lib: add mean and variance module.

This module provides a fast 64bit implementation of basic statistics
functions, including mean, variance and standard deviation in both
weighted and unweighted variants, the unweighted variant has a 32bit
limitation per sample to prevent overflow when squaring.

Signed-off-by: Daniel Hill <daniel@gluo.nz>

6 files changed, 400 insertions, 0 deletions

diff --git a/MAINTAINERS b/MAINTAINERS
index 64379c699903..5dd800e78290 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -12311,6 +12311,15 @@ F:	Documentation/devicetree/bindings/net/ieee802154/mcr20a.txt
 F:	drivers/net/ieee802154/mcr20a.c
 F:	drivers/net/ieee802154/mcr20a.h
 
+MEAN AND VARIANCE LIBRARY
+M:	Daniel B. Hill <daniel@gluo.nz>
+M:	Kent Overstreet <kent.overstreet@linux.dev>
+S:	Maintained
+T:	git https://github.com/YellowOnion/linux/
+F:	lib/math/mean_and_variance.c
+F:	lib/math/mean_and_variance_test.c
+F:	include/linux/mean_and_variance.h
+
 MEASUREMENT COMPUTING CIO-DAC IIO DRIVER
 M:	William Breathitt Gray <vilhelm.gray@gmail.com>
 L:	linux-iio@vger.kernel.org
diff --git a/include/linux/mean_and_variance.h b/include/linux/mean_and_variance.h
new file mode 100644
index 000000000000..0a64c1be9d20
--- /dev/null
+++ b/include/linux/mean_and_variance.h
@@ -0,0 +1,37 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef STATS_H_
+#define STATS_H_
+
+#include <linux/types.h>
+
+#define SQRT_U64_MAX 4294967295ULL
+
+struct mean_and_variance {
+	s64 n;
+	s64 sum;
+	u64 sum_squares;
+};
+
+/* expontentially weighted variant */
+struct mean_and_variance_weighted {
+	bool init;
+	u8 w;
+	s64 mean;
+	u64 variance;
+};
+
+#ifdef CONFIG_MEAN_AND_VARIANCE_UNIT_TEST
+s64 fast_divpow2(s64 n, u8 d);
+#endif
+
+struct mean_and_variance mean_and_variance_update(struct mean_and_variance s1, s64 v1);
+       s64		 mean_and_variance_get_mean(struct mean_and_variance s);
+       u64		 mean_and_variance_get_variance(struct mean_and_variance s1);
+       u32		 mean_and_variance_get_stddev(struct mean_and_variance s);
+
+struct mean_and_variance_weighted mean_and_variance_weighted_update(struct mean_and_variance_weighted s1, s64 v1);
+       s64			  mean_and_variance_weighted_get_mean(struct mean_and_variance_weighted s);
+       u64			  mean_and_variance_weighted_get_variance(struct mean_and_variance_weighted s);
+       u32			  mean_and_variance_weighted_get_stddev(struct mean_and_variance_weighted s);
+
+#endif // STATS_H_
diff --git a/lib/math/Kconfig b/lib/math/Kconfig
index 0634b428d0cb..12fe28622e82 100644
--- a/lib/math/Kconfig
+++ b/lib/math/Kconfig
@@ -15,3 +15,11 @@ config PRIME_NUMBERS
 
 config RATIONAL
 	tristate
+
+config MEAN_AND_VARIANCE
+	tristate
+
+config MEAN_AND_VARIANCE_UNIT_TEST
+	tristate "mean_and_variance unit tests" if !KUNIT_ALL_TESTS
+	depends on MEAN_AND_VARIANCE && KUNIT
+	default KUNIT_ALL_TESTS
diff --git a/lib/math/Makefile b/lib/math/Makefile
index bfac26ddfc22..2ef1487e01c2 100644
--- a/lib/math/Makefile
+++ b/lib/math/Makefile
@@ -4,6 +4,8 @@ obj-y += div64.o gcd.o lcm.o int_pow.o int_sqrt.o reciprocal_div.o
 obj-$(CONFIG_CORDIC)		+= cordic.o
 obj-$(CONFIG_PRIME_NUMBERS)	+= prime_numbers.o
 obj-$(CONFIG_RATIONAL)		+= rational.o
+obj-$(CONFIG_MEAN_AND_VARIANCE) += mean_and_variance.o
 
 obj-$(CONFIG_TEST_DIV64)	+= test_div64.o
 obj-$(CONFIG_RATIONAL_KUNIT_TEST) += rational-test.o
+obj-$(CONFIG_MEAN_AND_VARIANCE_UNIT_TEST)   += mean_and_variance_test.o
diff --git a/lib/math/mean_and_variance.c b/lib/math/mean_and_variance.c
new file mode 100644
index 000000000000..f15aefc0b4d0
--- /dev/null
+++ b/lib/math/mean_and_variance.c
@@ -0,0 +1,183 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Functions for incremental mean and variance.
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 as published by
+ * the Free Software Foundation.
+ *
+ * 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.
+ *
+ * Copyright © 2022 Daniel B. Hill
+ *
+ * Author: Daniel B. Hill <daniel@gluo.nz>
+ *
+ * Description:
+ *
+ * This is includes some incremental algorithms for mean and variance calculation
+ *
+ * Derived from the paper: https://fanf2.user.srcf.net/hermes/doc/antiforgery/stats.pdf
+ *
+ * Create a struct and if it's the weighted variant set the w field (weight = 2^k).
+ *
+ * Use mean_and_variance[_weighted]_update() on the struct to update it's state.
+ *
+ * Use the mean_and_variance[_weighted]_get_* functions to calculate the mean and variance, some computation
+ * is deferred to these functions for performance reasons.
+ *
+ * see lib/math/mean_and_variance_test.c for examples of usage.
+ *
+ * DO NOT access the mean and variance fields of the weighted variants directly.
+ * DO NOT change the weight after calling update.
+ */
+
+#include <linux/bug.h>
+#include <linux/compiler.h>
+#include <linux/export.h>
+#include <linux/limits.h>
+#include <linux/math.h>
+#include <linux/math64.h>
+#include <linux/mean_and_variance.h>
+#include <linux/module.h>
+#include <linux/printbuf.h>
+
+
+/**
+ * fast_divpow2() - fast approximation for n / (1 << d)
+ * @n: numerator
+ * @d: the power of 2 denominator.
+ *
+ * note: this rounds towards 0.
+ */
+inline s64 fast_divpow2(s64 n, u8 d)
+{
+	return (n + ((n < 0) ? ((1 << d) - 1) : 0)) >> d; // + (n < 0 ? 1 : 0);
+}
+
+/**
+ * mean_and_variance_update() - update a mean_and_variance struct @s1 with a new sample @v1
+ * and return it.
+ * @s1: the mean_and_variance to update.
+ * @v1: the new sample.
+ *
+ * see linked pdf equation 12.
+ */
+struct mean_and_variance mean_and_variance_update(struct mean_and_variance s1, s64 v1)
+{
+	struct mean_and_variance s2;
+	u64 v2 = abs(v1);
+
+	if (v2 > SQRT_U64_MAX) {
+		v2 = SQRT_U64_MAX;
+		WARN(true, "stats overflow! %lld^2 > U64_MAX", v1);
+	}
+
+	s2.n           = s1.n + 1;
+	s2.sum         = s1.sum + v1;
+	s2.sum_squares = s1.sum_squares + v2*v2;
+	return s2;
+}
+EXPORT_SYMBOL_GPL(mean_and_variance_update);
+
+/**
+ * mean_and_variance_get_mean() - get mean from @s
+ */
+s64 mean_and_variance_get_mean(struct mean_and_variance s)
+{
+	return div64_u64(s.sum, s.n);
+}
+EXPORT_SYMBOL_GPL(mean_and_variance_get_mean);
+
+/**
+ * mean_and_variance_get_variance() -  get variance from @s1
+ *
+ * see linked pdf equation 12.
+ */
+u64 mean_and_variance_get_variance(struct mean_and_variance s1)
+{
+	u64 s2 = s1.sum_squares / s1.n;
+	u64 s3 = abs(mean_and_variance_get_mean(s1));
+
+	WARN(s3 > SQRT_U64_MAX, "stats overflow %lld ^2 > S64_MAX", s3);
+	return s2 - s3*s3;
+}
+EXPORT_SYMBOL_GPL(mean_and_variance_get_variance);
+
+/**
+ * mean_and_variance_get_stddev() - get standard deviation from @s
+ */
+u32 mean_and_variance_get_stddev(struct mean_and_variance s)
+{
+	return int_sqrt64(mean_and_variance_get_variance(s));
+}
+EXPORT_SYMBOL_GPL(mean_and_variance_get_stddev);
+
+/**
+ * mean_and_variance_weighted_update() - exponentially weighted variant of mean_and_variance_update()
+ * @s1: ..
+ * @s2: ..
+ *
+ * see linked pdf: function derived from equations 140-143 where alpha = 2^w.
+ * values are stored bitshifted for performance and added precision.
+ */
+struct mean_and_variance_weighted mean_and_variance_weighted_update(struct mean_and_variance_weighted s1, s64 v1)
+{
+	struct mean_and_variance_weighted s2;
+	s64 m = s1.mean;
+	u64 var = s1.variance;
+	u8 w = s2.w = s1.w;
+	s64 v2 = v1 << w;
+	s64 d1 = (v2 - m);
+	s64 d2 = fast_divpow2(d1, w);
+	u64 d3 = (d1*d1) >> w;
+
+	if (!s1.init) {
+		s2.mean = v2;
+		s2.variance = 0;
+	} else {
+		s2.mean = m + d2;
+		s2.variance = var + ((d3 - (d3 >> w) - var) >> w);
+	}
+	s2.init = true;
+
+	#ifdef CONFIG_STATS_UNIT_TEST
+	pr_debug("v1 = %lld, v2 = %lld, d1 = %lld, d2 = %lld, d3 = %llu, m = %lld, var = %llu",
+		   v1, v2, d1, d2, d3, s2.mean, s2.variance);
+	#endif
+	return s2;
+}
+EXPORT_SYMBOL_GPL(mean_and_variance_weighted_update);
+
+/**
+ * mean_and_variance_weighted_get_mean() - get mean from @s
+ */
+s64 mean_and_variance_weighted_get_mean(struct mean_and_variance_weighted s)
+{
+	return fast_divpow2(s.mean, s.w);
+}
+EXPORT_SYMBOL_GPL(mean_and_variance_weighted_get_mean);
+
+/**
+ * mean_and_variance_weighted_get_variance() -- get variance from @s
+ */
+u64 mean_and_variance_weighted_get_variance(struct mean_and_variance_weighted s)
+{
+	// always positive don't need fast divpow2
+	return s.variance >> s.w;
+}
+EXPORT_SYMBOL_GPL(mean_and_variance_weighted_get_variance);
+
+/**
+ * mean_and_variance_weighted_get_stddev() - get standard deviation from @s
+ */
+u32 mean_and_variance_weighted_get_stddev(struct mean_and_variance_weighted s)
+{
+	return int_sqrt64(mean_and_variance_weighted_get_variance(s));
+}
+EXPORT_SYMBOL_GPL(mean_and_variance_weighted_get_stddev);
+
+MODULE_AUTHOR("Daniel B. Hill");
+MODULE_LICENSE("GPL");
diff --git a/lib/math/mean_and_variance_test.c b/lib/math/mean_and_variance_test.c
new file mode 100644
index 000000000000..215e128acd86
--- /dev/null
+++ b/lib/math/mean_and_variance_test.c
@@ -0,0 +1,161 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <kunit/test.h>
+#include <linux/mean_and_variance.h>
+
+#define MAX_SQR (SQRT_U64_MAX*SQRT_U64_MAX)
+
+static void mean_and_variance_basic_test(struct kunit *test)
+{
+	struct mean_and_variance s = {};
+
+	s = mean_and_variance_update(s, 2);
+	s = mean_and_variance_update(s, 2);
+
+	KUNIT_EXPECT_EQ(test, mean_and_variance_get_mean(s), 2);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_get_variance(s), 0);
+	KUNIT_EXPECT_EQ(test, s.n, 2);
+
+	s = mean_and_variance_update(s, 4);
+	s = mean_and_variance_update(s, 4);
+
+	KUNIT_EXPECT_EQ(test, mean_and_variance_get_mean(s), 3);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_get_variance(s), 1);
+	KUNIT_EXPECT_EQ(test, s.n, 4);
+
+	/*
+	 * Test overflow bounds
+	 */
+	s = (struct mean_and_variance){};
+
+	s = mean_and_variance_update(s, SQRT_U64_MAX);
+
+	KUNIT_EXPECT_EQ_MSG(test,
+			    s.sum_squares,
+			    MAX_SQR,
+			    "%llu == %llu, sqrt: %llu == %llu",
+			    s.sum_squares,
+			    MAX_SQR,
+			    int_sqrt64(s.sum_squares),
+			    SQRT_U64_MAX);
+
+	s = (struct mean_and_variance){};
+
+	s = mean_and_variance_update(s, -(s64)SQRT_U64_MAX);
+
+	KUNIT_EXPECT_EQ_MSG(test,
+			    s.sum_squares,
+			    MAX_SQR,
+			    "%llu == %llu, sqrt: %llu == %llu",
+			    s.sum_squares,
+			    MAX_SQR,
+			    int_sqrt64(s.sum_squares),
+			    SQRT_U64_MAX);
+
+	s = (struct mean_and_variance){};
+
+	s = mean_and_variance_update(s, (SQRT_U64_MAX + 1));
+
+	KUNIT_EXPECT_EQ(test, s.sum_squares, MAX_SQR);
+
+	s = (struct mean_and_variance){};
+
+	s = mean_and_variance_update(s, (-(s64)SQRT_U64_MAX) - 1);
+
+	KUNIT_EXPECT_EQ(test, s.sum_squares, MAX_SQR);
+}
+
+/*
+ * Test values computed using a spreadsheet from the psuedocode at the bottom:
+ * https://fanf2.user.srcf.net/hermes/doc/antiforgery/stats.pdf
+ */
+
+static void mean_and_variance_weighted_test(struct kunit *test)
+{
+	struct mean_and_variance_weighted s = {};
+
+	s.w = 2;
+
+	s = mean_and_variance_weighted_update(s, 10);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s), 10);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s), 0);
+
+	s = mean_and_variance_weighted_update(s, 20);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s), 12);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s), 18);
+
+	s = mean_and_variance_weighted_update(s, 30);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s), 16);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s), 71);
+
+	s = (struct mean_and_variance_weighted){};
+	s.w = 2;
+
+	s = mean_and_variance_weighted_update(s, -10);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s), -10);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s), 0);
+
+	s = mean_and_variance_weighted_update(s, -20);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s), -12);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s), 18);
+
+	s = mean_and_variance_weighted_update(s, -30);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s), -16);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s), 71);
+
+}
+
+static void mean_and_variance_weighted_advanced_test(struct kunit *test)
+{
+	struct mean_and_variance_weighted s = {};
+	s64 i;
+
+	s.w = 8;
+	for (i = 10; i <= 100; i += 10)
+		s = mean_and_variance_weighted_update(s, i);
+
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s), 11);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s), 107);
+
+	s = (struct mean_and_variance_weighted){};
+
+	s.w = 8;
+	for (i = -10; i >= -100; i -= 10)
+		s = mean_and_variance_weighted_update(s, i);
+
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s), -11);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s), 107);
+
+}
+
+static void mean_and_variance_fast_divpow2(struct kunit *test)
+{
+	s64 i;
+	u8 d;
+
+	for (i = 0; i < 100; i++) {
+		d = 0;
+		KUNIT_EXPECT_EQ(test, fast_divpow2(i, d), div_u64(i, 1LLU << d));
+		KUNIT_EXPECT_EQ(test, abs(fast_divpow2(-i, d)), div_u64(i, 1LLU << d));
+		for (d = 1; d < 32; d++) {
+			KUNIT_EXPECT_EQ_MSG(test, abs(fast_divpow2(i, d)),
+					    div_u64(i, 1 << d), "%lld %u", i, d);
+			KUNIT_EXPECT_EQ_MSG(test, abs(fast_divpow2(-i, d)),
+					    div_u64(i, 1 << d), "%lld %u", -i, d);
+		}
+	}
+}
+
+static struct kunit_case mean_and_variance_test_cases[] = {
+	KUNIT_CASE(mean_and_variance_basic_test),
+	KUNIT_CASE(mean_and_variance_weighted_test),
+	KUNIT_CASE(mean_and_variance_weighted_advanced_test),
+	KUNIT_CASE(mean_and_variance_fast_divpow2),
+	{}
+};
+
+static struct kunit_suite mean_and_variance_test_suite = {
+.name = "statistics",
+.test_cases = mean_and_variance_test_cases
+};
+
+kunit_test_suite(mean_and_variance_test_suite);