[libc] Add isqrt() function to find integer square roots

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/isqrt.c

@@ -0,0 +1,52 @@
+/*
+ * Copyright (C) 2014 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+/** @file
+ *
+ * Integer square root
+ *
+ */
+
+#include <ipxe/isqrt.h>
+
+/**
+ * Find integer square root
+ *
+ * @v value		Value
+ * @v isqrt		Integer square root of value
+ */
+unsigned long isqrt ( unsigned long value ) {
+	unsigned long result = 0;
+	unsigned long bit = ( 1UL << ( ( 8 * sizeof ( bit ) ) - 2 ) );
+
+	while ( bit > value )
+		bit >>= 2;
+	while ( bit ) {
+		if ( value >= ( result + bit ) ) {
+			value -= ( result + bit );
+			result = ( ( result >> 1 ) + bit );
+		} else {
+			result >>= 1;
+		}
+		bit >>= 2;
+	}
+	return result;
+}

src/include/ipxe/isqrt.h

@@ -0,0 +1,14 @@
+#ifndef _IPXE_ISQRT_H
+#define _IPXE_ISQRT_H
+
+/** @file
+ *
+ * Integer square root
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+extern unsigned long isqrt ( unsigned long value );
+
+#endif /* _IPXE_ISQRT_H */

src/tests/math_test.c

@@ -32,6 +32,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #include <strings.h>
 #include <assert.h>
 #include <ipxe/test.h>
+#include <ipxe/isqrt.h>
 
 /**
  * Force a call to the non-constant implementation of flsl()
@@ -269,6 +270,16 @@ static void math_test_exec ( void ) {
 		       -1LL, -0x9cb7d9d78556fb9LL );
 	u64divmod_ok ( 0, 0x5b2f2737f4ffULL, 0, 0 );
 	s64divmod_ok ( 0, 0xbb00ded72766207fULL, 0, 0 );
+
+	/* Test integer square root */
+	ok ( isqrt ( 0 ) == 0 );
+	ok ( isqrt ( 1 ) == 1 );
+	ok ( isqrt ( 255 ) == 15 );
+	ok ( isqrt ( 256 ) == 16 );
+	ok ( isqrt ( 257 ) == 16 );
+	ok ( isqrt ( 0xa53df2adUL ) == 52652 );
+	ok ( isqrt ( 0x123793c6UL ) == 17482 );
+	ok ( isqrt ( -1UL ) == ( -1UL >> ( 8 * sizeof ( unsigned long ) / 2 )));
 }
 
 /** Mathematical self-tests */