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py/parsenum: Improve parsing of floating point numbers.

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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.
This commit is contained in:
Damien George
2017-11-27 12:51:52 +11:00
parent f59c6b48ae
commit 84895f1a21
4 changed files with 60 additions and 6 deletions
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27
py/parsenum.c
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@@ -170,6 +170,14 @@ typedef enum {
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 @@ mp_obj_t mp_parse_num_decimal(const char *str, size_t len, bool allow_imag, bool
// 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 @@ mp_obj_t mp_parse_num_decimal(const char *str, size_t len, bool allow_imag, bool
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 @@ mp_obj_t mp_parse_num_decimal(const char *str, size_t len, bool allow_imag, bool
}
// 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