public class EinsteinOCRPredictionHandler {

    final static String VISION_API = 'https://api.einstein.ai/v2/vision';
    final static String OCREndpoint = VISION_API + '/ocr';
    final static String cmdtDeveloperName = 'Einstein_Vision_API_JWT';
    
    public class EinsteinOCRPredictionHandlerException extends Exception {}

    public static EinsteinOCRPredictionResponseWrapper predictViaURL(String fileURL, String detectionType, String accessToken, String model) {
        String response = detectText(fileURL, detectionType, accessToken, model, false);
        return parseResponse(response);
    }

    public static EinsteinOCRPredictionResponseWrapper predictViaBase64(String base64String, String detectionType, String accessToken, String model) {
        String response = detectText(base64String, detectionType, accessToken, model, true);
        return parseResponse(response);
    }
    
    public static EinsteinOCRPredictionResponseWrapper predictViaBlob(Blob fileBlob, String detectionType, String accessToken, String model) {
        String response = detectText(EncodingUtil.base64Encode(fileBlob), detectionType, accessToken, model, true);
        return parseResponse(response);
    }
    
    public static String detectText(String sample, String detectionType, String accessToken, String model, boolean isBase64) {
        string contentType = HttpFormBuilder.GetContentType();
        //  Compose the form
        string form64 = '';
        form64 += HttpFormBuilder.WriteBoundary();
        form64 += HttpFormBuilder.WriteBodyParameter('modelId', EncodingUtil.urlEncode(model, 'UTF-8'));
        form64 += HttpFormBuilder.WriteBoundary();
        form64 += HttpFormBuilder.WriteBodyParameter('task', EncodingUtil.urlEncode(detectionType, 'UTF-8'));
        form64 += HttpFormBuilder.WriteBoundary();
        if(isBase64) {
            form64 += HttpFormBuilder.WriteBodyParameter('sampleBase64Content', sample);
        } else {
            form64 += HttpFormBuilder.WriteBodyParameter('sampleLocation', sample);
        }
        form64 += HttpFormBuilder.WriteBoundary(HttpFormBuilder.EndingType.CrLf);
        blob formBlob = EncodingUtil.base64Decode(form64);
        string contentLength = string.valueOf(formBlob.size());
        //  Compose the http request
        HttpRequest httpRequest = new HttpRequest();
        httpRequest.setBodyAsBlob(formBlob);
        httpRequest.setHeader('Connection', 'keep-alive');
        httpRequest.setHeader('Content-Length', contentLength);
        httpRequest.setHeader('Content-Type', contentType); 
        httpRequest.setMethod('POST');
        //httpRequest.setTimeout(120000);
        httpRequest.setHeader('Authorization','Bearer ' + accessToken);
        httpRequest.setEndpoint(OCREndpoint);
        
        Http http = new Http();
        HTTPResponse res = http.send(httpRequest);
        System.debug(res.getStatus());
        String responseBody;
        Integer statusCode = res.getStatusCode();
        String requestStatus = res.getStatus();
        if ( statusCode == 200) {
            responseBody = res.getBody();
        } else {
            throw new EinsteinOCRPredictionHandlerException('Callout unsuccessful! Status code: '+statusCode + '. Status: '+requestStatus);
        }
        return responseBody;
    }

    static EinsteinOCRPredictionResponseWrapper parseResponse(String response) {
        if(String.isEmpty(response)) {
            throw new EinsteinOCRPredictionHandlerException('Empty response received');
        }
        EinsteinOCRPredictionResponseWrapper obj = new EinsteinOCRPredictionResponseWrapper();
        obj = (EinsteinOCRPredictionResponseWrapper)JSON.deserialize(response, EinsteinOCRPredictionResponseWrapper.class);
        return obj;
    }
}

public class HttpFormBuilder {
    //  The boundary is alligned so it doesn't produce padding characters when base64 encoded.
    private final static string Boundary = '1ff13444ed8140c7a32fc4e6451aa76d';

    /**
     *  Returns the request's content type for multipart/form-data requests.
     */
    public static string GetContentType() {
        return 'multipart/form-data; charset="UTF-8"; boundary="' + Boundary + '"';
    }

    /**
     *  Pad the value with spaces until the base64 encoding is no longer padded.
     */
    private static string SafelyPad(
        string value,
        string valueCrLf64,
        string lineBreaks) {
        string valueCrLf = '';
        blob valueCrLfBlob = null;

        while (valueCrLf64.endsWith('=')) {
            value += ' ';
            valueCrLf = value + lineBreaks;
            valueCrLfBlob = blob.valueOf(valueCrLf);
            valueCrLf64 = EncodingUtil.base64Encode(valueCrLfBlob);
        }

        return valueCrLf64;
    }

    /**
     *  Write a boundary between parameters to the form's body.
     */
    public static string WriteBoundary() {
        string value = '--' + Boundary + '\r\n';
        blob valueBlob = blob.valueOf(value);

        return EncodingUtil.base64Encode(valueBlob);
    }

    /**
     *  Write a boundary at the end of the form's body.
     */
    public static string WriteBoundary(
        EndingType ending) {
        string value = '';

        if (ending == EndingType.Cr) {
            //  The file's base64 was padded with a single '=',
            //  so it was replaced with '\r'. Now we have to
            //  prepend the boundary with '\n' to complete
            //  the line break.
            value += '\n';
        } else if (ending == EndingType.None) {
            //  The file's base64 was not padded at all,
            //  so we have to prepend the boundary with
            //  '\r\n' to create the line break.
            value += '\r\n';
        }
        //  Else:
        //  The file's base64 was padded with a double '=',
        //  so they were replaced with '\r\n'. We don't have to
        //  do anything to the boundary because there's a complete
        //  line break before it.

        value += '--' + Boundary + '--';

        blob valueBlob = blob.valueOf(value);

        return EncodingUtil.base64Encode(valueBlob);
    }

    /**
     *  Write a key-value pair to the form's body.
     */
    public static string WriteBodyParameter(
        string key,
        string value) {
        string contentDisposition = 'Content-Disposition: form-data; name="' + key + '"';
        string contentDispositionCrLf = contentDisposition + '\r\n\r\n';
        blob contentDispositionCrLfBlob = blob.valueOf(contentDispositionCrLf);
        string contentDispositionCrLf64 = EncodingUtil.base64Encode(contentDispositionCrLfBlob);
        string content = SafelyPad(contentDisposition, contentDispositionCrLf64, '\r\n\r\n');
        string valueCrLf = value + '\r\n';
        blob valueCrLfBlob = blob.valueOf(valueCrLf);
        string valueCrLf64 = EncodingUtil.base64Encode(valueCrLfBlob);

        content += SafelyPad(value, valueCrLf64, '\r\n');

        return content;
    }

    /**
     *  Helper enum indicating how a file's base64 padding was replaced.
     */
    public enum EndingType {
        Cr,
        CrLf,
        None
    }
}