Bit-shifting

Grains
Grains in Rust 
pub fn square(s: u32) -> u64 {
    if (s < 1) | (s > 64) {
        panic!("Square must be between 1 and 64");
    }
    1 << (s - 1)
}

pub fn total() -> u64 {
    ((1_u128 << 64) - 1) as u64
}

Instead of using math to calculate the number of grains on a square, you can simply set a bit in the correct position of a u64 or u128 value.

To understand how this works, consider just two squares that are represented in binary bits as 00.

You use the left-shift operator to set 1 at the position needed to make the correct decimal value.

  • To set the one grain on Square One you shift 1 for 0 positions to the left. So, if n is 1 for square One, you subtract n by 1 to get 0, which will not move it any positions to the left. The result is binary 01, which is decimal 1.
  • To set the two grains on Square Two you shift 1 for 1 position to the left. So, if n is 2 for square Two, you subtract n by 1 to get 1, which will move it 1 position to the left. The result is binary 10, which is decimal 2.
Square Shift Left By Binary Value Decimal Value
1 0 0001 1
2 1 0010 2
3 2 0100 4
4 3 1000 8

For total we want all of the 64 bits set to 1 to get the sum of grains on all sixty-four squares. The easy way to do this is to set the 65th bit to 1 and then subtract 1. However, we can't do this with a u64 which has only 64 bits, so we need to use a u128. To go back to our two-square example, if we can grow to three squares, then we can shift 1_u128 two positions to the left for binary 100, which is decimal 4.

Note

Note that the type of a binding can be defined by appending it to the binding name. It can optionally be appended using one or more _ separators between the value and the type. More info can be found in the Literals section of Rust by Example

By subtracting 1 we get 3, which is the total amount of grains on the two squares.

Square Binary Value Decimal Value
3 0100 4
Square Sum Binary Value Sum Decimal Value
1 0001 1
2 0011 3
28th Aug 2024 · Found it useful?

Other Approaches to Grains in Rust

Other ways our community solved this exercise
pub fn square(s: u32) -> u64 {
    // code snipped
    2u64.pow(s - 1)
}

pub fn total() -> u64 {
    (2_u128.pow(64) - 1) as u64
}
pow

Use pow to raise 2 by a specified power. 

pub fn total() -> u64 {
    u64::MAX
}
Max value

Use u64::MAX for total.