Avatar of rcanning

rcanning's solution

to Two Bucket in the Rust Track

Published at Sep 30 2019 · 0 comments
Instructions
Test suite
Solution

Given two buckets of different size, demonstrate how to measure an exact number of liters by strategically transferring liters of fluid between the buckets.

Since this mathematical problem is fairly subject to interpretation / individual approach, the tests have been written specifically to expect one overarching solution.

To help, the tests provide you with which bucket to fill first. That means, when starting with the larger bucket full, you are NOT allowed at any point to have the smaller bucket full and the larger bucket empty (aka, the opposite starting point); that would defeat the purpose of comparing both approaches!

Your program will take as input:

  • the size of bucket one
  • the size of bucket two
  • the desired number of liters to reach
  • which bucket to fill first, either bucket one or bucket two

Your program should determine:

  • the total number of "moves" it should take to reach the desired number of liters, including the first fill
  • which bucket should end up with the desired number of liters (let's say this is bucket A) - either bucket one or bucket two
  • how many liters are left in the other bucket (bucket B)

Note: any time a change is made to either or both buckets counts as one (1) move.

Example: Bucket one can hold up to 7 liters, and bucket two can hold up to 11 liters. Let's say bucket one, at a given step, is holding 7 liters, and bucket two is holding 8 liters (7,8). If you empty bucket one and make no change to bucket two, leaving you with 0 liters and 8 liters respectively (0,8), that counts as one "move". Instead, if you had poured from bucket one into bucket two until bucket two was full, leaving you with 4 liters in bucket one and 11 liters in bucket two (4,11), that would count as only one "move" as well.

To conclude, the only valid moves are:

  • pouring from one bucket to another
  • emptying one bucket and doing nothing to the other
  • filling one bucket and doing nothing to the other

Written with <3 at Fullstack Academy by Lindsay Levine.

Rust Installation

Refer to the exercism help page for Rust installation and learning resources.

Writing the Code

Execute the tests with:

$ cargo test

All but the first test have been ignored. After you get the first test to pass, open the tests source file which is located in the tests directory and remove the #[ignore] flag from the next test and get the tests to pass again. Each separate test is a function with #[test] flag above it. Continue, until you pass every test.

If you wish to run all ignored tests without editing the tests source file, use:

$ cargo test -- --ignored

To run a specific test, for example some_test, you can use:

$ cargo test some_test

If the specific test is ignored use:

$ cargo test some_test -- --ignored

To learn more about Rust tests refer to the online test documentation

Make sure to read the Modules chapter if you haven't already, it will help you with organizing your files.

Further improvements

After you have solved the exercise, please consider using the additional utilities, described in the installation guide, to further refine your final solution.

To format your solution, inside the solution directory use

cargo fmt

To see, if your solution contains some common ineffective use cases, inside the solution directory use

cargo clippy --all-targets

Submitting the solution

Generally you should submit all files in which you implemented your solution (src/lib.rs in most cases). If you are using any external crates, please consider submitting the Cargo.toml file. This will make the review process faster and clearer.

Feedback, Issues, Pull Requests

The exercism/rust repository on GitHub is the home for all of the Rust exercises. If you have feedback about an exercise, or want to help implement new exercises, head over there and create an issue. Members of the rust track team are happy to help!

If you want to know more about Exercism, take a look at the contribution guide.

Source

Water Pouring Problem http://demonstrations.wolfram.com/WaterPouringProblem/

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

two-bucket.rs

use two_bucket::{solve, Bucket, BucketStats};

#[test]
fn test_case_1() {
    assert_eq!(
        solve(3, 5, 1, &Bucket::One),
        Some(BucketStats {
            moves: 4,
            goal_bucket: Bucket::One,
            other_bucket: 5,
        })
    );
}

#[test]
#[ignore]
fn test_case_2() {
    assert_eq!(
        solve(3, 5, 1, &Bucket::Two),
        Some(BucketStats {
            moves: 8,
            goal_bucket: Bucket::Two,
            other_bucket: 3,
        })
    );
}

#[test]
#[ignore]
fn test_case_3() {
    assert_eq!(
        solve(7, 11, 2, &Bucket::One),
        Some(BucketStats {
            moves: 14,
            goal_bucket: Bucket::One,
            other_bucket: 11,
        })
    );
}

#[test]
#[ignore]
fn test_case_4() {
    assert_eq!(
        solve(7, 11, 2, &Bucket::Two),
        Some(BucketStats {
            moves: 18,
            goal_bucket: Bucket::Two,
            other_bucket: 7,
        })
    );
}

#[test]
#[ignore]
fn goal_equal_to_start_bucket() {
    assert_eq!(
        solve(1, 3, 3, &Bucket::Two),
        Some(BucketStats {
            moves: 1,
            goal_bucket: Bucket::Two,
            other_bucket: 0,
        })
    );
}

#[test]
#[ignore]
fn goal_equal_to_other_bucket() {
    assert_eq!(
        solve(2, 3, 3, &Bucket::One),
        Some(BucketStats {
            moves: 2,
            goal_bucket: Bucket::Two,
            other_bucket: 2,
        })
    );
}

#[test]
#[ignore]
fn not_possible_to_reach_the_goal() {
    assert_eq!(
        solve(6, 15, 5, &Bucket::One),
        None
    );
}

#[test]
#[ignore]
fn with_same_buckets_but_different_goal_then_it_is_possible() {
    assert_eq!(
        solve(6, 15, 9, &Bucket::One),
        Some(BucketStats {
            moves: 10,
            goal_bucket: Bucket::Two,
            other_bucket: 0,
        })
    );
}
use std::collections::HashSet;

#[derive(Copy, Clone, PartialEq, Eq, Debug)]
pub enum Bucket {
    One,
    Two,
}

/// A struct to hold your results in.
#[derive(PartialEq, Eq, Debug)]
pub struct BucketStats {
    /// The total number of "moves" it should take to reach the desired number of liters, including
    /// the first fill.
    pub moves: u8,
    /// Which bucket should end up with the desired number of liters? (Either "one" or "two")
    pub goal_bucket: Bucket,
    /// How many liters are left in the other bucket?
    pub other_bucket: u8,
}

#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
struct State {
    bucket_levels: [u8; 2],
}

struct Puzzle {
    capacity: [u8; 2],
    goal: u8,
    start_bucket: usize,
}

impl Puzzle {
    fn first_move(&self) -> State {
        if self.start_bucket == 0 {
            State {
                bucket_levels: [self.capacity[0], 0],
            }
        } else {
            State {
                bucket_levels: [0, self.capacity[1]],
            }
        }
    }

    fn forbidden_state(&self) -> State {
        if self.start_bucket == 1 {
            State {
                bucket_levels: [self.capacity[0], 0],
            }
        } else {
            State {
                bucket_levels: [0, self.capacity[1]],
            }
        }
    }
    // Returns the index of the bucket with the goal amount, or None if goal
    // not reached yet.
    fn is_finished_state(&self, state: &State) -> Option<usize> {
        for i in 0..2 {
            if state.bucket_levels[i] == self.goal {
                return Some(i);
            }
        }
        None
    }
}

/// Solve the bucket problem
pub fn solve(
    capacity_1: u8,
    capacity_2: u8,
    goal: u8,
    start_bucket: &Bucket,
) -> Option<BucketStats> {
    let puzzle = Puzzle {
        capacity: [capacity_1, capacity_2],
        goal,
        start_bucket: match start_bucket {
            Bucket::One => 0,
            Bucket::Two => 1,
        },
    };

    solve_puzzle(puzzle)
}

fn solve_puzzle(puzzle: Puzzle) -> Option<BucketStats> {
    const BUCKETS: [Bucket; 2] = [Bucket::One, Bucket::Two];

    let initial_state = puzzle.first_move();
    let forbidden_state = puzzle.forbidden_state();

    let mut states_seen = HashSet::new();
    states_seen.insert(initial_state);

    if let Some(goal_i) = puzzle.is_finished_state(&initial_state) {
        return Some(BucketStats {
            moves: 1,
            goal_bucket: BUCKETS[goal_i],
            other_bucket: initial_state.bucket_levels[1 - goal_i],
        });
    }

    // To ensure we find the smallest number of moves, we use a
    // breadth-first search.
    let mut unprocessed_states = vec![initial_state];

    // Move count starts from 2 because the initial fill counts as
    // move #1.
    for moves in 2.. {
        if unprocessed_states.is_empty() {
            return None;
        }

        let mut next_states = Vec::new();

        // Use a macro to clean up duplicated code.
        macro_rules! process_state {
            ($s: ident) => {
                if $s != forbidden_state && !states_seen.contains(&$s) {
                    if let Some(goal_i) = puzzle.is_finished_state(&$s) {
                        return Some(BucketStats {
                            moves,
                            goal_bucket: BUCKETS[goal_i],
                            other_bucket: $s.bucket_levels[1 - goal_i],
                        });
                    }
                    states_seen.insert($s);
                    next_states.push($s);
                }
            };
        }

        for s in &unprocessed_states {
            // Move #1: Pour from bucket 'i' to bucket 'j'.
            for i in 0..2 {
                let j = 1 - i;
                if s.bucket_levels[i] > 0 && s.bucket_levels[j] < puzzle.capacity[j] {
                    let diff =
                        std::cmp::min(s.bucket_levels[i], puzzle.capacity[j] - s.bucket_levels[j]);
                    assert!(diff > 0);
                    let mut new_state = *s;
                    new_state.bucket_levels[i] -= diff;
                    new_state.bucket_levels[j] += diff;
                    process_state!(new_state);
                }
            }

            // Move #2: Empty one bucket.
            for i in 0..2 {
                if s.bucket_levels[i] > 0 {
                    let mut new_state = *s;
                    new_state.bucket_levels[i] = 0;
                    process_state!(new_state);
                }
            }

            // Move #3: Fill one bucket.
            for i in 0..2 {
                if s.bucket_levels[i] < puzzle.capacity[i] {
                    let mut new_state = *s;
                    new_state.bucket_levels[i] = puzzle.capacity[i];
                    process_state!(new_state);
                }
            }
        }

        std::mem::swap(&mut unprocessed_states, &mut next_states);
    }

    unreachable!();
}

Community comments

Find this solution interesting? Ask the author a question to learn more.

What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that you could read more about to improve your understanding?