py/formatfloat: Improve accuracy of float formatting code.

Following discussions in PR micropython#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 <[email protected]>

Author: yoctopuce

ports/esp8266/mpconfigport.h

@@ -106,6 +106,7 @@
 #define MICROPY_PY_OS_URANDOM       (1)
 #define MICROPY_LONGINT_IMPL        (MICROPY_LONGINT_IMPL_MPZ)
 #define MICROPY_FLOAT_IMPL          (MICROPY_FLOAT_IMPL_FLOAT)
+#define MICROPY_FLOAT_FORMAT_IMPL   (MICROPY_FLOAT_FORMAT_IMPL_BASIC)
 #define MICROPY_WARNINGS            (1)
 #define MICROPY_PY_STR_BYTES_CMP_WARN (1)
 #define MICROPY_STREAMS_POSIX_API   (1)

ports/unix/coverage.c

@@ -612,26 +612,6 @@ static mp_obj_t extra_coverage(void) {
         mp_emitter_warning(MP_PASS_CODE_SIZE, "test");
     }
 
-    // format float
-    {
-        mp_printf(&mp_plat_print, "# format float\n");
-
-        // format with inadequate buffer size
-        char buf[5];
-        mp_format_float(1, buf, sizeof(buf), 'g', 0, '+');
-        mp_printf(&mp_plat_print, "%s\n", buf);
-
-        // format with just enough buffer so that precision must be
-        // set from 0 to 1 twice
-        char buf2[8];
-        mp_format_float(1, buf2, sizeof(buf2), 'g', 0, '+');
-        mp_printf(&mp_plat_print, "%s\n", buf2);
-
-        // format where precision is trimmed to avoid buffer overflow
-        mp_format_float(1, buf2, sizeof(buf2), 'e', 0, '+');
-        mp_printf(&mp_plat_print, "%s\n", buf2);
-    }
-
     // binary
     {
         mp_printf(&mp_plat_print, "# binary\n");