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: expand.go
    path of file: ./repos/sealer/vendor/sigs.k8s.io/kustomize/api/filters/refvar
    
    the code of the file until where you have to start completion: // Copyright 2019 The Kubernetes Authors.
// SPDX-License-Identifier: Apache-2.0

package refvar

import (
	"fmt"
	"log"
	"strings"
)

const (
	operator        = '$'
	referenceOpener = '('
	referenceCloser = ')'
)

// syntaxWrap returns the input string wrapped by the expansion syntax.
func syntaxWrap(input string) string {
	var sb strings.Builder
	sb.W
    
func DoReplacements(input string, mapping MappingFunc) interface{} {
	var buf strings.Builder
	checkpoint := 0
	for cursor := 0; cursor < len(input); cursor++ {
		if input[cursor] == operator && cursor+1 < len(input) {
			// Copy the portion of the input string since the last
			// checkpoint into the buffer
			buf.WriteString(input[checkpoint:cursor])

			// Attempt to read the variable name as defined by the
			// syntax from the input string
			read, isVar, advance := tryReadVariableName(input[cursor+1:])

			if isVar {
				// We were able to read a variable name correctly;
				// apply the mapping to the variable name and copy the
				// bytes into the buffer
				mapped := mapping(read)
				if input == syntaxWrap(read) {
					// Preserve the type of variable
					return mapped
				}

				// Variable is used in a middle of a string
				buf.WriteString(fmt.Sprintf("%v", mapped))
			} else {
				// Not a variable name; copy the read bytes into the buffer
				buf.WriteString(read)
			}

			// Advance the cursor in the input string to account for
			// bytes consumed to read the variable name expression
			cursor += advance

			// Advance the checkpoint in the input string
			checkpoint = cursor + 1
		}
	}

	// Return the buffer and any remaining unwritten bytes in the
	// input string.
	return buf.String() + input[checkpoint:]
}