json_preprocessor.go

package learn

import (
	"io"
)

type stripControlCharactersReader struct {
	hasPrecedingBackslash bool
	wrapped               io.Reader
}

func newStripControlCharactersReader(wrapped io.Reader) *stripControlCharactersReader {
	return &stripControlCharactersReader{wrapped: wrapped}
}

// Read up to len(p) bytes, removing any control characters found.
// Removed characters do not count toward the total bytes read.
//
// If the control character is preceded by a backslash, it is replaced with a
// backslash rather than removed, e.g. "\<CR>" becomes "\\".  This prevents
// the JSON parser from applying the backslash to escape the next character.
//
// Returns the number of bytes read.
func (r *stripControlCharactersReader) Read(p []byte) (n int, err error) {
	pIdx := 0
	buf := make([]byte, len(p))

	// Read up to len(p) bytes, then discard any control characters.  Continue
	// reading (and discarding control characters) until p is full or there are
	// no more bytes to read.
	for pIdx < len(p) {
		remaining := len(p) - pIdx
		bufSlice := buf[:remaining]

		var bufN int
		bufN, err = r.wrapped.Read(bufSlice)

		// Copy from buf to p, skipping control characters.
		for _, c := range bufSlice[:bufN] {
			toWrite := c

			if c <= 0x1f {
				if r.hasPrecedingBackslash {
					// If the control character is escaped, replace it with a
					// backslash.
					toWrite = '\\'
				} else {
					// Otherwise, just remove it.
					continue
				}
			}

			if c == '\\' {
				r.hasPrecedingBackslash = !r.hasPrecedingBackslash
			} else {
				r.hasPrecedingBackslash = false
			}

			p[pIdx] = toWrite
			pIdx += 1
		}

		// If we hit an error or read fewer bytes than the size of the buffer,
		// don't bother trying to read more from the underlying reader.
		if err != nil || bufN < remaining {
			break
		}

		// Otherwise, we loop to replace CRs we dropped.
	}

	return pIdx, err
}