forked from open-telemetry/opentelemetry-go
/
exponent.go
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/
exponent.go
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// Copyright The OpenTelemetry Authors
//
// 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.
package exponent // import "go.opentelemetry.io/otel/data/exponential/mapping/exponent"
import (
"fmt"
"math"
"go.opentelemetry.io/otel/data/exponential/mapping"
"go.opentelemetry.io/otel/data/exponential/mapping/internal"
)
const (
// MinScale defines the point at which the exponential mapping
// function becomes useless for float64. With scale -10, ignoring
// subnormal values, bucket indices range from -1 to 1.
MinScale int32 = -10
// MaxScale is the largest scale supported in this code. Use
// ../logarithm for larger scales.
MaxScale int32 = 0
)
type exponentMapping struct {
shift uint8 // equals negative scale
}
// exponentMapping is used for negative scales, effectively a
// mapping of the base-2 logarithm of the exponent.
var prebuiltMappings = [-MinScale + 1]exponentMapping{
{10},
{9},
{8},
{7},
{6},
{5},
{4},
{3},
{2},
{1},
{0},
}
// NewMapping constructs an exponential mapping function, used for scales <= 0.
func NewMapping(scale int32) (mapping.Mapping, error) {
if scale > MaxScale {
return nil, fmt.Errorf("exponent mapping requires scale <= 0")
}
if scale < MinScale {
return nil, fmt.Errorf("scale too low")
}
return &prebuiltMappings[scale-MinScale], nil
}
// minNormalLowerBoundaryIndex is the largest index such that
// base**index is <= MinValue. A histogram bucket with this index
// covers the range (base**index, base**(index+1)], including
// MinValue.
func (e *exponentMapping) minNormalLowerBoundaryIndex() int32 {
idx := int32(internal.MinNormalExponent) >> e.shift
if e.shift < 2 {
// For scales -1 and 0 the minimum value 2**-1022
// is a power-of-two multiple, meaning it belongs
// to the index one less.
idx--
}
return idx
}
// maxNormalLowerBoundaryIndex is the index such that base**index
// equals the largest representable boundary. A histogram bucket with this
// index covers the range (0x1p+1024/base, 0x1p+1024], which includes
// MaxValue; note that this bucket is incomplete, since the upper
// boundary cannot be represented. One greater than this index
// corresponds with the bucket containing values > 0x1p1024.
func (e *exponentMapping) maxNormalLowerBoundaryIndex() int32 {
return int32(internal.MaxNormalExponent) >> e.shift
}
// MapToIndex implements mapping.Mapping.
func (e *exponentMapping) MapToIndex(value float64) int32 {
// Note: we can assume not a 0, Inf, or NaN; positive sign bit.
if value < internal.MinValue {
return e.minNormalLowerBoundaryIndex()
}
// Extract the raw exponent.
rawExp := internal.GetNormalBase2(value)
// In case the value is an exact power of two, compute a
// correction of -1:
correction := int32((internal.GetSignificand(value) - 1) >> internal.SignificandWidth)
// Note: bit-shifting does the right thing for negative
// exponents, e.g., -1 >> 1 == -1.
return (rawExp + correction) >> e.shift
}
// LowerBoundary implements mapping.Mapping.
func (e *exponentMapping) LowerBoundary(index int32) (float64, error) {
if min := e.minNormalLowerBoundaryIndex(); index < min {
return 0, mapping.ErrUnderflow
}
if max := e.maxNormalLowerBoundaryIndex(); index > max {
return 0, mapping.ErrOverflow
}
return math.Ldexp(1, int(index<<e.shift)), nil
}
// Scale implements mapping.Mapping.
func (e *exponentMapping) Scale() int32 {
return -int32(e.shift)
}