0025602: It is important for IGES and STEP file format to add check, if wrong data are read from the file

1. Functions Precision::IsIllegal(...) and Precision::IsInfinitesimal(...) were added.
2. Detection incorrect numbers read from *.stp and/or *.igs files was added.

src/Precision/Precision.cxx

@@ -16,3 +16,47 @@
 
 
 #include <Precision.hxx>
+
+//Binary representation of double numbers: expands it to MANTISSA, EXPONENT and SIGN-bit.
+struct stDBLType
+{
+  unsigned int manl:32;
+  unsigned int manh:20;
+  unsigned int exp:11;
+  unsigned int sign:1;
+};
+
+
+//=======================================================================
+//function : IsIllegal
+//purpose  : Returns True if R is infinite, NAN or another number
+//            which is not supported by OCCT.
+//            Fragment of fpclassify(...) function is used, which is taken
+//            from here: http://www.jbox.dk/sanos/source/lib/math.c.html
+//=======================================================================
+Standard_Boolean Precision::IsIllegal (const Standard_Real R)
+{
+  stDBLType* aDoubleBits = (struct stDBLType*) &R;
+  if (aDoubleBits->exp == 0)
+  {
+    if (aDoubleBits->manh != 0 || aDoubleBits->manl != 0)
+    {//_FPCLASS_ND (-DBL_MIN/2.0) or _FPCLASS_PD (+DBL_MIN/2.0)
+      return Standard_True;
+    }
+  }
+
+  if (aDoubleBits->exp == 0x7ff)
+  {//_FPCLASS_NINF (-1/0), _FPCLASS_PINF (+1/0), _FPCLASS_QNAN or _FPCLASS_SNAN
+    return Standard_True;
+  }
+
+  const Standard_Real anINf = 2.0*Infinite();
+
+  if(R >= anINf)
+    return Standard_True;
+
+  if(R <= -anINf)
+    return Standard_True;
+
+  return Standard_False;
+}