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.
diff --git a/py/parsenum.c b/py/parsenum.c
index b62029f..98e7736 100644
--- a/py/parsenum.c
+++ b/py/parsenum.c
@@ -170,6 +170,14 @@
mp_obj_t mp_parse_num_decimal(const char *str, size_t len, bool allow_imag, bool force_complex, mp_lexer_t *lex) {
#if MICROPY_PY_BUILTINS_FLOAT
+
+// DEC_VAL_MAX only needs to be rough and is used to retain precision while not overflowing
+#if MICROPY_FLOAT_IMPL == MICROPY_FLOAT_IMPL_FLOAT
+#define DEC_VAL_MAX 1e20F
+#elif MICROPY_FLOAT_IMPL == MICROPY_FLOAT_IMPL_DOUBLE
+#define DEC_VAL_MAX 1e200
+#endif
+
const char *top = str + len;
mp_float_t dec_val = 0;
bool dec_neg = false;
@@ -214,8 +222,8 @@
// string should be a decimal number
parse_dec_in_t in = PARSE_DEC_IN_INTG;
bool exp_neg = false;
- mp_float_t frac_mult = 0.1;
mp_int_t exp_val = 0;
+ mp_int_t exp_extra = 0;
while (str < top) {
mp_uint_t dig = *str++;
if ('0' <= dig && dig <= '9') {
@@ -223,11 +231,18 @@
if (in == PARSE_DEC_IN_EXP) {
exp_val = 10 * exp_val + dig;
} else {
- if (in == PARSE_DEC_IN_FRAC) {
- dec_val += dig * frac_mult;
- frac_mult *= MICROPY_FLOAT_CONST(0.1);
- } else {
+ if (dec_val < DEC_VAL_MAX) {
+ // dec_val won't overflow so keep accumulating
dec_val = 10 * dec_val + dig;
+ if (in == PARSE_DEC_IN_FRAC) {
+ --exp_extra;
+ }
+ } else {
+ // dec_val might overflow and we anyway can't represent more digits
+ // of precision, so ignore the digit and just adjust the exponent
+ if (in == PARSE_DEC_IN_INTG) {
+ ++exp_extra;
+ }
}
}
} else if (in == PARSE_DEC_IN_INTG && dig == '.') {
@@ -261,7 +276,7 @@
}
// apply the exponent
- dec_val *= MICROPY_FLOAT_C_FUN(pow)(10, exp_val);
+ dec_val *= MICROPY_FLOAT_C_FUN(pow)(10, exp_val + exp_extra);
}
// negate value if needed