Filtered readers w/out extra buffer allocations (golang)

streamingio.go

package main

import (
	"fmt"
	"io"
	"log"
	"strings"
)

func main() {

  // FilterReader filters out non-alphanumeric bytes.
	reader := newFilterReader(strings.NewReader("a2bc123basgh1241vb113f"))

	p := make([]byte, 7) // constant memory pressure achieved via buffer limit
	for {
		n, err := reader.Read(p)
		if err != nil {
			if err != io.EOF {
				log.Panic(err)
			}
			break
		}
		fmt.Println(string(p[:n]))
	}
}

type filterReader struct {
	reader io.Reader
}

func newFilterReader(reader io.Reader) *filterReader {
	return &filterReader{reader}
}

func (r *filterReader) Read(p []byte) (int, error) {
	n, err := r.reader.Read(p)
	if err != nil {
		return n, err
	}
	// re-use p by packing bytes into buffer
	j := 0
	for i := 0; i < n; i++ {
		if char := alpha(p[i]); char != 0 {
			p[j] = char
			j++
		}
	}
	if j == 0 {
		return 0, io.EOF
	}
	return j, nil
}

func alpha(r byte) byte {
	if (r >= 'A' && r <= 'Z') || (r >= 'a' && r <= 'z') {
		return r
	}
	return 0
}