dup2 from The Go Programming Language (ISBN-13: 978-0134190440)

dup2_ex1_4.go

// dup2
//
// Prints the count and text of the lines that appear more than once in the input.
// It reads from stdin or from a list of named files.
//
// The original version simply printed the line and total count across all inputs.
// The exercise asked you to modify it to print all the anmes of the files in which
// each duplicated line occurs.
//
package main

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

// LineInfo is a struct for keeping track
// of line information as we process files.
type LineInfo struct {
	Files map[string]int
	Count int
}

func main() {
	counts := make(map[string]LineInfo)
	files := os.Args[1:]
	if len(files) == 0 {
		countLines(os.Stdin, counts)
	} 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)
			f.Close()
		}
	}
	for line, lineInfo := range counts {
		fmt.Printf("Line: %s\n", line)
		fmt.Printf("\tTotal Usage: %d\n", lineInfo.Count)
		for fileName, count := range lineInfo.Files {
			fmt.Printf("\t\t%s\t%d\n", fileName, count)
		}
	}
}

func countLines(f *os.File, counts map[string]LineInfo) {
	input := bufio.NewScanner(f)
	for input.Scan() {
		lineInfo := counts[input.Text()]
		lineInfo.Count++
		if lineInfo.Files == nil {
			lineInfo.Files = make(map[string]int)
		}
		lineInfo.Files[f.Name()]++
		counts[input.Text()] = lineInfo

	}
}