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woodpecker/vendor/github.com/lucas-clemente/quic-go/internal/utils/float16.go

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2017-07-25 01:15:25 +02:00
package utils
import (
"bytes"
"io"
"math"
)
// We define an unsigned 16-bit floating point value, inspired by IEEE floats
// (http://en.wikipedia.org/wiki/Half_precision_floating-point_format),
// with 5-bit exponent (bias 1), 11-bit mantissa (effective 12 with hidden
// bit) and denormals, but without signs, transfinites or fractions. Wire format
// 16 bits (little-endian byte order) are split into exponent (high 5) and
// mantissa (low 11) and decoded as:
// uint64_t value;
// if (exponent == 0) value = mantissa;
// else value = (mantissa | 1 << 11) << (exponent - 1)
const uFloat16ExponentBits = 5
const uFloat16MaxExponent = (1 << uFloat16ExponentBits) - 2 // 30
const uFloat16MantissaBits = 16 - uFloat16ExponentBits // 11
const uFloat16MantissaEffectiveBits = uFloat16MantissaBits + 1 // 12
const uFloat16MaxValue = ((uint64(1) << uFloat16MantissaEffectiveBits) - 1) << uFloat16MaxExponent // 0x3FFC0000000
// ReadUfloat16 reads a float in the QUIC-float16 format and returns its uint64 representation
func ReadUfloat16(b io.ByteReader) (uint64, error) {
val, err := ReadUint16(b)
if err != nil {
return 0, err
}
res := uint64(val)
if res < (1 << uFloat16MantissaEffectiveBits) {
// Fast path: either the value is denormalized (no hidden bit), or
// normalized (hidden bit set, exponent offset by one) with exponent zero.
// Zero exponent offset by one sets the bit exactly where the hidden bit is.
// So in both cases the value encodes itself.
return res, nil
}
exponent := val >> uFloat16MantissaBits // No sign extend on uint!
// After the fast pass, the exponent is at least one (offset by one).
// Un-offset the exponent.
exponent--
// Here we need to clear the exponent and set the hidden bit. We have already
// decremented the exponent, so when we subtract it, it leaves behind the
// hidden bit.
res -= uint64(exponent) << uFloat16MantissaBits
res <<= exponent
return res, nil
}
// WriteUfloat16 writes a float in the QUIC-float16 format from its uint64 representation
func WriteUfloat16(b *bytes.Buffer, value uint64) {
var result uint16
if value < (uint64(1) << uFloat16MantissaEffectiveBits) {
// Fast path: either the value is denormalized, or has exponent zero.
// Both cases are represented by the value itself.
result = uint16(value)
} else if value >= uFloat16MaxValue {
// Value is out of range; clamp it to the maximum representable.
result = math.MaxUint16
} else {
// The highest bit is between position 13 and 42 (zero-based), which
// corresponds to exponent 1-30. In the output, mantissa is from 0 to 10,
// hidden bit is 11 and exponent is 11 to 15. Shift the highest bit to 11
// and count the shifts.
exponent := uint16(0)
for offset := uint16(16); offset > 0; offset /= 2 {
// Right-shift the value until the highest bit is in position 11.
// For offset of 16, 8, 4, 2 and 1 (binary search over 1-30),
// shift if the bit is at or above 11 + offset.
if value >= (uint64(1) << (uFloat16MantissaBits + offset)) {
exponent += offset
value >>= offset
}
}
// Hidden bit (position 11) is set. We should remove it and increment the
// exponent. Equivalently, we just add it to the exponent.
// This hides the bit.
result = (uint16(value) + (exponent << uFloat16MantissaBits))
}
WriteUint16(b, result)
}