// ## Line Filter

// A line filter program reads input on stdin,
// processes it, and prints results to stdout.
// Here's an example line filter that writes
// a capitalized version of all text it reads.
package main

// Package `bufio` will help us read line-by-line.
import "bufio"                              
import "bytes"
import "os"
import "io"

// We'll need to add our own newlines between
// processed lines.
func main() {                               
    newline := []byte("\n")                

    // The buffered reader gives us `ReadLine`.
    in  := bufio.NewReader(os.Stdin)        
    out := os.Stdout

    // If successful, each `ReadLine` returns bytes and a
    // boolean indicating if don't have the whole line yet.
    for {                                   
        inBytes, pfx, err := in.ReadLine()  
        
        // The `EOF` error is expected when we reach the end
        // of the input, so exit gracefully in that case.
        // Otherwise there is a problem.
        if err == io.EOF {                  
            return                          
        }                                   
        if err != nil {
            panic (err)
        }       

        // `bytes.ToUpper` works directly on bytes, so we don't
        // need to convert to a string for `strings.ToUpper`.
        outBytes := bytes.ToUpper(inBytes)  
                                            
        // Write out the upercased bytes, checking for an error
        // here as well.
        _, err = out.Write(outBytes)        
        if err != nil {                    
            panic(err)
        }

        // Unless this read was for a prefix (not the full
        // line), we need to add our own newline.
        if !pfx {                           
            out.Write(newline)              
        }        
    }
}