// Copyright 2013 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import 'dart:io' as io;
import 'package:image/image.dart';

/// How to compares pixels within the image.
///
/// Keep this enum in sync with the one defined in `golden_tester.dart`.
enum PixelComparison {
  /// Allows minor blur and anti-aliasing differences by comparing a 3x3 grid
  /// surrounding the pixel rather than direct 1:1 comparison.
  fuzzy,

  /// Compares one pixel at a time.
  ///
  /// Anti-aliasing or blur will result in higher diff rate.
  precise,
}

void main(List<String> args) {
  final io.File fileA = io.File(args[0]);
  final io.File fileB = io.File(args[1]);
  final Image imageA = decodeNamedImage(fileA.readAsBytesSync(), 'a.png')!;
  final Image imageB = decodeNamedImage(fileB.readAsBytesSync(), 'b.png')!;
  final ImageDiff diff = ImageDiff(
      golden: imageA, other: imageB, pixelComparison: PixelComparison.fuzzy);
  print('Diff: ${(diff.rate * 100).toStringAsFixed(4)}%');
}

/// This class encapsulates visually diffing an Image with any other.
/// Both images need to be the exact same size.
class ImageDiff {
  /// The image to match
  final Image golden;

  /// The image being compared
  final Image other;

  /// Algorithm used for comparing pixels.
  final PixelComparison pixelComparison;

  /// The output of the comparison
  /// Pixels in the output image can have 3 different colors depending on the comparison
  /// between golden pixels and other pixels:
  ///  * white: when both pixels are the same
  ///  * red: when a pixel is found in other, but not in golden
  ///  * green: when a pixel is found in golden, but not in other
  late Image diff;

  /// The ratio of wrong pixels to all pixels in golden (between 0 and 1)
  /// This gets set to 1 (100% difference) when golden and other aren't the same size.
  double get rate => _wrongPixels / _pixelCount;

  /// Image diff constructor which requires two [Image]s to compare and
  /// [PixelComparison] algorithm.
  ImageDiff({
    required this.golden,
    required this.other,
    required this.pixelComparison,
  }) {
    _computeDiff();
  }

  int _pixelCount = 0;
  int _wrongPixels = 0;

  /// That would be the distance between black and white.
  static final double _maxTheoreticalColorDistance = Color.distance(
    <num>[255, 255, 255], // white
    <num>[0, 0, 0], // black
    false,
  ).toDouble();

  // If the normalized color difference of a pixel is greater than this number,
  // we consider it a wrong pixel.
  static const double _kColorDistanceThreshold = 0.1;

  final int _colorOk = Color.fromRgb(255, 255, 255);
  final int _colorBadPixel = Color.fromRgb(255, 0, 0);
  final int _colorExpectedPixel = Color.fromRgb(0, 255, 0);

  /// Reads a pixel value out of [image] at [x] and [y].
  ///
  /// If the pixel is out of bounds, reflects the [x] and [y] coordinates off
  /// the border back into the image treating the border like a mirror.
  static int _reflectedPixel(Image image, int x, int y) {
    x = x.abs();
    if (x == image.width) {
      x = image.width - 2;
    }

    y = y.abs();
    if (y == image.height) {
      y = image.height - 2;
    }

    return image.getPixel(x, y);
  }

  static int _average(Iterable<int> values) {
    return values.reduce((int a, int b) => a + b) ~/ values.length;
  }

  /// The value of the pixel at [x] and [y] coordinates.
  ///
  /// If [pixelComparison] is [PixelComparison.precise], reads the RGB value of
  /// the pixel.
  ///
  /// If [pixelComparison] is [PixelComparison.fuzzy], reads the RGB values of
  /// the average of the 3x3 box of pixels centered at [x] and [y].
  List<int> _getPixelRgbForComparison(Image image, int x, int y) {
    switch (pixelComparison) {
      case PixelComparison.fuzzy:
        final List<int> pixels = <int>[
          _reflectedPixel(image, x - 1, y - 1),
          _reflectedPixel(image, x - 1, y),
          _reflectedPixel(image, x - 1, y + 1),
          _reflectedPixel(image, x, y - 1),
          _reflectedPixel(image, x, y),
          _reflectedPixel(image, x, y + 1),
          _reflectedPixel(image, x + 1, y - 1),
          _reflectedPixel(image, x + 1, y),
          _reflectedPixel(image, x + 1, y + 1),
        ];
        return <int>[
          _average(pixels.map((int p) => getRed(p))),
          _average(pixels.map((int p) => getGreen(p))),
          _average(pixels.map((int p) => getBlue(p))),
        ];
      case PixelComparison.precise:
        final int pixel = image.getPixel(x, y);
        return <int>[
          getRed(pixel),
          getGreen(pixel),
          getBlue(pixel),
        ];
      default:
        throw 'Unrecognized pixel comparison value: $pixelComparison';
    }
  }

  void _computeDiff() {
    final int goldenWidth = golden.width;
    final int goldenHeight = golden.height;

    _pixelCount = goldenWidth * goldenHeight;
    diff = Image(goldenWidth, goldenHeight);

    if (goldenWidth == other.width && goldenHeight == other.height) {
      for (int y = 0; y < goldenHeight; y++) {
        for (int x = 0; x < goldenWidth; x++) {
          final bool isExactlySame =
              golden.getPixel(x, y) == other.getPixel(x, y);
          final List<int> goldenPixel = _getPixelRgbForComparison(golden, x, y);
          final List<int> otherPixel = _getPixelRgbForComparison(other, x, y);
          final double colorDistance =
              Color.distance(goldenPixel, otherPixel, false) /
                  _maxTheoreticalColorDistance;
          final bool isFuzzySame = colorDistance < _kColorDistanceThreshold;
          if (isExactlySame || isFuzzySame) {
            diff.setPixel(x, y, _colorOk);
          } else {
            final int goldenLuminance =
                getLuminanceRgb(goldenPixel[0], goldenPixel[1], goldenPixel[2]);
            final int otherLuminance =
                getLuminanceRgb(otherPixel[0], otherPixel[1], otherPixel[2]);
            if (goldenLuminance < otherLuminance) {
              diff.setPixel(x, y, _colorExpectedPixel);
            } else {
              diff.setPixel(x, y, _colorBadPixel);
            }
            _wrongPixels++;
          }
        }
      }
    } else {
      // Images are completely different resolutions. Bail out big time.
      _wrongPixels = _pixelCount;
    }
  }
}

/// Returns text explaining pixel difference rate.
String getPrintableDiffFilesInfo(double diffRate, double maxRate) =>
    '(${(diffRate * 100).toStringAsFixed(4)}% of pixels were different. '
    'Maximum allowed rate is: ${(maxRate * 100).toStringAsFixed(4)}%).';