py/parsenum: Improve parsing of floating point numbers.

This patch improves parsing of floating point numbers by converting all the
digits (integer and fractional) together into a number 1 or greater, and
then applying the correct power of 10 at the very end.  In particular the
multiple "multiply by 0.1" operations to build a fraction are now combined
together and applied at the same time as the exponent, at the very end.

This helps to retain precision during parsing of floats, and also includes
a check that the number doesn't overflow during the parsing.  One benefit
is that a float will have the same value no matter where the decimal point
is located, eg 1.23 == 123e-2.

1 files changed, 16 insertions, 0 deletions

diff --git a/tests/float/float_parse_doubleprec.py b/tests/float/float_parse_doubleprec.py
new file mode 100644
index 000000000..356601130
--- /dev/null
+++ b/tests/float/float_parse_doubleprec.py
@@ -0,0 +1,16 @@
+# test parsing of floats, requiring double-precision
+
+# very large integer part with a very negative exponent should cancel out
+print(float('9' * 400 + 'e-100'))
+print(float('9' * 400 + 'e-200'))
+print(float('9' * 400 + 'e-400'))
+
+# many fractional digits
+print(float('.' + '9' * 400))
+print(float('.' + '9' * 400 + 'e100'))
+print(float('.' + '9' * 400 + 'e-100'))
+
+# tiny fraction with large exponent
+print(float('.' + '0' * 400 + '9e100'))
+print(float('.' + '0' * 400 + '9e200'))
+print(float('.' + '0' * 400 + '9e400'))