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: metaphor.go
    path of file: ./repos/langchaingo/tools/metaphor
    
    the code of the file until where you have to start completion: package metaphor

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"regexp"
	"strings"

	"github.com/metaphorsystems/metaphor-go"
	"github.com/tmc/langchaingo/tools"
)

var _ tools.Tool = &API{}

// API defines a tool implementation for the Metaphor API.
type API struct {
	client *metaphor.Client
}

// ToolInput defines a struct the tool expects as input.
type ToolInp
    
func (tool *API) Call(ctx context.Context, input string) (string, error) {
	var toolInput ToolInput

	re := regexp.MustCompile(`(?s)\{.*\}`)
	jsonString := re.FindString(input)

	err := json.Unmarshal([]byte(jsonString), &toolInput)
	if err != nil {
		return "", err
	}

	switch toolInput.Operation {
	case "Search":
		return tool.performSearch(ctx, toolInput)
	case "FindSimilar":
		return tool.findSimilar(ctx, toolInput)
	case "GetContents":
		return tool.getContents(ctx, toolInput)
	}

	return "", nil
}