SF bug #444510: int() should guarantee truncation.

tim-one · tim-one · commit 7321ec437b0a · 2001-07-26T20:02:17.000Z
It's guaranteed now, assuming the platform modf() works correctly.
diff --git a/Doc/lib/libfuncs.tex b/Doc/lib/libfuncs.tex
@@ -338,9 +338,7 @@ \section{Built-in Functions \label{built-in-funcs}}
   \exception{TypeError} is raised.
   Otherwise, the argument may be a plain or
   long integer or a floating point number.  Conversion of floating
-  point numbers to integers is defined by the C semantics; normally
-  the conversion truncates towards zero.\footnote{This is ugly --- the
-  language definition should require truncation towards zero.}
+  point numbers to integers truncates (towards zero).
 \end{funcdesc}
 
 \begin{funcdesc}{intern}{string}
diff --git a/Lib/test/test_b1.py b/Lib/test/test_b1.py
@@ -366,6 +366,19 @@ def f(): pass
     pass
 else:
     raise TestFailed, "int(%s)" % `s[1:]` + " should raise ValueError"
+try:
+    int(1e100)
+except OverflowError:
+    pass
+else:
+    raise TestFailed("int(1e100) expected OverflowError")
+try:
+    int(-1e100)
+except OverflowError:
+    pass
+else:
+    raise TestFailed("int(-1e100) expected OverflowError")
+
 
 # SF bug 434186:  0x80000000/2 != 0x80000000>>1.
 # Worked by accident in Windows release build, but failed in debug build.
diff --git a/Objects/floatobject.c b/Objects/floatobject.c
@@ -606,13 +606,19 @@ static PyObject *
 float_int(PyObject *v)
 {
 	double x = PyFloat_AsDouble(v);
-	if (x < 0 ? (x = ceil(x)) < (double)LONG_MIN
-	          : (x = floor(x)) > (double)LONG_MAX) {
-		PyErr_SetString(PyExc_OverflowError,
-				"float too large to convert");
-		return NULL;
-	}
-	return PyInt_FromLong((long)x);
+	double wholepart;	/* integral portion of x, rounded toward 0 */
+	long aslong;		/* (long)wholepart */
+
+	(void)modf(x, &wholepart);
+	/* doubles may have more bits than longs, or vice versa; and casting
+	   to long may yield gibberish in either case.  What really matters
+	   is whether converting back to double again reproduces what we
+	   started with. */
+	aslong = (long)wholepart;
+	if ((double)aslong == wholepart)
+		return PyInt_FromLong(aslong);
+	PyErr_SetString(PyExc_OverflowError, "float too large to convert");
+	return NULL;
 }
 
 static PyObject *
