Re: [gentoo-dev] [PATCH 3/5] qmail.eclass: simplify is_prime()

[Ulrich Mueller]

>>>>> On Thu, 12 Aug 2021, Rolf Eike Beer wrote:

> -# @FUNCTION: primes
> -# @USAGE: <min> <max>
> +# @FUNCTION: is_prime
> +# @USAGE: <number>
>  # @DESCRIPTION:
> -# Prints a list of primes between min and max inclusive
> -# Note: this functions gets very slow when used with large numbers.
> -primes() {
> -	local min=${1} max=${2}
> -	local result= primelist=2 i p
> +# Checks wether a number is a valid prime number for queue split
> +is_prime() {
> +	local number=${1} i
> +
> +	if [[ ${number} < 7 ]]; then
> +		# too small
> +		return 0
> +	fi

So e.g. all numbers between 100 and 699 qualify as primes? I doubt that
this is what was intended. :)

>
> -	[[ ${min} -le 2 ]] && result="${result} 2"
> +	if [[ $[number % 2] == 0 ]]; then
> +		return 1
> +	fi
>
> -	for ((i = 3; i <= max; i += 2))
> +	# let i run up to the square root of number
> +	for ((i = 3; i * i <= number; i += 2))
>  	do
> -		for p in ${primelist}
> -		do
> -			[[ $[i % p] == 0 || $[p * p] -gt ${i} ]] && \
> -				break
> -		done
> -		if [[ $[i % p] != 0 ]]
> -		then
> -			primelist="${primelist} ${i}"
> -			[[ ${i} -ge ${min} ]] && \
> -				result="${result} ${i}"
> +		if [[ $[number % i ] == 0 ]]; then
> +			return 1
>  		fi
>  	done
>
> -	echo ${result}
> -}

This function asks for a unit test in eclass/tests/.

Ulrich