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

[Rolf Eike Beer]

The previous algorithm would scan for all primes for a given number, which
takes needlessly long.

Signed-off-by: Rolf Eike Beer <eike@×××××××.de>
---
 eclass/qmail.eclass   | 51 +++++++++++++++---------------------------
 eclass/tests/qmail.sh | 52 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 70 insertions(+), 33 deletions(-)
 create mode 100755 eclass/tests/qmail.sh

diff --git a/eclass/qmail.eclass b/eclass/qmail.eclass
index 40130a502cb..9f644dd74c8 100644
--- a/eclass/qmail.eclass
+++ b/eclass/qmail.eclass
@@ -29,46 +29,31 @@ GENQMAIL_S="${WORKDIR}"/genqmail-${GENQMAIL_PV}
 QMAIL_SPP_F=qmail-spp-${QMAIL_SPP_PV}.tar.gz
 QMAIL_SPP_S="${WORKDIR}"/qmail-spp-${QMAIL_SPP_PV}
 
-# @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} -lt 7 ]]; then
+		# too small
+		return 1
+	fi
 
-	[[ ${min} -le 2 ]] && result="${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 ${result}
-}
-
-# @FUNCTION: is_prima
-# @USAGE: <number>
-# @DESCRIPTION:
-# Checks wether a number is a prime number
-is_prime() {
-	local number=${1} i
-	for i in $(primes ${number} ${number})
-	do
-		[[ ${i} == ${number} ]] && return 0
-	done
-	return 1
+	return 0
 }
 
 dospp() {
diff --git a/eclass/tests/qmail.sh b/eclass/tests/qmail.sh
new file mode 100755
index 00000000000..3520ed2a9d5
--- /dev/null
+++ b/eclass/tests/qmail.sh
@@ -0,0 +1,52 @@
+#!/bin/bash
+# Copyright 2020-2021 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+EAPI=8
+source tests-common.sh
+
+inherit qmail
+
+# some numbers are blocked because they are to small even if prime
+test_low_numbers() {
+	tbegin "low numbers"
+
+	for i in $(seq 0 6); do
+		if is_prime ${i}; then
+			return tend 1 "${i} badly accepted"
+		fi
+	done
+
+	tend 0
+}
+
+# test a given number for being prime
+check_prime_number() {
+	# use factor from coreutils to count the factors
+	if [[ $(factor $1 | cut -d: -f2 | wc -w) == 1 ]]; then
+		return $(is_prime $1)
+	else
+		return $(is_prime $1 && false || true)
+	fi
+}
+
+test_primes() {
+	tbegin "factorizations from ${1} to ${2}"
+
+	for i in $(seq ${1:?} ${2:?}); do
+		if ! check_prime_number $i; then
+			tend 1 "${i} returned bad factorization"
+			return 1
+		fi
+	done
+
+	tend 0
+}
+
+test_low_numbers
+test_primes 7 99
+for i in $(seq 100 100 1000); do
+	test_primes $i $((i + 99))
+done
+
+texit
-- 
2.26.2