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# PatrickMcSweeny's solution

## to Grains in the Rust Track

Published at Jul 19 2019 · 0 comments
Instructions
Test suite
Solution

#### Note:

This exercise has changed since this solution was written.

Calculate the number of grains of wheat on a chessboard given that the number on each square doubles.

There once was a wise servant who saved the life of a prince. The king promised to pay whatever the servant could dream up. Knowing that the king loved chess, the servant told the king he would like to have grains of wheat. One grain on the first square of a chess board, with the number of grains doubling on each successive square.

There are 64 squares on a chessboard (where square 1 has one grain, square 2 has two grains, and so on).

Write code that shows:

• how many grains were on a given square, and
• the total number of grains on the chessboard

## For bonus points

Did you get the tests passing and the code clean? If you want to, these are some additional things you could try:

• Optimize for speed.

Then please share your thoughts in a comment on the submission. Did this experiment make the code better? Worse? Did you learn anything from it?

## 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 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
``````

## 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

JavaRanch Cattle Drive, exercise 6 http://www.javaranch.com/grains.jsp

## Submitting Incomplete Solutions

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

### grains.rs

``````use grains;

fn process_square_case(input: u32, expected: u64) {
assert_eq!(grains::square(input), expected);
}

#[test]
/// 1
fn test_1() {
process_square_case(1, 1);
}

#[test]
#[ignore]
/// 2
fn test_2() {
process_square_case(2, 2);
}

#[test]
#[ignore]
/// 3
fn test_3() {
process_square_case(3, 4);
}

#[test]
#[ignore]
/// 4
fn test_4() {
process_square_case(4, 8);
}

//NEW
#[test]
#[ignore]
/// 16
fn test_16() {
process_square_case(16, 32_768);
}

#[test]
#[ignore]
/// 32
fn test_32() {
process_square_case(32, 2_147_483_648);
}

#[test]
#[ignore]
/// 64
fn test_64() {
process_square_case(64, 9_223_372_036_854_775_808);
}

#[test]
#[ignore]
#[should_panic(expected = "Square must be between 1 and 64")]
fn test_square_0_raises_an_exception() {
grains::square(0);
}

#[test]
#[ignore]
#[should_panic(expected = "Square must be between 1 and 64")]
fn test_square_greater_than_64_raises_an_exception() {
grains::square(65);
}

#[test]
#[ignore]
fn test_returns_the_total_number_of_grains_on_the_board() {
assert_eq!(grains::total(), 18_446_744_073_709_551_615);
}``````
``````const SQUARES: u32 = 64;

pub fn square(s: u32) -> u64 {
if s < 1 || s > SQUARES {
panic!("Square must be between 1 and {}", SQUARES);
}
2_u64.pow(s - 1)
}

pub fn total() -> u64 {
(1..=SQUARES).map(|number| square(number)).sum()
}``````