department-of-veterans-affairs/vets-website

const fs = require('fs');
const resemble = require('node-resemble-js');

const { getFileNames, createDirectoryIfNotExist } = require('./get-file-names');

const DIFF_THRESHOLD = 0.01;

// A wrapper around fs.readFile to return a promise
async function readFile(fileName) {
  return new Promise((resolve, reject) => {
    fs.readFile(
      fileName,
      (err, result) => (err ? reject(err) : resolve(result)),
    );
  });
}

// A wrapper around Nightwatch's screenshot function to return a promise
// Note - Nightwatch's screenshot stores the screenshot only in memory as opposed to saveScreenshot
async function takeScreenshot(browser) {
  return browser.screenshot({ fullPage: true });
}

// Compares the values for the baseline image with the screenshot of the browser's current page.
async function executeComparison(baselineImageBuffer, screenshotResult) {
  const baselineImage = Buffer.from(baselineImageBuffer, 'base64');
  const screenshot = Buffer.from(screenshotResult, 'base64');

  // Execute ResembleJS to compare the images
  return new Promise(resolve => {
    resemble(baselineImage)
      .compareTo(screenshot)
      .onComplete(resolve);
  });
}

// Writes the diff image file used in the event the baseline doesn't match the screenshot of the browser's current page.
async function createDiffImage(diffFileName, comparisonResult) {
  const diffImageStream = comparisonResult.getDiffImage();
  const writer = fs.createWriteStream(diffFileName);

  // A wrapper around the stream to return a promise
  return new Promise(resolve => {
    diffImageStream
      .pack()
      .pipe(writer)
      .once('finish', resolve);
  });
}

// After executing the comparison operation, inspect the result object to create a diff image and run the test.
async function computeComparisonResult(
  browser,
  route,
  diffFileName,
  comparisonResult,
) {
  const misMatchPercentage = parseFloat(comparisonResult.misMatchPercentage);
  const changesExceedThreshold = misMatchPercentage > DIFF_THRESHOLD;

  // Execution the test assertion

  // When the images differ, chain additional operations to create the diff image file
  if (changesExceedThreshold) {
    // Create the directory first to prevent errors
    await createDirectoryIfNotExist(diffFileName);
    // Then actually write the diff file
    await createDiffImage(diffFileName, comparisonResult);
    return {
      route,
      diffFileName,
      misMatchPercentage,
    };
  }

  // For consistency, return a resolved promise.
  return null;
}

// The entry point for this module as a route handler
async function calculateDiff(browser, route) {
  // const timeout = 5000;
  const [baselineFileName, diffFileName] = getFileNames(route);
  const fileBuffer = await readFile(baselineFileName);
  const screenshotBuffer = await takeScreenshot(browser);
  // After reading both images into memory, run the operation used to compare their contents
  const comparisonResult = await executeComparison(
    fileBuffer,
    screenshotBuffer,
  );
  return computeComparisonResult(
    browser,
    route,
    diffFileName,
    comparisonResult,
  );
}

module.exports = calculateDiff;