src/internal/strconv/ctoa.go

// Copyright 2020 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.

package strconv

// FormatComplex converts the complex number c to a string of the
// form (a+bi) where a and b are the real and imaginary parts,
// formatted according to the format fmt and precision prec.
//
// The format fmt and precision prec have the same meaning as in [FormatFloat].
// It rounds the result assuming that the original was obtained from a complex
// value of bitSize bits, which must be 64 for complex64 and 128 for complex128.
func FormatComplex(c complex128, fmt byte, prec, bitSize int) string {
	var buf [64]byte
	return string(AppendComplex(buf[:0], c, fmt, prec, bitSize))
}

// AppendComplex appends the result of FormatComplex to dst.
// It is here for the runtime.
// There is no public strconv.AppendComplex.
func AppendComplex(dst []byte, c complex128, fmt byte, prec, bitSize int) []byte {
	if bitSize != 64 && bitSize != 128 {
		panic("invalid bitSize")
	}
	bitSize >>= 1 // complex64 uses float32 internally

	dst = append(dst, '(')
	dst = AppendFloat(dst, real(c), fmt, prec, bitSize)
	i := len(dst)
	dst = AppendFloat(dst, imag(c), fmt, prec, bitSize)
	// Check if imaginary part has a sign. If not, add one.
	if dst[i] != '+' && dst[i] != '-' {
		dst = append(dst, 0)
		copy(dst[i+1:], dst[i:])
		dst[i] = '+'
	}
	dst = append(dst, "i)"...)
	return dst
}