til/go/write-a-custom-scan-function-for-file-io.md

Write A Custom Scan Function For File IO

By default a bufio.Scanner will scan input line-by-line. In other words, splitting on newlines such that each iteration will emit everything up to the next newline character.

We can write our own SplitFunc and override the default one by calling scanner.Split with it. Our custom scan function needs to match the type signature of SplitFunc.

Here is a custom one that emits each individual character but omits the newlines.

func ScanChar(data []byte, atEOF bool) (int, []byte, error) {
	if atEOF || len(data) == 0 {
		return 0, nil, nil
	}

	start := 0
	for start < len(data) {
		if !utf8.FullRune(data[start:]) {
			return 0, nil, nil
		}

		r, size := utf8.DecodeRune(data[start:])
		if r == utf8.RuneError {
			return 0, nil, fmt.Errorf("invalid UTF-8 encoding")
		}

		if r != '\n' {
			return start + size, data[start:start+size], nil
		}

		// found a \n, advance the start position
		start += size
	}

	return start, nil, nil
}

We can then use thi ScanChar function with a bufio.Scanner like so:

func ReadFileByCharacter(file io.Reader) {
	scanner := bufio.NewScanner(file)

	// override default SplitFunc
	scanner.Split(scanChar)

	for scanner.Scan() {
		char := scanner.Text()

		fmt.Printf("- %s\n", char)
	}
}