// When Go is encoding a particular type to JSON,
// it looks to see if the type has a MarshalJSON()
// method implemented on it. If it has, then Go will
// call this method to determine how to encode it.
package main

import (
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
)

type envelope map[string]any

// This was the original function signature without enveloping
// func (app *application) writeJSON(w http.ResponseWriter, status int, data any, headers http.Header) error {

func (app *application) writeJSON(w http.ResponseWriter, status int, data envelope, headers http.Header) error {
	// TODO set a flag for wether to indent or not. Indenting cost CPU time, more memory, and more allocs
	// approximately 65% more time 30% more mem.
	//js, err := json.Marshal(data)

	// Use the json.MarshalIndent() function so that whitespace is added to the encoded
	// JSON. Here we use no line prefix ("") and tab indents ("\t") for each element.
	js, err := json.MarshalIndent(data, "", "\t")

	if err != nil {
		return err
	}

	js = append(js, '\n')

	// At this point, we know that we won't encounter any more errors before writing the
	// response, so it's safe to add any headers that we want to include. We loop
	// through the header map and add each header to the http.ResponseWriter header map.
	// Note that it's OK if the provided header map is nil. Go doesn't throw an error
	// if you try to range over (or generally, read from) a nil map.
	for key, value := range headers {
		w.Header()[key] = value
	}

	// Add the "Content-Type: application/json" header, then write the status code and
	// JSON response.
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(status)
	w.Write(js)

	return nil
}

func (app *application) readJSON(w http.ResponseWriter, r *http.Request, dst any) error {
	// Decode the request body into the target destination.
	err := json.NewDecoder(r.Body).Decode(dst)
	if err != nil {
		// If there is an error during decoding, start the triage...
		var syntaxError *json.SyntaxError
		var unmarshalTypeError *json.UnmarshalTypeError
		var invalidUnmarshalError *json.InvalidUnmarshalError

		switch {
		// Use the errors.As() function to check whether the error has the type
		// *json.SyntaxError. If it does, then return a plain-english error message
		// which includes the location of the problem.
		case errors.As(err, &syntaxError):
			return fmt.Errorf("body contains badly-formed JSON (at character %d)", syntaxError.Offset)

		// In some circumstances Decode() may also return an io.ErrUnexpectedEOF error
		// for syntax errors in the JSON. So we check for this using errors.Is() and
		// return a generic error message. There is an open issue regarding this at
		// https://github.com/golang/go/issues/25956.
		case errors.Is(err, io.ErrUnexpectedEOF):
			return errors.New("body contains badly-formed JSON")

		// Likewise, catch any *json.UnmarshalTypeError errors. These occur when the
		// JSON value is the wrong type for the target destination. If the error relates
		// to a specific field, then we include that in our error message to make it
		// easier for the client to debug.
		case errors.As(err, &unmarshalTypeError):
			if unmarshalTypeError.Field != "" {
				return fmt.Errorf("body contains incorrect JSON type for field %q", unmarshalTypeError.Field)
			}
			return fmt.Errorf("body contains incorrect JSON type (at character %d)", unmarshalTypeError.Offset)

		// An io.EOF error will be returned by Decode() if the request body is empty. We
		// check for this with errors.Is() and return a plain-english error message
		// instead.
		case errors.Is(err, io.EOF):
			return errors.New("body must not be empty")

		// A json.InvalidUnmarshalError error will be returned if we pass something
		// that is not a non-nil pointer to Decode(). We catch this and panic,
		// rather than returning an error to our handler.
		case errors.As(err, &invalidUnmarshalError):
			panic(err)

		// For anything else, return the error message as-is.
		default:
			return err
		}
	}

	return nil
}