/
unitconv.go
318 lines (284 loc) · 8.16 KB
/
unitconv.go
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// Copyright 2015, Joe Tsai. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE.md file.
// Package unitconv implements string conversion functionality for
// unit prefixes such as those from the SI and IEC standards.
//
// See https://wikipedia.org/wiki/unit_prefix
package unitconv
import (
"errors"
"math"
"strconv"
"strings"
)
// Mode is the base conversion for strings.
type Mode int
// These are the different modes for Prefix string conversion.
const (
// AutoParse will parse the input as either the SI, IEC, or regular float
// notation as accepted by ParseFloat.
//
// This is an invalid mode for Append and Format.
AutoParse Mode = iota
// SI uses a scaling of 1000x, and uses a single letter symbol to denote
// the scaling. The prefixes ranges from Yocto (1E-24) to Yotta (1E+24).
//
// The output uses SI prefixes, and uses 'k' for Kilo and 'μ' for Micro.
// The input accepts SI prefixes along with 'k' and 'K' for Kilo and
// 'μ' and 'u' for Micro.
// It does not support the Deca, Hecto, Deci, or Centi prefixes.
SI
// Base1024 uses a scaling of 1024x, but uses the same prefixes as SI. This
// is a non-standard prefix notation and exists because many legacy systems
// unfortunately operate in this way.
//
// The output uses SI prefixes, but uses 'K' for Kilo and 'u' for Micro.
// The input accepts SI prefixes along with 'k' and 'K' for Kilo and
// 'μ' and 'u' for Micro. It also accepts IEC notation.
Base1024
// IEC uses a scaling of 1024x, and uses a two-letter symbol to denote the
// scaling in a system similar to SI. Instead of Kilo denoted as 'k', it
// uses Kibi denoted as 'Ki'. The prefixes ranges from Unit (1) to Yobi
// (1<<80). Representing values less than 1 will not use any of the divisor
// prefixes. IEC notation is easy to identify since the prefix symbols
// always end with the letter 'i'.
//
// The output uses IEC prefixes that are Unit and greater.
// The input accepts IEC prefixes only.
IEC
)
func (m Mode) base() float64 {
switch m {
case SI:
return 1000.0
case Base1024, IEC:
return 1024.0
default:
return math.NaN()
}
}
// Prefix factors according to IEC standards.
const (
// These are not standard IEC prefixes, but used internally.
yocbi = 1.0 / (1 << 80)
zepbi = 1.0 / (1 << 70)
attbi = 1.0 / (1 << 60)
fembi = 1.0 / (1 << 50)
picbi = 1.0 / (1 << 40)
nanbi = 1.0 / (1 << 30)
micbi = 1.0 / (1 << 20)
milbi = 1.0 / (1 << 10)
_ = 1 << 0
Kibi = 1 << 10
Mebi = 1 << 20
Gibi = 1 << 30
Tebi = 1 << 40
Pebi = 1 << 50
Exbi = 1 << 60
Zebi = 1 << 70
Yobi = 1 << 80
)
// Prefix factors according to SI standards.
const (
Yocto = 1e-24
Zepto = 1e-21
Atto = 1e-18
Femto = 1e-15
Pico = 1e-12
Nano = 1e-9
Micro = 1e-6
Milli = 1e-3
Unit = 1e0 // Not a standard SI prefix.
Kilo = 1e+3
Mega = 1e+6
Giga = 1e+9
Tera = 1e+12
Peta = 1e+15
Exa = 1e+18
Zetta = 1e+21
Yotta = 1e+24
)
const (
prefixes = divPrefixes + string(unitPrefix) + mulPrefixes
divPrefixes = "yzafpnum"
unitPrefix = '.'
mulPrefixes = "KMGTPEZY"
parsePrefixes = divPrefixes + mulPrefixes + string(kiloAlt) + string(microAlt)
maxExp = +len(mulPrefixes)
minExp = -len(divPrefixes)
microNorm, kiloNorm = 'u', 'K'
microAlt, kiloAlt = 'μ', 'k'
)
var (
mapNormToAlt = map[rune]rune{microNorm: microAlt, kiloNorm: kiloAlt}
mapAltToNorm = map[rune]rune{microAlt: microNorm, kiloAlt: kiloNorm}
scaleSI = []float64{
Yocto, Zepto, Atto, Femto, Pico, Nano, Micro, Milli,
Unit,
Kilo, Mega, Giga, Tera, Peta, Exa, Zetta, Yotta,
}
scaleIEC = []float64{
yocbi, zepbi, attbi, fembi, picbi, nanbi, micbi, milbi,
Unit,
Kibi, Mebi, Gibi, Tebi, Pebi, Exbi, Zebi, Yobi,
}
)
// Using a combination of the math.LogX and math.Pow functions can be lossy.
// This leads to slightly wrong values around the prefix boundaries. Thus, we
// look up the computed exponent in an authoritative list of scalings and
// adjust accordingly. We only check up to 3 values in the relevant section,
// ensuring a runtime of O(1).
func adjustLog(val float64, scales []float64, minExp, exp, maxExp int) int {
exp++
if exp > maxExp {
exp = maxExp
}
for exp >= minExp && scales[exp+len(scales)/2] > math.Abs(val) {
exp--
}
if exp < minExp {
exp = minExp
}
return exp
}
// AppendPrefix appends the string form of the floating-point number val, as
// generated by FormatPrefix, to dst and returns the extended buffer.
func AppendPrefix(dst []byte, val float64, m Mode, prec int) (out []byte) {
if math.IsNaN(val) || math.IsInf(val, 0) {
return strconv.AppendFloat(dst, val, 'f', -1, 64)
}
// Compute the prefix exponent.
var exp int
var adjustValue bool
if val != 0 {
switch m {
case SI:
exp = int(math.Floor(math.Log10(math.Abs(val)) / 3))
exp = adjustLog(val, scaleSI, minExp, exp, maxExp)
val /= scaleSI[exp+len(scaleSI)/2] // Same as: Pow(1000, exp)
adjustValue = minExp < exp && exp < maxExp
case Base1024:
exp = int(math.Floor(math.Log2(math.Abs(val)) / 10))
exp = adjustLog(val, scaleIEC, minExp, exp, maxExp)
val /= scaleIEC[exp+len(scaleSI)/2] // Same as: Pow(1024, exp)
adjustValue = minExp < exp && exp < maxExp
case IEC:
exp = int(math.Floor(math.Log2(math.Abs(val)) / 10))
exp = adjustLog(val, scaleIEC, 0, exp, maxExp)
val /= scaleIEC[exp+len(scaleSI)/2] // Same as: Pow(1024, exp)
adjustValue = 0 < exp && exp < maxExp
default:
return strconv.AppendInt(append(dst, '%'), int64(m), 10)
}
} else {
exp = 0
}
if adjustValue {
if math.Abs(val) < 1 {
val = math.Copysign(1, val)
}
if math.Abs(val) >= m.base() {
val = math.Copysign(math.Nextafter(m.base(), 0), val)
}
}
// Print the actual number.
dst = strconv.AppendFloat(dst, val, 'f', prec, 64)
// Print the prefix symbol.
if exp != 0 {
sym := prefixes[exp+len(prefixes)/2]
if alt := mapNormToAlt[rune(sym)]; m == SI && alt != 0 {
dst = append(dst, string(alt)...)
} else {
dst = append(dst, sym)
}
if m == IEC {
dst = append(dst, 'i')
}
}
return dst
}
// FormatPrefix converts the floating-point number val to a string, according
// to the prefix notation specified by mode. The prec specifies the precision
// used by the numeric portion.
//
// Even if prec is -1, formatting a value and parsing it does not guarantee that
// the exact value will be returned. It will however be extremely accurate.
//
// It is valid to format +Inf, -Inf, and NaN.
func FormatPrefix(val float64, m Mode, prec int) (str string) {
return string(AppendPrefix(nil, val, m, prec))
}
// ParsePrefix converts the string str to a floating-point number, according to
// the prefix notation specified by mode.
//
// It is valid to parse +Inf, -Inf, and NaN.
func ParsePrefix(str string, m Mode) (val float64, err error) {
val, err = strconv.ParseFloat(str, 64)
if err == nil && (m == AutoParse || math.IsNaN(val) || math.IsInf(val, 0)) {
return val, nil
}
err = nil // Reset the error
// If mode is AutoParse, detect the format to use.
if m == AutoParse {
if len(str) > 0 && str[len(str)-1] == 'i' {
m = IEC
} else {
m = SI
}
}
// Parse the prefix symbol.
var exp int
var saveStr = str
if i := strings.IndexAny(str, parsePrefixes); i >= 0 {
strPre := str[i:]
str = str[:i]
if m == IEC && len(strPre) != 2 {
goto fail
}
for si, ch := range strPre {
switch si {
case 0:
norm, usedAlt := mapAltToNorm[ch]
if usedAlt {
ch = norm
}
exp = strings.IndexByte(prefixes, byte(ch)) - len(prefixes)/2
if m == IEC && (usedAlt || exp < 0) {
goto fail
}
case 1:
if m == SI || ch != 'i' {
goto fail
}
default:
goto fail
}
}
}
// Parse the number part.
if strings.Trim(str, "-+.0123456789") != "" {
goto fail
}
if val, err = strconv.ParseFloat(str, 64); err != nil {
goto fail
}
// Compute the actual value.
switch m {
case SI:
return val * scaleSI[exp+len(scaleSI)/2], nil
case Base1024, IEC:
return val * scaleIEC[exp+len(scaleSI)/2], nil
default:
return 0, errors.New("unitconv: invalid mode")
}
fail:
if err == nil {
err = strconv.ErrSyntax
}
if nerr, ok := err.(*strconv.NumError); ok {
err = nerr.Err
}
err = &strconv.NumError{Func: "ParsePrefix", Num: saveStr, Err: err}
return val, err
}