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support log1p and expm1 jet

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Currently, it is not possible to accurately evaluate the derivative of
d/dx log(1 + x) under all circumstances and significant deviations from
the actual derivative d/dx log1p(x) can occur. This changeset introduces
the necessary Jet overload and its inverse, expm1.

Change-Id: Ifcf88f6d684f61ba86bbe49f0d551b703f34ad0d
This commit is contained in:
Sergiu Deitsch
2021-11-14 17:28:30 +01:00
parent a668cabbcf
commit 2fba61434b
2 changed files with 71 additions and 0 deletions
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internal/ceres/jet_test.cc
+56
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@@ -128,6 +128,22 @@ TEST(Jet, Jet) {
ExpectJetsClose(w, x);
}
{ // Check that expm1(log1p(x)) == x.
J z = expm1(x);
J w = log1p(z);
VL << "z = " << z;
VL << "w = " << w;
ExpectJetsClose(w, x);
}
{ // Check that log1p(expm1(x)) == x.
J z = log1p(x);
J w = expm1(z);
VL << "z = " << z;
VL << "w = " << w;
ExpectJetsClose(w, x);
}
{ // Check that (x * y) / x == y.
J z = x * y;
J w = z / x;
@@ -631,6 +647,46 @@ TEST(Jet, Jet) {
NumericalTest("cbrt", cbrt<double, 2>, 1e-5);
NumericalTest("cbrt", cbrt<double, 2>, 1.0);
{ // Check that log1p(x) == log(1 + x)
J z = log1p(x);
J w = log(J{1} + x);
VL << "z = " << z;
VL << "w = " << w;
ExpectJetsClose(z, w);
}
{ // Check that log1p(x) does not loose precision for small x
J x = MakeJet(1e-16, 1e-8, 1e-4);
J z = log1p(x);
J w = MakeJet(9.9999999999999998e-17, 1e-8, 1e-4);
VL << "z = " << z;
VL << "w = " << w;
ExpectJetsClose(z, w);
// log(1 + x) collapes to 0
J v = log(J{1} + x);
EXPECT_TRUE(v.a == 0);
}
{ // Check that expm1(x) == exp(x) - 1
J z = expm1(x);
J w = exp(x) - J{1};
VL << "z = " << z;
VL << "w = " << w;
ExpectJetsClose(z, w);
}
{ // Check that expm1(x) does not loose precision for small x
J x = MakeJet(9.9999999999999998e-17, 1e-8, 1e-4);
J z = expm1(x);
J w = MakeJet(1e-16, 1e-8, 1e-4);
VL << "z = " << z;
VL << "w = " << w;
ExpectJetsClose(z, w);
// exp(x) - 1 collapes to 0
J v = exp(x) - J{1};
EXPECT_TRUE(v.a == 0);
}
{ // Check that exp2(x) == exp(x * log(2))
J z = exp2(x);
J w = exp(x * log(2.0));