captcha.go

package gosii

import (
	"encoding/base64"
	"encoding/json"
	"errors"
	"io"
	"strings"
)

// siiCaptchaURL is the URL from where the CAPTCHA is fetched.
var siiCaptchaURL = "https://zeus.sii.cl/cvc_cgi/stc/CViewCaptcha.cgi"
var ErrMaxCaptchaAttempts = errors.New("max captcha attempts reached")

type Captcha struct {
	Text     string `json:"text"`
	Solution string `json:"solution"`
}

// fetchCaptcha is responsible for retrieving a captcha challenge from the SII's service.
//
// The captcha can be reused for multiple requests, as long as the captcha is not expired.
// The captcha is considered expired after 1 minute.
//
// The method makes a POST request to the SII's captcha service with the "oper=0" payload,
// which instructs the service to generate a new captcha.
//
// The response from the service is expected to be a JSON object which includes an attribute
// "TxtCaptcha", a Base64 encoded string which represents the captcha image. The captcha text
// (i.e., the solution to the captcha) is embedded in the Base64 encoded string.
//
// This method then decodes the Base64 encoded string to retrieve the captcha text. Specifically,
// the method slices the decoded string from the 36th to 40th byte, as this slice is observed to
// represent the captcha text. This observation is based on the assumption that the structure of
// the Base64 encoded string remains consistent.
//
// The method returns a pointer to a Captcha structure, which contains the Base64 encoded string
// and the decoded captcha text, or an error if any step of the request or processing fails.
//
// Please note that this method relies on the structure of SII's captcha service and its response.
// If the service URL or the response structure changes, this method may not work as expected.
func (c *siiHTTPClient) fetchCaptcha() (*Captcha, error) {
	remAttempts := 3
	for {
		captcha, err := c.fetchCaptchaAtt()
		if err == nil && captcha != nil && captcha.Text != "" {
			return captcha, nil
		}
		remAttempts--
		if remAttempts == 0 {
			return nil, ErrMaxCaptchaAttempts
		}
	}
}

func (c *siiHTTPClient) fetchCaptchaAtt() (*Captcha, error) {
	c.requestCount.Add(1)
	resp, err := c.httpClient.Post(
		siiCaptchaURL, "application/json", strings.NewReader("oper=0"),
	)
	if err != nil {
		return nil, err
	}
	captchaResp := CaptchaResp{}
	respBody, err := io.ReadAll(resp.Body)
	if err != nil {
		return nil, err
	}
	err = json.Unmarshal(respBody, &captchaResp)
	if err != nil {
		return nil, err
	}
	txtCaptcha := captchaResp.TxtCaptcha
	return solveCaptcha(txtCaptcha)
}

func solveCaptcha(txtCaptcha string) (*Captcha, error) {
	decodedCaptcha, err := base64.StdEncoding.DecodeString(txtCaptcha)
	if err != nil {
		return nil, err
	}
	if len(decodedCaptcha) < 40 {
		return nil, err
	}
	solution := decodedCaptcha[36:40]

	return &Captcha{Text: txtCaptcha, Solution: string(solution)}, nil
}