Persistent Reader() object - string

Question

In Go, I am trying to create a function that reads and processes the next line of input:
// Read a string of hex from stdin and parse to an array of bytes
func ReadHex() []byte {
r := bufio.NewReader(os.Stdin)
t, _ := r.ReadString('\n')
data, _ := hex.DecodeString(strings.TrimSpace(t))
return data
}
Unfortunately, this only works the first time it is called. It captures the first line but is unable to capture subsequent lines piped via standard input.
I suspect, if the same persistent bufio.Reader() object was used on each subsequent call, it would work but I haven't been able to achieve this without passing it manually on each function call.

Answer 1

Yes, try this:
package main
import (
"bufio"
"encoding/hex"
"fmt"
"log"
"os"
"strings"
)
func ReadFunc() func() []byte {
r := bufio.NewReader(os.Stdin)
return func() []byte {
t, err := r.ReadString('\n')
if err != nil {
log.Fatal(err)
}
data, err := hex.DecodeString(strings.TrimSpace(t))
if err != nil {
log.Fatal(err)
}
return data
}
}
func main() {
r, w, err := os.Pipe()
if err != nil {
log.Fatal(err)
}
os.Stdin = r
w.Write([]byte(`ffff
cafebabe
ff
`))
w.Close()
ReadHex := ReadFunc()
fmt.Println(ReadHex())
fmt.Println(ReadHex())
fmt.Println(ReadHex())
}
Output:
[255 255]
[202 254 186 190]
[255]
Using a struct, try this:
package main
import (
"bufio"
"encoding/hex"
"fmt"
"io"
"log"
"os"
"strings"
)
// InputReader struct
type InputReader struct {
bufio.Reader
}
// New creates an InputReader
func New(rd io.Reader) *InputReader {
return &InputReader{Reader: *bufio.NewReader(rd)}
}
// ReadHex returns a string of hex from stdin and parse to an array of bytes
func (r *InputReader) ReadHex() []byte {
t, err := r.ReadString('\n')
if err != nil {
log.Fatal(err)
}
data, err := hex.DecodeString(strings.TrimSpace(t))
if err != nil {
log.Fatal(err)
}
return data
}
func main() {
r, w, err := os.Pipe()
if err != nil {
log.Fatal(err)
}
os.Stdin = r
w.Write([]byte(`ffff
cafebabe
ff
`))
w.Close()
rdr := New(os.Stdin)
fmt.Println(rdr.ReadHex())
fmt.Println(rdr.ReadHex())
fmt.Println(rdr.ReadHex())
}