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|
package strconv
import (
"math"
)
// ParseDecimal parses number of the format 1.2
func ParseDecimal(b []byte) (float64, int) {
// float64 has up to 17 significant decimal digits and an exponent in [-1022,1023]
i := 0
sign := 1.0
if 0 < len(b) && b[0] == '-' {
sign = -1.0
i++
}
start := -1
dot := -1
n := uint64(0)
for ; i < len(b); i++ {
// parse up to 18 significant digits (with dot will be 17) ignoring zeros before/after
c := b[i]
if '0' <= c && c <= '9' {
if start == -1 {
if '1' <= c && c <= '9' {
n = uint64(c - '0')
start = i
}
} else if i-start < 18 {
n *= 10
n += uint64(c - '0')
}
} else if c == '.' {
if dot != -1 {
break
}
dot = i
} else {
break
}
}
if i == 1 && dot == 0 {
return 0.0, 0 // only dot
} else if start == -1 {
return 0.0, i // only zeros and dot
} else if dot == -1 {
dot = i
}
exp := (dot - start) - LenUint(n)
if dot < start {
exp++
}
if 1023 < exp {
if sign == 1.0 {
return math.Inf(1), i
} else {
return math.Inf(-1), i
}
} else if exp < -1022 {
return 0.0, i
}
f := sign * float64(n)
if 0 <= exp && exp < 23 {
return f * float64pow10[exp], i
} else if 23 < exp && exp < 0 {
return f / float64pow10[exp], i
}
return f * math.Pow10(exp), i
}
// AppendDecimal appends a float to `b` with `dec` the maximum number of decimals.
func AppendDecimal(b []byte, f float64, dec int) []byte {
if math.IsNaN(float64(f)) || math.IsInf(float64(f), 0) {
return b
}
if dec < 0 || 17 < dec {
dec = 17
}
f *= math.Pow10(dec)
// correct rounding
if 0.0 <= f {
f += 0.5
} else {
f -= 0.5
}
// calculate mantissa and exponent
num := int64(f)
if num == 0 {
return append(b, '0')
}
for 0 < dec && num%10 == 0 {
num /= 10
dec-- // remove trailing zeros
}
i, n := len(b), LenInt(num)
if 0 < dec {
if n < dec {
n = dec // number has zero after dot
}
n++ // dot
if lim := int64pow10[dec]; 0 < num && num < lim || num < 0 && -lim < num {
n++ // zero at beginning
}
}
if cap(b) < i+n {
b = append(b, make([]byte, n)...)
} else {
b = b[:i+n]
}
// print sign
if num < 0 {
num = -num
b[i] = '-'
}
i += n - 1
// print number
if 0 < dec {
b[i] = byte(num%10) + '0'
num /= 10
dec--
i--
for 0 < dec {
b[i] = byte(num%10) + '0'
num /= 10
dec--
i--
}
b[i] = '.'
i--
}
if num == 0 {
b[i] = '0'
} else {
for num != 0 {
b[i] = byte(num%10) + '0'
num /= 10
i--
}
}
return b
}
|
