aoc/year2018/
day07.rs

1//! # The Sum of Its Parts
2//!
3//! Part one is a [topological sort](https://en.wikipedia.org/wiki/Topological_sorting)
4//! of the steps based on the dependencies between them.
5use crate::util::hash::*;
6use std::collections::BTreeMap;
7
8type Input = FastMap<u8, Step>;
9
10#[derive(Clone, Default)]
11pub struct Step {
12    remaining: u32,
13    children: Vec<u8>,
14}
15
16pub fn parse(input: &str) -> Input {
17    let mut steps: Input = FastMap::new();
18
19    for line in input.lines().map(str::as_bytes) {
20        // Each step is a single uppercase letter.
21        let from = line[5];
22        let to = line[36];
23
24        // Add all steps that depend on this one to children vec.
25        steps.entry(from).or_default().children.push(to);
26
27        // Count how many steps must finish before this step is ready.
28        // We only need the total count, the exact steps are not necessary.
29        steps.entry(to).or_default().remaining += 1;
30    }
31
32    steps
33}
34
35pub fn part1(input: &Input) -> String {
36    // Move all steps with no dependencies to the `ready` map. A `BTreeMap` is sorted by key
37    // so will retrieve steps in alphabetical order.
38    let mut ready = BTreeMap::new();
39    let mut blocked = FastMap::new();
40
41    for (key, step) in input.clone() {
42        if step.remaining == 0 {
43            ready.insert(key, step);
44        } else {
45            blocked.insert(key, step);
46        }
47    }
48
49    let mut done = String::new();
50
51    while let Some((key, step)) = ready.pop_first() {
52        // Keep track of the order of completed tasks.
53        done.push(key as char);
54
55        // For each dependent step, decrease the remaining count by one. Once a step reaches zero
56        // then all its dependencies have been completed and we can move it to the `ready` map.
57        for key in step.children {
58            let mut step = blocked.remove(&key).unwrap();
59            step.remaining -= 1;
60
61            if step.remaining == 0 {
62                ready.insert(key, step);
63            } else {
64                blocked.insert(key, step);
65            }
66        }
67    }
68
69    done
70}
71
72pub fn part2(input: &Input) -> u32 {
73    part2_testable(input, 5, 60)
74}
75
76pub fn part2_testable(input: &Input, max_workers: usize, base_duration: u32) -> u32 {
77    // Same as part one, move all tasks that are root nodes to the `ready` map.
78    let mut ready = BTreeMap::new();
79    let mut blocked = FastMap::new();
80
81    for (key, step) in input.clone() {
82        if step.remaining == 0 {
83            ready.insert(key, step);
84        } else {
85            blocked.insert(key, step);
86        }
87    }
88
89    // Loop until there are no more steps available and all workers are idle.
90    let mut time = 0;
91    let mut workers = Vec::new();
92
93    while !ready.is_empty() || !workers.is_empty() {
94        // Assign any steps to available workers until one or the other runs out first.
95        while !ready.is_empty() && workers.len() < max_workers {
96            let (key, step) = ready.pop_first().unwrap();
97            let finish = time + base_duration + (key - 64) as u32;
98
99            // Sort workers in reverse order, so that the worker that will finish first is at
100            // the end of the vec.
101            workers.push((finish, step));
102            workers.sort_unstable_by_key(|(time, _)| u32::MAX - time);
103        }
104
105        // Fast forward time until the earliest available worker finishes their step.
106        // This may not unblock a dependent step right away in which case the outer loop will
107        // bring things back here for another worker to complete.
108        let (finish, step) = workers.pop().unwrap();
109        time = finish;
110
111        // Update dependent tasks the same as part one.
112        for key in step.children {
113            let mut step = blocked.remove(&key).unwrap();
114            step.remaining -= 1;
115
116            if step.remaining == 0 {
117                ready.insert(key, step);
118            } else {
119                blocked.insert(key, step);
120            }
121        }
122    }
123
124    time
125}
aoc/year2018/day07.rs

aoc/year2018/
day07.rs
