My Go Journey

func percentage(input []byte, letter byte) float64 {
    var letterCount int
    for _, b := range input {
        if b == letter {
            letterCount++
        }
    }
    return 100.0 * float64(letterCount) / float64(len(input))
}

func main() {
    const requestedLetter = 'a'
    const dataDir = "../data"
    files, _ := os.ReadDir(dataDir)
    for _, file := range files {
        filePath := path.Join(dataDir, file.Name())
        fileContents, err := os.ReadFile(filePath)
        if err != nil { /* error handling */ }
        pct := percentage(fileContents, requestedLetter)
        fmt.Printf("%f%% of %q in file %s\n", pct, requestedLetter, filePath)
    }
}

    requestedLetterIn := flag.String("l", "", "letter that we want to count")
    dataDir := flag.String("d", "", "the directory")
    flag.Parse()
    if len(*requestedLetterIn) != 1 {
        return
    }
    requestedLetter := (*requestedLetterIn)[0]

        f, err := os.Open(filePath)
        if err != nil { /* error handling */ }
        pct := percentage(f, requestedLetter)
        if err = f.Close(); err != nil { /* error handling */ }
        fmt.Printf("%f%% of %q in file %s\n", pct, requestedLetter, filePath)

func percentage(r io.Reader, letter byte) float64 {
    var letterCount, allCount int
    bufR := bufio.NewReader(r)
    for {
        b, err := bufR.ReadByte()
        if err != nil {
            break
        }
        if b == letter {
            letterCount++
        }
        allCount++
    }
    return 100.0 * float64(letterCount) / float64(allCount)
}

    filePathCh := make(chan string)
    var workerWg sync.WaitGroup
    // We create a worker pool and thereby limit the maximal number of go routines
    for i := 0; i < runtime.NumCPU(); i++ {
        workerWg.Add(1)
        go func(workerNo int) {
            defer workerWg.Done()
            for filePath := range filePathCh {
                f, err := os.Open(filePath)
                if err != nil { /* error handling */
                }
                pct := percentage(f, requestedLetter)
                if err = f.Close(); err != nil { /* error handling */
                }
                fmt.Printf("%f%% of %q in file %s processed by worker %d\n",
                    pct, requestedLetter, filePath, workerNo)
            }
        }(i)
    }
    for _, file := range files {
        filePathCh <- path.Join(*dataDir, file.Name())
    }
    close(filePathCh)
    workerWg.Wait()