How can you prepend data to a buffer in Golang?

Appending and Prepending to a Buffer in Golang

When working with buffers in Golang, it's common to append data to the end of the buffer using methods like WriteString. However, in certain scenarios, it may be necessary to write to the beginning of a buffer.

Modifying the Buffer Internally

Since the underlying buf slice in bytes.Buffer is not exported, it's not possible to directly modify the buffer contents. To work around this, you can follow these steps:

1. Append the data you want to prepend to the end of the buffer, as seen in the example:

buffer.WriteString("B")

2. Extract the resulting buffer's contents as a string:

s := buffer.String()

3. Reset the buffer to clear its contents:

buffer.Reset()

4. Write the prepended data back to the beginning of the buffer, followed by the original string:

buffer.WriteString("A" + s)

This solution effectively prepends data to the buffer.

Example and Output

The following Go Playground code demonstrates this technique:

package main

import (
    "bytes"
    "fmt"
)

func main() {
    var buffer bytes.Buffer
    buffer.WriteString("B")
    s := buffer.String()
    buffer.Reset()
    buffer.WriteString("A" + s)
    fmt.Println(buffer.String())
}

Running the code above yields the output:

AB

In this example, the letter 'A' is prepended to the 'B' initially written to the buffer, resulting in the string "AB."

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