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Rearrange left/right DCB tail integral evaluation for numerical stability when n1 or n2 is very close to 1 #1253

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Rearrange left/right DCB tail integral evaluation for numerical stability when n1 or n2 is very close to 1 #1253

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12 changes: 6 additions & 6 deletions src/RooDoubleCBFast.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -138,15 +138,15 @@
double n1invalpha1 = n1*my_inv(fabs(alpha1));
if(fabs(n1-1.0)>1.e-5) {
double invn1m1 = my_inv(n1-1.);
double leftpow = gbrmath::fast_pow(n1invalpha1,-n1*invn1m1);
double leftpow = gbrmath::fast_pow(n1invalpha1, n1);
double left0 = width*my_exp(-0.5*alpha1*alpha1)*invn1m1;
double left1, left2;

if (xmax>(mean-alpha1*width)) left1 = n1invalpha1;
else left1 = gbrmath::fast_pow( leftpow*(n1invalpha1 - alpha1 - thigh), 1.-n1);
else left1 = leftpow * gbrmath::fast_pow(n1invalpha1 - alpha1 - thigh, 1. - n1);

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if (tmin<-1000.) left2 = 0.;
else left2 = gbrmath::fast_pow( leftpow*(n1invalpha1 - alpha1 - tmin ), 1.-n1);
else left2 = leftpow * gbrmath::fast_pow(n1invalpha1 - alpha1 - tmin, 1. - n1);

left = left0*(left1-left2);

Expand Down Expand Up @@ -175,15 +175,15 @@
double n2invalpha2 = n2*my_inv(fabs(alpha2));
if(fabs(n2-1.0)>1.e-5) {
double invn2m2 = my_inv(n2-1.);
double rightpow = gbrmath::fast_pow(n2invalpha2,-n2*invn2m2);
double rightpow = gbrmath::fast_pow(n2invalpha2, n2);
double right0 = width*my_exp(-0.5*alpha2*alpha2)*invn2m2;
double right1, right2;

if (xmin<(mean+alpha2*width)) right1 = n2invalpha2;
else right1 = gbrmath::fast_pow( rightpow*(n2invalpha2 - alpha2 + tlow), 1.-n2);
else right1 = rightpow * gbrmath::fast_pow(n2invalpha2 - alpha2 + tlow, 1. - n2);

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if (tmax>1000.) right2 = 0.;
else right2 = gbrmath::fast_pow( rightpow*(n2invalpha2 - alpha2 + tmax), 1.-n2);
else right2 = rightpow * gbrmath::fast_pow(n2invalpha2 - alpha2 + tmax, 1. - n2);

right = right0*(right1-right2);

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