You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: codec.go
    path of file: ./repos/ko/vendor/github.com/spf13/viper/internal/encoding/dotenv
    
    the code of the file until where you have to start completion: package dotenv

import (
	"bytes"
	"fmt"
	"sort"
	"strings"

	"github.com/subosito/gotenv"
)

const keyDelimiter = "_"

// Codec implements the encoding.Encoder and encoding.Decoder interfaces for
    
func (Codec) Encode(v map[string]any) ([]byte, error) {
	flattened := map[string]any{}

	flattened = flattenAndMergeMap(flattened, v, "", keyDelimiter)

	keys := make([]string, 0, len(flattened))

	for key := range flattened {
		keys = append(keys, key)
	}

	sort.Strings(keys)

	var buf bytes.Buffer

	for _, key := range keys {
		_, err := buf.WriteString(fmt.Sprintf("%v=%v\n", strings.ToUpper(key), flattened[key]))
		if err != nil {
			return nil, err
		}
	}

	return buf.Bytes(), nil
}