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LibM: Make the gamma family of functions more accurate and conformant

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This patch makes tgamma use an approximation that is more accurate with
regards to floating point arithmetic, and fixes some issues when tgamma
was called with positive integer values.

It also makes lgamma set signgam to the correct value, and makes its
return value be more inline with what the C standard defines.
This commit is contained in:
Mițca Dumitru 2021-03-15 16:27:14 +02:00 • committed by Andreas Kling
parent 2d0f334e5d
commit 987cc904c2
2 changed files with 56 additions and 15 deletions
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27
Userland/Tests/LibM/test-math.cpp
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@ -220,4 +220,31 @@ TEST_CASE(scalbn)
EXPECT_EQ(scalbn(2.0, 4), 32.0);
}
TEST_CASE(gamma)
{
EXPECT(isinf(tgamma(+0.0)) && !signbit(tgamma(+0.0)));
EXPECT(isinf(tgamma(-0.0)) && signbit(tgamma(-0.0)));
EXPECT(isinf(tgamma(INFINITY)) && !signbit(tgamma(INFINITY)));
EXPECT(isnan(tgamma(NAN)));
EXPECT(isnan(tgamma(-INFINITY)));
EXPECT(isnan(tgamma(-5)));
EXPECT_APPROXIMATE(tgamma(0.5), sqrt(M_PI));
EXPECT_EQ(tgammal(21.0l), 2'432'902'008'176'640'000.0l);
EXPECT_EQ(tgamma(19.0), 6'402'373'705'728'000.0);
EXPECT_EQ(tgammaf(11.0f), 3628800.0f);
EXPECT_EQ(tgamma(4.0), 6);
EXPECT_EQ(lgamma(1.0), 0.0);
EXPECT_EQ(lgamma(2.0), 0.0);
EXPECT(isinf(lgamma(0.0)));
EXPECT(!signbit(lgamma(-0.0)));
EXPECT(isnan(lgamma(NAN)));
EXPECT(isinf(lgamma(INFINITY)));
EXPECT(isinf(lgamma(-INFINITY)));
EXPECT_EQ(signgam, 1);
lgamma(-2.5);
EXPECT_EQ(signgam, -1);
}
TEST_MAIN(Math)