// Dup2 prints the count and text  of lines that appear more than once
// in the input.  It  reads from stdin or from a  list of named files.
// In Exercise  1.4. modify dup2  to print the  names of all  files in
// which each duplicated line occurs.
package main

import (
        "bufio"
        "fmt"
        "os"
)

func main() {
        counts := make(map[string]int)
        files := os.Args[1:]

        if len(files) == 0 {
                fileName := os.Stdin
                countLines(os.Stdin, counts)
        } else {
                for _, arg := range files {
                        fileName := arg
                        f, err := os.Open(arg)
                        if err != nil {
                                fmt.Fprintf(os.Stderr, "dup2: %v\n", err)
                                continue
                        }
                        countLines(f, counts)
                        f.Close()
                }
        }
        for line, n := range counts {
                if n > 1 {
                        fmt.Printf("%v: %d\t%s\n", fileName, n, line)
                }
        }
}

func countLines(f *os.File, counts map[string]int) {
        input := bufio.NewScanner(f)
        for input.Scan() {
                counts[input.Text()]++
        }
        // NOTE: ignoring potential errors from input.Err()
}