py/formatfloat: Improve accuracy of float formatting code.

Following discussions in PR #16666, this commit updates the float
formatting code to improve the `repr` reversibility, i.e. the percentage of
valid floating point numbers that do parse back to the same number when
formatted by `repr` (in CPython it's 100%).

This new code offers a choice of 3 float conversion methods, depending on
the desired tradeoff between code size and conversion precision:

- BASIC method is the smallest code footprint

- APPROX method uses an iterative method to approximate the exact
  representation, which is a bit slower but but does not have a big impact
  on code size.  It provides `repr` reversibility on >99.8% of the cases in
  double precision, and on >98.5% in single precision (except with REPR_C,
  where reversibility is 100% as the last two bits are not taken into
  account).

- EXACT method uses higher-precision floats during conversion, which
  provides perfect results but has a higher impact on code size.  It is
  faster than APPROX method, and faster than the CPython equivalent
  implementation.  It is however not available on all compilers when using
  FLOAT_IMPL_DOUBLE.

Here is the table comparing the impact of the three conversion methods on
code footprint on PYBV10 (using single-precision floats) and reversibility
rate for both single-precision and double-precision floats.  The table
includes current situation as a baseline for the comparison:

              PYBV10  REPR_C   FLOAT  DOUBLE
    current = 364688   12.9%   27.6%   37.9%
    basic   = 364812   85.6%   60.5%   85.7%
    approx  = 365080  100.0%   98.5%   99.8%
    exact   = 366408  100.0%  100.0%  100.0%

Signed-off-by: Yoctopuce dev <dev@yoctopuce.com>

1 files changed, 1 insertions, 17 deletions

diff --git a/py/objfloat.c b/py/objfloat.c
index 81b0daa62..125b576fb 100644
--- a/py/objfloat.c
+++ b/py/objfloat.c
@@ -110,23 +110,7 @@ mp_int_t mp_float_hash(mp_float_t src) {
 static void float_print(const mp_print_t *print, mp_obj_t o_in, mp_print_kind_t kind) {
     (void)kind;
     mp_float_t o_val = mp_obj_float_get(o_in);
-    #if MICROPY_FLOAT_IMPL == MICROPY_FLOAT_IMPL_FLOAT
-    char buf[16];
-    #if MICROPY_OBJ_REPR == MICROPY_OBJ_REPR_C
-    const int precision = 6;
-    #else
-    const int precision = 7;
-    #endif
-    #else
-    char buf[32];
-    const int precision = 16;
-    #endif
-    mp_format_float(o_val, buf, sizeof(buf), 'g', precision, '\0');
-    mp_print_str(print, buf);
-    if (strchr(buf, '.') == NULL && strchr(buf, 'e') == NULL && strchr(buf, 'n') == NULL) {
-        // Python floats always have decimal point (unless inf or nan)
-        mp_print_str(print, ".0");
-    }
+    mp_print_float(print, o_val, 'g', PF_FLAG_ALWAYS_DECIMAL, '\0', -1, MP_FLOAT_REPR_PREC);
 }
 
 static mp_obj_t float_make_new(const mp_obj_type_t *type_in, size_t n_args, size_t n_kw, const mp_obj_t *args) {