linux/lib/math/tests/polynomial_kunit.c

// SPDX-License-Identifier: GPL-2.0-only

#include <kunit/test.h>
#include <linux/polynomial.h>

struct polynomial_test_param {
	const struct polynomial *poly;
	long data;
	long expected;
	const char *name;
};

/* f(x) = 5 */
static const struct polynomial poly_constant = {
	.total_divider = 1,
	.terms = {
		{0, 5, 1, 1},
	}
};

/* f(x) = 2x^2 + 3x + 5 */
static const struct polynomial poly_simple = {
	.total_divider = 1,
	.terms = {
		{2, 2, 1, 1},
		{1, 3, 1, 1},
		{0, 5, 1, 1},
	}
};

/* f(x) = -5x + 100 */
static const struct polynomial poly_negative_coef = {
	.total_divider = 1,
	.terms = {
		{1, -5, 1, 1},
		{0, 100, 1, 1},
	}
};

/* f(x) = (150x + 50) / 10 */
static const struct polynomial poly_total_divider = {
	.total_divider = 10,
	.terms = {
		{1, 150, 1, 1},
		{0,  50, 1, 1},
	}
};

/*
 * f(x) = x / 2
 * divider=2 applied once per multiply: mult_frac(coef, data, 2) = coef*data/2
 */
static const struct polynomial poly_step_divider = {
	.total_divider = 1,
	.terms = {
		{1, 1, 2, 1},
		{0, 0, 1, 1},
	}
};

/*
 * f(x) = (100/500) * x^2 = 0.2 * x^2
 * Encoded as coef=100, divider=10, divider_leftover=5:
 *   denom = 10^2 * 5 = 500
 */
static const struct polynomial poly_leftover = {
	.total_divider = 1,
	.terms = {
		{2, 100, 10, 5},
		{0,   0,  1, 1},
	}
};

/*
 * f(x) = 2x^3  (single high-degree term, no constant)
 * Used to exercise the power loop alone.
 */
static const struct polynomial poly_cubic = {
	.total_divider = 1,
	.terms = {
		{3, 2, 1, 1},
		{0, 0, 1, 1},
	}
};

/*
 * f(x) = 4x + 1  with a zero-coefficient quadratic term.
 * The deg-2 term contributes nothing regardless of input.
 */
static const struct polynomial poly_zero_coef = {
	.total_divider = 1,
	.terms = {
		{2, 0, 1, 1},
		{1, 4, 1, 1},
		{0, 1, 1, 1},
	}
};

/*
 * f(x) = 9  with total_divider = 0.
 * The implementation treats 0 as 1 via `total_divider ?: 1`, so the
 * result must equal the constant term unchanged.
 */
static const struct polynomial poly_zero_total_divider = {
	.total_divider = 0,
	.terms = {
		{0, 9, 1, 1},
	}
};


static const struct polynomial_test_param test_params[] = {
	{
		.poly     = &poly_constant,
		.data     = 0,
		.expected = 5,
		.name     = "Constant polynomial at x=0",
	},
	{
		.poly     = &poly_constant,
		.data     = 42,
		.expected = 5,
		.name     = "Constant polynomial is independent of input",
	},
	{
		.poly     = &poly_simple,
		.data     = 0,
		.expected = 5,	/* zero input collapses all power terms */
		.name     = "Zero input yields constant term only",
	},
	{
		.poly     = &poly_simple,
		.data     = 10,
		.expected = 235,	/* 2*100 + 3*10 + 5 */
		.name     = "Simple quadratic at x=10",
	},
	{
		.poly     = &poly_negative_coef,
		.data     = 10,
		.expected = 50,		/* -5*10 + 100 */
		.name     = "Negative coefficient at x=10",
	},
	{
		.poly     = &poly_negative_coef,
		.data     = 20,
		.expected = 0,		/* -5*20 + 100 = 0 */
		.name     = "Negative coefficient result is zero",
	},
	{
		.poly     = &poly_total_divider,
		.data     = 3,
		.expected = 50,		/* (150*3 + 50) / 10 = 500/10 */
		.name     = "total_divider scales the final sum",
	},
	{
		.poly     = &poly_step_divider,
		.data     = 100,
		.expected = 50,		/* 1*100/2 */
		.name     = "Per-step divider halves input",
	},
	{
		.poly     = &poly_leftover,
		.data     = 30,
		.expected = 180,	/* 100*30^2 / (10^2 * 5) = 90000/500 */
		.name     = "divider_leftover with quadratic term",
	},
	/* Boundary: unit and negative-unit input */
	{
		/*
		 * data=1: each mult_frac(tmp, 1, divider) strips one factor of
		 * divider from coef per degree, so coef is left-shifted right
		 * until intermediate precision is exhausted.
		 * 2*1 + 3*1 + 5 = 10
		 */
		.poly     = &poly_simple,
		.data     = 1,
		.expected = 10,
		.name     = "Boundary: data=1 (unit input)",
	},
	{
		/*
		 * data=-1: even degrees produce positive contributions,
		 * odd degrees produce negative ones.
		 * 2*(-1)^2 + 3*(-1) + 5 = 2 - 3 + 5 = 4
		 */
		.poly     = &poly_simple,
		.data     = -1,
		.expected = 4,
		.name     = "Boundary: data=-1 (negative unit input)",
	},

	/* Boundary: negative non-trivial input */
	{
		/*
		 * 2*(-3)^2 + 3*(-3) + 5 = 18 - 9 + 5 = 14
		 * Verifies sign handling for negative data across all degrees.
		 */
		.poly     = &poly_simple,
		.data     = -3,
		.expected = 14,
		.name     = "Boundary: negative data with quadratic",
	},

	/* Boundary: total_divider = 0 is treated as 1 */
	{
		.poly     = &poly_zero_total_divider,
		.data     = 42,
		.expected = 9,
		.name     = "Boundary: total_divider=0 defaults to 1",
	},

	/* Boundary: zero-coefficient high-degree term */
	{
		/*
		 * The deg-2 term has coef=0, so it contributes 0 regardless
		 * of data. Result: 0 + 4*10 + 1 = 41
		 */
		.poly     = &poly_zero_coef,
		.data     = 10,
		.expected = 41,
		.name     = "Boundary: zero-coefficient term is inert",
	},

	/* Boundary: single high-degree term, no constant */
	{
		/* 2 * 5^3 = 250; also verifies the loop terminates on deg-0 */
		.poly     = &poly_cubic,
		.data     = 5,
		.expected = 250,
		.name     = "Boundary: single cubic term",
	},
	{
		/* 2 * (-2)^3 = -16; odd power preserves sign of negative data */
		.poly     = &poly_cubic,
		.data     = -2,
		.expected = -16,
		.name     = "Boundary: single cubic term, negative data",
	},

};

static void get_desc(const struct polynomial_test_param *param, char *desc)
{
	strscpy(desc, param->name, KUNIT_PARAM_DESC_SIZE);
}

KUNIT_ARRAY_PARAM(polynomial, test_params, get_desc);

static void polynomial_calc_test(struct kunit *test)
{
	const struct polynomial_test_param *param = test->param_value;

	KUNIT_EXPECT_EQ(test, polynomial_calc(param->poly, param->data),
			param->expected);
}

static struct kunit_case polynomial_test_cases[] = {
	KUNIT_CASE_PARAM(polynomial_calc_test, polynomial_gen_params),
	{}
};

static struct kunit_suite polynomial_test_suite = {
	.name = "math-polynomial",
	.test_cases = polynomial_test_cases,
};

kunit_test_suites(&polynomial_test_suite);

MODULE_DESCRIPTION("math.polynomial_calc KUnit test suite");
MODULE_LICENSE("GPL");