Feature #4897 ยป tau.patch
ext/bigdecimal/lib/bigdecimal/math.rb  

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# sin (x, prec) 
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# cos (x, prec) 
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# atan(x, prec) Note: x<1, x=0.9999 may not converge. 
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# TAU (prec) 

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# PI (prec) 
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# E (prec) == exp(1.0,prec) 
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# 
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y 
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end 
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# See http://tauday.com/ 

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def TAU(prec) 

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raise ArgumentError, "Zero or negative argument for TAU" if prec <= 0 

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PI(prec)*BigDecimal("2") 

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end 

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# Computes the value of pi to the specified number of digits of precision. 
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def PI(prec) 
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raise ArgumentError, "Zero or negative argument for PI" if prec <= 0 
math.c  

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#ifdef M_PI 
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rb_define_const(rb_mMath, "PI", DBL2NUM(M_PI)); 
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rb_define_const(rb_mMath, "TAU", DBL2NUM(M_PI*2.0)); 

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#else 
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rb_define_const(rb_mMath, "PI", DBL2NUM(atan(1.0)*4.0)); 
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rb_define_const(rb_mMath, "TAU", DBL2NUM(atan(1.0)*8.0)); 

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#endif 
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#ifdef M_E 