src/math/big/ftoa.go

// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// This file implements Float-to-string conversion functions.
// It is closely following the corresponding implementation
// in strconv/ftoa.go, but modified and simplified for Float.

package big

import (
	"strconv"
	"strings"
)

// Format converts the floating-point number x to a string according
// to the given format and precision prec. The format is one of:
//
//	'e'	-d.dddde±dd, decimal exponent, at least two (possibly 0) exponent digits
//	'E'	-d.ddddE±dd, decimal exponent, at least two (possibly 0) exponent digits
//	'f'	-ddddd.dddd, no exponent
//	'g'	like 'e' for large exponents, like 'f' otherwise
//	'G'	like 'E' for large exponents, like 'f' otherwise
//	'b'	-ddddddp±dd, binary exponent
//	'p'	-0x.dddp±dd, binary exponent, hexadecimal mantissa
//
// For the binary exponent formats, the mantissa is printed in normalized form:
//
//	'b'	decimal integer mantissa using x.Prec() bits, or -0
//	'p'	hexadecimal fraction with 0.5 <= 0.mantissa < 1.0, or -0
//
// The precision prec controls the number of digits (excluding the exponent)
// printed by the 'e', 'E', 'f', 'g', and 'G' formats. For 'e', 'E', and 'f'
// it is the number of digits after the decimal point. For 'g' and 'G' it is
// the total number of digits. A negative precision selects the smallest
// number of digits necessary such that ParseFloat will return f exactly.
// The prec value is ignored for the 'b' or 'p' format.
//
// BUG(gri) Float.Format does not accept negative precisions.
// BUG(gri) The Float.Format signature conflicts with Format(f fmt.State, c rune).
//          (https://github.com/golang/go/issues/10938)
func (x *Float) Format(format byte, prec int) string {
	const extra = 10 // TODO(gri) determine a good/better value here
	return string(x.Append(make([]byte, 0, prec+extra), format, prec))
}

// String formats x like x.Format('g', 10).
func (x *Float) String() string {
	return x.Format('g', 10)
}

// Append appends to buf the string form of the floating-point number x,
// as generated by x.Format, and returns the extended buffer.
func (x *Float) Append(buf []byte, fmt byte, prec int) []byte {
	// sign
	if x.neg {
		buf = append(buf, '-')
	}

	// Inf
	if x.IsInf() {
		if !x.neg {
			buf = append(buf, '+')
		}
		return append(buf, "Inf"...)
	}

	// pick off easy formats
	switch fmt {
	case 'b':
		return x.fmtB(buf)
	case 'p':
		return x.fmtP(buf)
	}

	// Algorithm:
	//   1) convert Float to multiprecision decimal
	//   2) round to desired precision
	//   3) read digits out and format

	// 1) convert Float to multiprecision decimal
	var mant nat
	if x.form == finite {
		mant = x.mant
	}
	var d decimal
	d.init(mant, int(x.exp)-x.mant.bitLen())

	// 2) round to desired precision
	shortest := false
	if prec < 0 {
		shortest = true
		panic("unimplemented")
		// TODO(gri) complete this
		// roundShortest(&d, f.mant, int(f.exp))
		// Precision for shortest representation mode.
		switch fmt {
		case 'e', 'E':
			prec = len(d.mant) - 1
		case 'f':
			prec = max(len(d.mant)-d.exp, 0)
		case 'g', 'G':
			prec = len(d.mant)
		}
	} else {
		// round appropriately
		switch fmt {
		case 'e', 'E':
			// one digit before and number of digits after decimal point
			d.round(1 + prec)
		case 'f':
			// number of digits before and after decimal point
			d.round(d.exp + prec)
		case 'g', 'G':
			if prec == 0 {
				prec = 1
			}
			d.round(prec)
		}
	}

	// 3) read digits out and format
	switch fmt {
	case 'e', 'E':
		return fmtE(buf, fmt, prec, d)
	case 'f':
		return fmtF(buf, prec, d)
	case 'g', 'G':
		// trim trailing fractional zeros in %e format
		eprec := prec
		if eprec > len(d.mant) && len(d.mant) >= d.exp {
			eprec = len(d.mant)
		}
		// %e is used if the exponent from the conversion
		// is less than -4 or greater than or equal to the precision.
		// If precision was the shortest possible, use eprec = 6 for
		// this decision.
		if shortest {
			eprec = 6
		}
		exp := d.exp - 1
		if exp < -4 || exp >= eprec {
			if prec > len(d.mant) {
				prec = len(d.mant)
			}
			return fmtE(buf, fmt+'e'-'g', prec-1, d)
		}
		if prec > d.exp {
			prec = len(d.mant)
		}
		return fmtF(buf, max(prec-d.exp, 0), d)
	}

	// unknown format
	if x.neg {
		buf = buf[:len(buf)-1] // sign was added prematurely - remove it again
	}
	return append(buf, '%', fmt)
}

// %e: d.ddddde±dd
func fmtE(buf []byte, fmt byte, prec int, d decimal) []byte {
	// first digit
	ch := byte('0')
	if len(d.mant) > 0 {
		ch = d.mant[0]
	}
	buf = append(buf, ch)

	// .moredigits
	if prec > 0 {
		buf = append(buf, '.')
		i := 1
		m := min(len(d.mant), prec+1)
		if i < m {
			buf = append(buf, d.mant[i:m]...)
			i = m
		}
		for ; i <= prec; i++ {
			buf = append(buf, '0')
		}
	}

	// e±
	buf = append(buf, fmt)
	var exp int64
	if len(d.mant) > 0 {
		exp = int64(d.exp) - 1 // -1 because first digit was printed before '.'
	}
	if exp < 0 {
		ch = '-'
		exp = -exp
	} else {
		ch = '+'
	}
	buf = append(buf, ch)

	// dd...d
	if exp < 10 {
		buf = append(buf, '0') // at least 2 exponent digits
	}
	return strconv.AppendInt(buf, exp, 10)
}

// %f: ddddddd.ddddd
func fmtF(buf []byte, prec int, d decimal) []byte {
	// integer, padded with zeros as needed
	if d.exp > 0 {
		m := min(len(d.mant), d.exp)
		buf = append(buf, d.mant[:m]...)
		for ; m < d.exp; m++ {
			buf = append(buf, '0')
		}
	} else {
		buf = append(buf, '0')
	}

	// fraction
	if prec > 0 {
		buf = append(buf, '.')
		for i := 0; i < prec; i++ {
			ch := byte('0')
			if j := d.exp + i; 0 <= j && j < len(d.mant) {
				ch = d.mant[j]
			}
			buf = append(buf, ch)
		}
	}

	return buf
}

// fmtB appends the string of x in the format mantissa "p" exponent
// with a decimal mantissa and a binary exponent, or 0" if x is zero,
// and returns the extended buffer.
// The mantissa is normalized such that is uses x.Prec() bits in binary
// representation.
// The sign of x is ignored, and x must not be an Inf.
func (x *Float) fmtB(buf []byte) []byte {
	if x.form == zero {
		return append(buf, '0')
	}

	if debugFloat && x.form != finite {
		panic("non-finite float")
	}
	// x != 0

	// adjust mantissa to use exactly x.prec bits
	m := x.mant
	switch w := uint32(len(x.mant)) * _W; {
	case w < x.prec:
		m = nat(nil).shl(m, uint(x.prec-w))
	case w > x.prec:
		m = nat(nil).shr(m, uint(w-x.prec))
	}

	buf = append(buf, m.decimalString()...)
	buf = append(buf, 'p')
	e := int64(x.exp) - int64(x.prec)
	if e >= 0 {
		buf = append(buf, '+')
	}
	return strconv.AppendInt(buf, e, 10)
}

// fmtP appends the string of x in the format 0x." mantissa "p" exponent
// with a hexadecimal mantissa and a binary exponent, or 0" if x is zero,
// ad returns the extended buffer.
// The mantissa is normalized such that 0.5 <= 0.mantissa < 1.0.
// The sign of x is ignored, and x must not be an Inf.
func (x *Float) fmtP(buf []byte) []byte {
	if x.form == zero {
		return append(buf, '0')
	}

	if debugFloat && x.form != finite {
		panic("non-finite float")
	}
	// x != 0

	// remove trailing 0 words early
	// (no need to convert to hex 0's and trim later)
	m := x.mant
	i := 0
	for i < len(m) && m[i] == 0 {
		i++
	}
	m = m[i:]

	buf = append(buf, "0x."...)
	buf = append(buf, strings.TrimRight(x.mant.hexString(), "0")...)
	buf = append(buf, 'p')
	if x.exp >= 0 {
		buf = append(buf, '+')
	}
	return strconv.AppendInt(buf, int64(x.exp), 10)
}

func min(x, y int) int {
	if x < y {
		return x
	}
	return y
}