ext/gmp: Add gmp_prevprime()

NEWS

@@ -6,6 +6,7 @@ PHP                                                                        NEWS
   . Implemented partial function application RFC. (Arnaud)
 
 - GMP:
+  . Added gmp_prevprime(). (Weilin Du, David Carlier)
   . Fixed GMP power and shift operators to reject GMP right operands outside
     the unsigned long range instead of silently truncating them. (Weilin Du)
   . Fixed GMP integer string parsing to reject strings containing NUL bytes

UPGRADING

@@ -260,6 +260,11 @@ PHP 8.6 UPGRADE NOTES
 - Fileinfo:
   . finfo_file() now works with remote streams.
 
+- GMP:
+  . Added gmp_prevprime() to get the largest prime smaller than the given
+    number. A ValueError is thrown if no such prime exists. Requires GNU MP
+    6.3.0 or later.
+
 - Intl:
   . Added Locale::getDisplayKeyword() and Locale::getDisplayKeywordValue(),
     with the alias of locale_get_display_keyword() and
@@ -401,6 +406,9 @@ PHP 8.6 UPGRADE NOTES
 6. New Functions
 ========================================
 
+- GMP:
+  . gmp_prevprime()
+
 - Intl:
   . grapheme_strrev()
     RFC: https://wiki.php.net/rfc/grapheme_strrev

ext/gmp/config.m4

@@ -22,6 +22,7 @@ if test "$PHP_GMP" != "no"; then
   LIBS="$LIBS $GMP_LIBS"
   gmp_check=no
   AC_CHECK_HEADER([gmp.h], [AC_CHECK_FUNC([__gmpz_rootrem], [gmp_check=yes])])
+  AC_CHECK_FUNCS([__gmpz_prevprime])
   CFLAGS=$CFLAGS_SAVED
   LIBS=$LIBS_SAVED
 

ext/gmp/config.w32

@@ -5,6 +5,9 @@ ARG_WITH("gmp", "Include GNU MP support.", "no");
 if (PHP_GMP != "no") {
 	if (CHECK_LIB("mpir_a.lib", "gmp", PHP_GMP) &&
 		CHECK_HEADER("gmp.h", "CFLAGS_GMP", PHP_GMP +  ";" + PHP_PHP_BUILD + "\\include\\mpir")) {
+		if (GREP_HEADER("gmp.h", "mpz_prevprime", PHP_GMP +  ";" + PHP_PHP_BUILD + "\\include\\mpir")) {
+			AC_DEFINE('HAVE___GMPZ_PREVPRIME', 1, "Define to 1 if GMP has the 'mpz_prevprime' function.");
+		}
 		EXTENSION("gmp", "gmp.c", null, "/DZEND_ENABLE_STATIC_TSRMLS_CACHE=1");
 		PHP_INSTALL_HEADERS("ext/gmp", "php_gmp_int.h");
 		AC_DEFINE('HAVE_GMP', 1, "Define to 1 if the PHP extension 'gmp' is available.");

ext/gmp/gmp.c

@@ -1082,6 +1082,37 @@ GMP_UNARY_OP_FUNCTION(com);
 /* {{{ Finds next prime of a */
 GMP_UNARY_OP_FUNCTION(nextprime);
 
+#ifdef HAVE___GMPZ_PREVPRIME
+/* {{{ Finds previous prime of a */
+ZEND_FUNCTION(gmp_prevprime)
+{
+	mpz_ptr gmpnum_a, gmpnum_result;
+	int res;
+
+	ZEND_PARSE_PARAMETERS_START(1, 1)
+		GMP_Z_PARAM_INTO_MPZ_PTR(gmpnum_a)
+	ZEND_PARSE_PARAMETERS_END();
+
+	if (mpz_cmp_ui(gmpnum_a, 2) <= 0) {
+		/*
+		 * mpz_prevprime() returns 0 when no previous prime exists, which happens
+		 * for operands not greater than 2.
+		 * https://gmplib.org/manual/Number-Theoretic-Functions#index-mpz_005fprevprime
+		 * However. since returning a prime number smaller than 2 is mathemeticaly
+		 * impossible, throwing a ValueError instead is a more widely used solution
+		 * throughout the php code base.
+		 */
+		zend_argument_value_error(1, "must be greater than 2");
+		RETURN_THROWS();
+	}
+
+	INIT_GMP_RETVAL(gmpnum_result);
+	res = mpz_prevprime(gmpnum_result, gmpnum_a);
+	ZEND_ASSERT(res);
+}
+/* }}} */
+#endif
+
 /* Add a and b */
 GMP_BINARY_OP_FUNCTION(add);
 /* Subtract b from a */

ext/gmp/gmp.stub.php

@@ -183,4 +183,8 @@ function gmp_hamdist(GMP|int|string $num1, GMP|int|string $num2): int {}
 
 function gmp_nextprime(GMP|int|string $num): GMP {}
 
+#ifdef HAVE___GMPZ_PREVPRIME
+function gmp_prevprime(GMP|int|string $num): GMP {}
+#endif
+
 function gmp_binomial(GMP|int|string $n, int $k): GMP {}

ext/gmp/tests/bug80560.phpt

@@ -24,6 +24,9 @@ $functions1 = [
     'gmp_com',
     'gmp_nextprime',
 ];
+if (function_exists('gmp_prevprime')) {
+    $functions1[] = 'gmp_prevprime';
+}
 $functions1_need_int_2 = [
     'gmp_testbit',
     'gmp_scan0',

ext/gmp/tests/gh13661.phpt

@@ -0,0 +1,34 @@
+--TEST--
+gmp_prevprime()
+--EXTENSIONS--
+gmp
+--SKIPIF--
+<?php
+if (!function_exists('gmp_prevprime')) {
+    die('skip gmp_prevprime() is not available');
+}
+?>
+--FILE--
+<?php
+
+foreach ([-1, 0, 1, 2] as $value) {
+    try {
+        var_dump(gmp_prevprime($value));
+    } catch (\ValueError $e) {
+        echo $e->getMessage() . \PHP_EOL;
+    }
+}
+
+var_dump(gmp_strval(gmp_prevprime(3)));
+var_dump(gmp_strval(gmp_prevprime(4)));
+var_dump(gmp_strval(gmp_prevprime(10000)));
+
+?>
+--EXPECT--
+gmp_prevprime(): Argument #1 ($num) must be greater than 2
+gmp_prevprime(): Argument #1 ($num) must be greater than 2
+gmp_prevprime(): Argument #1 ($num) must be greater than 2
+gmp_prevprime(): Argument #1 ($num) must be greater than 2
+string(1) "2"
+string(1) "3"
+string(4) "9973"