Iterate once

const SIGNS: [&'static str; 4] = ["wink", "double blink", "close your eyes", "jump"];
const REVERSE_SIGNS: u8 = 16;

pub fn actions(n: u8) -> Vec<&'static str> {
    let (mut action, action_incr, end) = match n & REVERSE_SIGNS {
        0 => (0, 1, 4),
        _ => (3, -1, -1),
    };
    let mut output: Vec<&'static str> = Vec::new();

    loop {
        if action == end {
            break;
        }
        if (n & (1 << action)) != 0 {
            output.push(SIGNS[action as usize])
        }
        action += action_incr
    }
    output
}

This approach starts by defining a fixed-size array to hold the signal values in normal order.

The [&'static str; 4] is used to give the type and length of the array. To be a const, the size of the array must be known at compile time, so setting the type and length must be done explicitly, so the size in bytes of the fixed array can be deduced by the compiler from that and not by inspecting the element types and counting the elements itself.

The value of 16 is defined as a const with a meaningful name so it won't be used as a magic number.

The actions function uses multiple assignment with a match expression to define the variables that control iterating through the signals array, setting their values to iterate in either the normal or reverse order.

The bitwise AND operator is used to check if the input number contains the signal for reversing the order of the other signals.

For example, if the number passed in is 19, which is 10011 in binary, then it is ANDed with 16, which is 10000 in binary. The 1 in 10000 is also at the same position in 10011, so the two values ANDed will not be 0.

• 10011 AND
• 10000 =
• 10000

If the number passed in is 3, which is 00011 in binary, then it is ANDed with 16, which is 10000 in binary. The 1 in 10000 is not at the same position in 00011, so the two values ANDed will be 0.

• 00011 AND
• 10000 =
• 00000

If the number passed in does not contain the signal for reverse, then the iteration variables are set to iterate through the array of signals in their normal order, otherwise they are set to iterate through the arrray backwards..

The output vector is defined, and then the loop begins.

Normal iteration will start at index 0. Reverse iteration will start at index 3.

Normal iteration will terminate when the index equals 4. Reverse iteration will terminate when the index equals -1.

Normal iteration will increase the index by 1 for each iteration. Reverse iteration will decrease the index by 1 for each iteration.

For each iteration of the loop, the AND operator is used to check if the number passed in contains 1 shifted left (<<) for the number of positions as the value being iterated.

if (n & (1 << action)) != 0 {
    output.push(SIGNS[action as usize])
}

For example, if the number being iterated is 0, then 1 is shifted left 0 times (so not shifted at all), and the number passed in is ANDed with 00001. If the number passed in is 3, which is 00011 in binary, then it is ANDed with 00001. 00011 ANDed with 00001 is not equal to 0, so the signal at the index of the array of signals is added to the output vector. The index used is the number being iterated, which is 0, so the element at index 0 ("wink") would be added to the output vector using the push function.

If the number being iterated is 1, then 1 is shifted left 1 time, and the number passed in is ANDed with 00010. If the number passed in is 3, which is 00011 in binary, then it is ANDed with 00010. 00011 ANDed with 00010 is not equal to 0, so the signal at the index of the array of signals is added to the output vector. The index used is the number being iterated, which is 1, so the element at index 1 ("double blink") would be added to the output vector.

If the number passed in ANDed with the number being iterated is equal to 0, then the signal in the array for that index is not added to the output vector.

After iterating through the array of signals is done, the output vector is returned from the function.