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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`. 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 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 parent 465b967 commit ab1173c

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lines changed

‎ports/esp8266/mpconfigport.h

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

‎ports/unix/coverage.c

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@@ -607,26 +607,6 @@ static mp_obj_t extra_coverage(void) {
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mp_emitter_warning(MP_PASS_CODE_SIZE, "test");
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}
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// format float
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{
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mp_printf(&mp_plat_print, "# format float\n");
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// format with inadequate buffer size
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char buf[5];
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mp_format_float(1, buf, sizeof(buf), 'g', 0, '+');
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mp_printf(&mp_plat_print, "%s\n", buf);
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// format with just enough buffer so that precision must be
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// set from 0 to 1 twice
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char buf2[8];
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mp_format_float(1, buf2, sizeof(buf2), 'g', 0, '+');
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mp_printf(&mp_plat_print, "%s\n", buf2);
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// format where precision is trimmed to avoid buffer overflow
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mp_format_float(1, buf2, sizeof(buf2), 'e', 0, '+');
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mp_printf(&mp_plat_print, "%s\n", buf2);
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}
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// binary
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{
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mp_printf(&mp_plat_print, "# binary\n");

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