Slice indexes

// Package secret is a small library for interpreting signals for a secret greeting
package secret

// Handshake interprets what sequence of signals are contained within the number passed in
func Handshake(code uint) []string {
	signs := []string{"wink", "double blink", "close your eyes", "jump"}
	const reverseSigns = 16
	action, actionIncr, end := 0, 1, len(signs)

	if (code & reverseSigns) != 0 {
		action = 3
		actionIncr = -1
		end = -1
	}
	output := []string{}

	for ; action != end; action += actionIncr {
		if (code & (1 << action)) != 0 {
			output = append(output, signs[action])
		}
	}
	return output
}

This approach starts by defining a slice of the signals in their normal order. A const is defined to represent the signal for reversing the order of the other signals. It is given a meaningful name so that 16 won't be used as a magic number.

Some variables are defined with values for iterating the slice of signals in their normal 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 contains the signal for reverse, then the iteration variables are set to iterate backwards through the slice of signals.

The output slice is defined, and then the for loop begins.

for ; action != end; action += actionIncr {

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, 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 (code & (1 << action)) != 0 {
  output = append(output, signs[action])
}

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 slice of signals is added to the output slice. The index used is the number being iterated, which is 0, so the element at index 0 ("wink") would be added to the output slice using the append 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 slice of signals is added to the output slice. 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 slice.

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

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