Exercise 1.4 from "The Go Programming Language." Prints duplicate lines along with the file name and number of occurrences

duplicates.go

 /* 	To Run: go run duplicates.go <file1 file2 file3 ...>
 
	Example Output:
	test was found in the following files:
	tester1.txt    (3 times)
	-----
	asdf was found in the following files:
	tester2.txt   (5 times)
	tester3.txt   (2 times)
	-----
*/
package main

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

func main() {
	counts := make(map[string]map[string]int)
	files := os.Args[1:]
	if len(files) == 0 {
		countLines(os.Stdin, counts, "os.Stdin")
	} else {
		for _, arg := range files {
			f, err := os.Open(arg)
			if err != nil {
				fmt.Fprintf(os.Stderr, "dup2: %v\n", err)
				continue
			}
			countLines(f, counts, arg)
			f.Close()
		}
	}

	for line, numCount := range counts {
		total := 0
		for _, counter := range numCount {
			if counter > 1 {
				total++
			}
		}

		if total > 0 {
			fmt.Printf("-----\n%s was found in the following files:\n", line)
		}

		for file, counter := range numCount {
			if counter > 1 {
				fmt.Printf("%s\t(%d times)\n", file, counter)
			}
		}
	}
}

func countLines(f *os.File, counts map[string]map[string]int, filename string) {
	input := bufio.NewScanner(f)
	for input.Scan() {
		line := input.Text()
		if line == "" {
			continue
		}

		if counts[line] == nil {
			counts[line] = make(map[string]int)
		}

		counts[line][filename]++
	}
}