src/float16.h

/* Copyright 2016 Google Inc. All Rights Reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
==============================================================================*/

#ifndef FLOAT16_H_
#define FLOAT16_H_

#include <string.h>  // for memcpy

#include "base.h"
#include "casts.h"

namespace chrome_lang_id {

// Compact 16-bit encoding of floating point numbers. This
// representation uses 1 bit for the sign, 8 bits for the exponent and
// 7 bits for the mantissa.  It is assumed that floats are in IEEE 754
// format so a float16 is just bits 16-31 of a single precision float.
//
// NOTE: The IEEE floating point standard defines a float16 format that
// is different than this format (it has fewer bits of exponent and more
// bits of mantissa).  We don't use that format here because conversion
// to/from 32-bit floats is more complex for that format, and the
// conversion for this format is very simple.
//
// <---------float16------------>
// s e e e e e e e e f f f f f f f f f f f f f f f f f f f f f f f
// <------------------------------float-------------------------->
// 3 3             2 2             1 1                           0
// 1 0             3 2             5 4                           0

typedef uint16 float16;

static inline float16 Float32To16(float f) {
  // Note that we just truncate the mantissa bits: we make no effort to
  // do any smarter rounding.
  return (lang_id_bit_cast<uint32>(f) >> 16) & 0xffff;
}

static inline float Float16To32(float16 f) {
  // We fill in the new mantissa bits with 0, and don't do anything smarter.
  return lang_id_bit_cast<float>(f << 16);
}

}  // namespace chrome_lang_id

#endif  // FLOAT16_H_