ieee754_string.h

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#pragma once

// Integer-only string <-> IEEE 754 single-precision codec.
//
// Motivation: on no-FPU ARM (Cortex-M0+ / LPC845 etc.) the libgcc soft-FP
// helpers (__aeabi_d* / __aeabi_f* and their ~7 KB transitive closure) get
// pulled into any link that runs decimal arithmetic through `float` or
// `double` operators. FastLED #3022 (and the gap-tracking issue #3029) ask
// the library to never structurally reach those helpers from the JSON
// parser / serializer or the variant's float-vector storage path.
//
// This codec satisfies that constraint:
//   * Parse: digits feed an integer mantissa, scaled by a precomputed table
//     of `10^k` in 64-bit-mantissa + binary-exponent form. The widening
//     multiply that follows uses only 32x32->64 native ops (no `__aeabi_d*`).
//   * Format: mantissa / exponent are unpacked from the IEEE-754 bit
//     pattern via shifts and masks; decimal digits emerge from integer
//     scaling against the same `10^k` table.
//
// API surface deliberately works on `fl::u32` bit-patterns (not `float`).
// The caller can `fl::bit_cast<float>(bits)` at the API boundary -- that is
// a `memcpy` and never anchors libgcc. Doing the conversion *inside* this
// codec would defeat the point.
//
// Precision target: ~6 significant decimal digits with at-most +/-1 ULP
// drift from `strtof` / `snprintf("%g")` at the edges. Sufficient for
// FastLED's JSON-RPC use case (brightness, ratios, coordinates). Callers
// that need bit-exact round-trip on hosts should use the platform `strtof`
// / `snprintf` directly -- those are *intentionally* not wired through this
// header.
//
// See FastLED #3022 (parent), #3029 (gap), #3027 (superseded gate PR).
 
#include "fl/stl/stdint.h"
#include "fl/stl/cstddef.h"
#include "fl/stl/noexcept.h"
 
namespace fl {
 
// Forward declaration to avoid pulling fl/stl/string.h here.
class string;

u32 ieee754_parse_decimal(const char* s, fl::size len, fl::size* consumed = nullptr) FL_NO_EXCEPT;

fl::string ieee754_format_decimal(u32 bits, int precision = 6) FL_NO_EXCEPT;
 
} // namespace fl