winapi-util",
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]
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[[package]]
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name = "wasi"
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version = "0.10.2+wasi-snapshot-preview1"
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source = "registry+https://github.com/rust-lang/crates.io-index"
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checksum = "fd6fbd9a79829dd1ad0cc20627bf1ed606756a7f77edff7b66b7064f9cb327c6"
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[[package]]
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name = "wasm-bindgen"
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version = "0.2.78"
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@ -9,6 +9,7 @@ edition = "2021"
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[dependencies]
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aoc = { path = "../aoc" }
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anyhow = "1.0"
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rand = "0.8"
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[lib]
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path = "src/lib.rs"
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@ -1,6 +1,7 @@
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use std::fmt::Write;
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use anyhow::{Context, Result};
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use rand::Rng;
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const INPUT: &str = include_str!("../input/day07.txt");
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@ -20,9 +21,8 @@ fn part1(input: &str) -> Result<u64> {
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.map(|n| n.parse::<u64>().context("couldn't parse position"))
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.collect::<Result<Vec<_>>>()?;
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// TODO: try linear selection algorithm
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horizontal_positions.sort_unstable();
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let median = horizontal_positions[horizontal_positions.len() / 2];
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let median_rank = horizontal_positions.len() / 2;
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let median = selection(&mut horizontal_positions, median_rank);
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Ok(horizontal_positions
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.iter()
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@ -31,6 +31,52 @@ fn part1(input: &str) -> Result<u64> {
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.sum())
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}
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fn selection<T>(data: &mut [T], i: usize) -> T
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where
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T: Copy + Ord,
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{
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if data.len() == 1 {
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return data[0];
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}
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let mid = random_partition(data);
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if i < mid {
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selection(&mut data[..mid], i)
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} else {
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selection(&mut data[mid..], i - mid)
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}
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}
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fn random_partition<T>(data: &mut [T]) -> usize
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where
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T: Copy + Ord,
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{
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let pivot_index = rand::thread_rng().gen_range(0..data.len());
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let pivot = data[pivot_index];
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let mut i = 0;
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let mut j = data.len() - 1;
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loop {
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while data[i] < pivot {
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i += 1;
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}
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while data[j] > pivot {
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j -= 1;
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}
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if i >= j {
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return usize::max(i, 1);
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}
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data.swap(i, j);
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i += 1;
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j -= 1;
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}
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}
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fn part2(input: &str) -> Result<u64> {
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let horizontal_positions = input
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.trim()
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@ -90,4 +136,15 @@ mod tests {
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fn part2_real() {
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assert_eq!(part2(INPUT).unwrap(), 96592275);
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}
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#[test]
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fn test_selection() {
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for _ in 0..4200 {
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for i in 0..=9 {
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let mut data = vec![9, 2, 7, 3, 5, 4, 6, 1, 8, 0];
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let res = selection(&mut data, i);
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assert_eq!(res, i);
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}
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}
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}
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}
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