From 6003243ef3622395513da90919c225a4a9cfe2a8 Mon Sep 17 00:00:00 2001
From: Pawel Dziepak <pdziepak@quarnos.org>
Date: Thu, 11 Apr 2013 04:22:22 +0200
Subject: [PATCH] util: introduce kernel utils for pseudorandom number
 generation

Currently there are two generators. The fast one is the same one the scheduler
is using. The standard one is the same algorithm libroot's rand() uses. Should
there be a need for more cryptographically PRNG MD4 or MD5 might be a good
candidates.
---
 headers/private/kernel/util/Random.h | 69 ++++++++++++++++++++++++++++
 src/system/kernel/util/Jamfile       |  1 +
 src/system/kernel/util/Random.cpp    | 54 ++++++++++++++++++++++
 3 files changed, 124 insertions(+)
 create mode 100644 headers/private/kernel/util/Random.h
 create mode 100644 src/system/kernel/util/Random.cpp

diff --git a/headers/private/kernel/util/Random.h b/headers/private/kernel/util/Random.h
new file mode 100644
index 0000000000..7ca4f05312
--- /dev/null
+++ b/headers/private/kernel/util/Random.h
@@ -0,0 +1,69 @@
+/*
+ * Copyright 2013 Haiku, Inc. All rights reserved.
+ * Distributed under the terms of the MIT License.
+ *
+ * Authors:
+ *		Paweł Dziepak, pdziepak@quarnos.org
+ */
+#ifndef KERNEL_UTIL_RANDOM_H
+#define KERNEL_UTIL_RANDOM_H
+
+
+#include <smp.h>
+#include <SupportDefs.h>
+
+
+#define MAX_FAST_RANDOM_VALUE		0x7fff
+#define MAX_RANDOM_VALUE			0x7fffffffu
+
+const int	kFastRandomShift		= 15;
+const int	kRandomShift			= 31;
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+unsigned int	fast_random_value(void);
+unsigned int	random_value(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+
+#ifdef __cplusplus
+
+template<typename T>
+T
+fast_get_random()
+{
+	size_t shift = 0;
+	T random = 0;
+	while (shift < sizeof(T) * 8) {
+		random |= (T)fast_random_value() << shift;
+		shift += kFastRandomShift;
+	}
+
+	return random;
+}
+
+
+template<typename T>
+T
+get_random()
+{
+	size_t shift = 0;
+	T random = 0;
+	while (shift < sizeof(T) * 8) {
+		random |= (T)random_value() << shift;
+		shift += kRandomShift;
+	}
+
+	return random;
+}
+
+
+#endif	// __cplusplus
+
+#endif	// KERNEL_UTIL_RANDOM_H
+
diff --git a/src/system/kernel/util/Jamfile b/src/system/kernel/util/Jamfile
index 02ce7ce690..a8f49b9d2a 100644
--- a/src/system/kernel/util/Jamfile
+++ b/src/system/kernel/util/Jamfile
@@ -14,6 +14,7 @@ KernelMergeObject kernel_util.o :
 	queue.cpp
 	ring_buffer.cpp
 	RadixBitmap.cpp
+	Random.cpp
 
 	: $(TARGET_KERNEL_PIC_CCFLAGS) -DUSING_LIBGCC
 ;
diff --git a/src/system/kernel/util/Random.cpp b/src/system/kernel/util/Random.cpp
new file mode 100644
index 0000000000..417abe7324
--- /dev/null
+++ b/src/system/kernel/util/Random.cpp
@@ -0,0 +1,54 @@
+/*
+ * Copyright 2013 Haiku, Inc. All rights reserved.
+ * Distributed under the terms of the MIT License.
+ *
+ * Authors:
+ *		Paweł Dziepak, pdziepak@quarnos.org
+ */
+
+
+#include <util/Random.h>
+
+#include <OS.h>
+
+
+static uint32	fast_last	= 0;
+static uint32	last		= 0;
+
+// In the following functions there are race conditions when many threads
+// attempt to update static variable last. However, since such conflicts
+// are non-deterministic it is not a big problem.
+
+
+// A simple linear congruential generator
+unsigned int
+fast_random_value()
+{
+	if (fast_last == 0)
+		fast_last = system_time();
+
+	uint32 random = fast_last * 1103515245 + 12345;
+	fast_last = random;
+	return (random >> 16) & 0x7fff;
+}
+
+
+// Taken from "Random number generators: good ones are hard to find",
+// Park and Miller, Communications of the ACM, vol. 31, no. 10,
+// October 1988, p. 1195.
+unsigned int
+random_value()
+{
+	if (last == 0)
+		last = system_time();
+
+	uint32 hi = last / 127773;
+	uint32 lo = last % 127773;
+
+	int32 random = 16807 * lo - 2836 * hi;
+	if (random <= 0)
+		random += MAX_RANDOM_VALUE;
+	last = random;
+	return random % (MAX_RANDOM_VALUE + 1);
+}
+