py/formatfloat: Calculate reference values with pow().

dpwe · dpwe · commit 701f3ba0dbe5 · 2022-08-02T14:24:52.000-04:00
Formerly, ```formatfloat``` struggled with formatting exact powers of 10 as created by ```parsenum``` because of small differences in how those powers of 10 were calculated: ```formatfloat``` multiplied together a few values of 10^(2^Y) from a lookup table, but ```parsenum``` used ```pow(10, X)```. This was mitigated in micropython#8905 by adding 2eps of extra margin when comparing values ("aggressive rounding up"). This patch instead removes the discrepency by using ```pow()``` in ```formatfloat``` also. This eliminates the need for the 2eps margin, as well as the lookup tables. It is likely slower to run, however. In addition, this patch directly estimates the power-of-10 exponent from the power-of-2 exponent in the floating-point representation. This change surfaced a previously-noted problem with parsing numbers including trailing zeros. The fix to that problem, micropython#8980, is included in this PR to make the tests pass. Signed-off-by: Dan Ellis <dan.ellis@gmail.com>
diff --git a/py/parsenum.c b/py/parsenum.c
@@ -309,6 +309,8 @@ mp_obj_t mp_parse_num_float(const char *str, size_t len, bool allow_imag, mp_lex
                         }
                         accept_digit(&dec_val, dig, &exp_extra, in);
                     }
+                    //DEBUG_printf("dec_val %f exp_extra %d trail_z_i %d trail_z_f %d\n",
+                    //             (double)dec_val, exp_extra, trailing_zeros_intg, trailing_zeros_frac);
                 }
             } else if (in == PARSE_DEC_IN_INTG && dig == '.') {
                 in = PARSE_DEC_IN_FRAC;

Original file line number	Diff line number	Diff line change
`@@ -309,6 +309,8 @@ mp_obj_t mp_parse_num_float(const char *str, size_t len, bool allow_imag, mp_lex`
`309`	`309`	`}`
`310`	`310`	`accept_digit(&dec_val, dig, &exp_extra, in);`
`311`	`311`	`}`
	`312`	`+ //DEBUG_printf("dec_val %f exp_extra %d trail_z_i %d trail_z_f %d\n",`
	`313`	`+ // (double)dec_val, exp_extra, trailing_zeros_intg, trailing_zeros_frac); 47BF`
`312`	`314`	`}`
`313`	`315`	`} else if (in == PARSE_DEC_IN_INTG && dig == '.') {`
`314`	`316`	`in = PARSE_DEC_IN_FRAC;`