How to Read Unicode Files with and Without BOMs in Go?

Reading Files with BOM in Go

Question:

How can I read Unicode files containing or lacking byte-order marks (BOMs) in Go? Is there a standard method for handling this?

Answer:

Go's standard libraries do not provide a dedicated method for BOM handling. Here are two approaches to implement this functionality yourself:

Buffered Reader Approach:

The bufio package offers a convenient solution for handling BOMs. You can wrap a buffered reader around your data stream and inspect the first rune:

<code class="go">import (
    "bufio"
    "os"
)

func main() {
    fd, err := os.Open("filename")
    if err != nil {
        // Handle error
    }

    br := bufio.NewReader(fd)
    r, _, err := br.ReadRune()
    if err != nil {
        // Handle error
    }

    if r != '\uFEFF' {
        br.UnreadRune() // Not a BOM -- put the rune back
    }
}</code>

If the first rune is not a BOM, you can continue reading from the buffered reader as expected.

Seeker Interface Approach:

For objects implementing the io.Seeker interface (such as os.File), you can check the first three bytes directly and seek back to the start if there is no BOM:

<code class="go">import (
    "os"
)

func main() {
    fd, err := os.Open("filename")
    if err != nil {
        // Handle error
    }

    bom := [3]byte
    _, err = io.ReadFull(fd, bom[:])
    if err != nil {
        // Handle error
    }

    if bom[0] != 0xef || bom[1] != 0xbb || bom[2] != 0xbf {
        _, err = fd.Seek(0, 0) // Not a BOM -- seek back to the beginning
        if err != nil {
            // Handle error
        }
    }
}</code>

Note that this approach assumes UTF-8 encoding. For other encodings, more complex handling is required.

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