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package bom

import (
	"bytes"
	"io"
	"io/ioutil"
)

const (
	bom0 = 0xef
	bom1 = 0xbb
	bom2 = 0xbf
)

// CleanBom returns b with the 3 byte BOM stripped off the front if it is present.
// If the BOM is not present, then b is returned.
func CleanBom(b []byte) []byte {
	if len(b) >= 3 &&
		b[0] == bom0 &&
		b[1] == bom1 &&
		b[2] == bom2 {
		return b[3:]
	}
	return b
}

// NewReaderWithoutBom returns an io.Reader that will skip over initial UTF-8 byte order marks.
func NewReaderWithoutBom(r io.Reader) (io.Reader, error) {
	bs, err := ioutil.ReadAll(r)
	if err != nil {
		return nil, err
	}
	return bytes.NewReader(CleanBom(bs)), nil
}