Added algorithm for color contrast ratio. (#1794)

* Added algorithm for color contrast ratio.

* Added comment and example usage in main.

* Fixed calling method in main without creating object.

* Formatted with Google Java Formatter

* Add imports for ColorContrastRatio.java

* updating DIRECTORY.md

* Updated to follow repo conventions. Undid formatting of documents.

* Follow repository conventions.

* Formatted with Google Java Formatter

* Added test method with assetions.

* Formatted with Google Java Formatter

* Update Misc/ColorContrastRatio.java

Co-authored-by: Ayaan Khan <ayaankhan98@gmail.com>

* updating DIRECTORY.md

* Correct javadocs and parameter names.

* Formatted with Google Java Formatter

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: Ayaan Khan <ayaankhan98@gmail.com>

DIRECTORY.md

@@ -129,9 +129,14 @@
   * [Average](https://github.com/TheAlgorithms/Java/blob/master/Maths/Average.java)
   * [BinaryPow](https://github.com/TheAlgorithms/Java/blob/master/Maths/BinaryPow.java)
   * [Ceil](https://github.com/TheAlgorithms/Java/blob/master/Maths/Ceil.java)
+  * [CircularConvolutionFFT](https://github.com/TheAlgorithms/Java/blob/master/Maths/CircularConvolutionFFT.java)
   * [Combinations](https://github.com/TheAlgorithms/Java/blob/master/Maths/Combinations.java)
+  * [Convolution](https://github.com/TheAlgorithms/Java/blob/master/Maths/Convolution.java)
+  * [ConvolutionFFT](https://github.com/TheAlgorithms/Java/blob/master/Maths/ConvolutionFFT.java)
   * [Factorial](https://github.com/TheAlgorithms/Java/blob/master/Maths/Factorial.java)
   * [FactorialRecursion](https://github.com/TheAlgorithms/Java/blob/master/Maths/FactorialRecursion.java)
+  * [FFT](https://github.com/TheAlgorithms/Java/blob/master/Maths/FFT.java)
+  * [FFTBluestein](https://github.com/TheAlgorithms/Java/blob/master/Maths/FFTBluestein.java)
   * [FibonacciNumber](https://github.com/TheAlgorithms/Java/blob/master/Maths/FibonacciNumber.java)
   * [FindMax](https://github.com/TheAlgorithms/Java/blob/master/Maths/FindMax.java)
   * [FindMaxRecursion](https://github.com/TheAlgorithms/Java/blob/master/Maths/FindMaxRecursion.java)
@@ -165,6 +170,7 @@
   * [MinimizingLateness](https://github.com/TheAlgorithms/Java/blob/master/MinimizingLateness/MinimizingLateness.java)
 
 ## Misc
+  * [ColorContrastRatio](https://github.com/TheAlgorithms/Java/blob/master/Misc/ColorContrastRatio.java)
   * [MedianOfRunningArray](https://github.com/TheAlgorithms/Java/blob/master/Misc/MedianOfRunningArray.java)
   * [PalindromePrime](https://github.com/TheAlgorithms/Java/blob/master/Misc/PalindromePrime.java)
   * [RangeInSortedArray](https://github.com/TheAlgorithms/Java/blob/master/Misc/RangeInSortedArray.java)

Maths/ConvolutionFFT.java

@@ -53,7 +53,7 @@ public class ConvolutionFFT {
     convolved
         .subList(convolvedSize, convolved.size())
         .clear(); // Remove the remaining zeros after the convolvedSize. These extra zeros came from
-                  // paddingPowerOfTwo() method inside the fft() method.
+    // paddingPowerOfTwo() method inside the fft() method.
 
     return convolved;
   }

Misc/ColorContrastRatio.java

@@ -0,0 +1,104 @@
+package Misc;
+
+import java.awt.Color;
+
+/**
+ * @brief A Java implementation of the offcial W3 documented procedure to calculate contrast ratio
+ *     between colors on the web. This is used to calculate the readability of a foreground color on
+ *     top of a background color.
+ * @since 2020-10-15
+ * @see [Color Contrast Ratio](https://www.w3.org/TR/WCAG20-TECHS/G17.html#G17-procedure)
+ * @author [Seth Falco](https://github.com/SethFalco)
+ */
+public class ColorContrastRatio {
+
+  /**
+   * @brief Calculates the contrast ratio between two given colors.
+   * @param a Any color, used to get the red, green, and blue values.
+   * @param b Another color, which will be compared against the first color.
+   * @return The contrast ratio between the two colors.
+   */
+  public double getContrastRatio(Color a, Color b) {
+    final double aColorLuminance = getRelativeLuminance(a);
+    final double bColorLuminance = getRelativeLuminance(b);
+
+    if (aColorLuminance > bColorLuminance)
+      return (aColorLuminance + 0.05) / (bColorLuminance + 0.05);
+
+    return (bColorLuminance + 0.05) / (aColorLuminance + 0.05);
+  }
+
+  /**
+   * @brief Calculates the relative luminance of a given color.
+   * @param color Any color, used to get the red, green, and blue values.
+   * @return The relative luminance of the color.
+   * @see [More info on relative
+   *     luminance.](https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef)
+   */
+  public double getRelativeLuminance(Color color) {
+    final double red = getColor(color.getRed());
+    final double green = getColor(color.getGreen());
+    final double blue = getColor(color.getBlue());
+
+    return 0.2126 * red + 0.7152 * green + 0.0722 * blue;
+  }
+
+  /**
+   * @brief Calculates the final value for a color to be used in the relative luminance formula as
+   *     described in step 1.
+   * @param color8Bit 8-bit representation of a color component value.
+   * @return Value for the provided color component to be used in the relative luminance formula.
+   */
+  public double getColor(int color8Bit) {
+    final double sRgb = getColorSRgb(color8Bit);
+    return (sRgb <= 0.03928) ? sRgb / 12.92 : Math.pow((sRgb + 0.055) / 1.055, 2.4);
+  }
+
+  /**
+   * @brief Calculates the Color sRGB value as denoted in step 1 of the procedure document.
+   * @param color8Bit 8-bit representation of a color component value.
+   * @return A percentile value of the color component.
+   */
+  private double getColorSRgb(double color8Bit) {
+    return color8Bit / 255.0;
+  }
+
+  /**
+   * You can check this example against another open-source implementation available on GitHub.
+   *
+   * @see [Online Contrast
+   *     Ratio](https://contrast-ratio.com/#rgb%28226%2C%20229%2C%20248-on-rgb%2823%2C%20103%2C%20154%29)
+   * @see [GitHub Repository for Online Contrast Ratio](https://github.com/LeaVerou/contrast-ratio)
+   */
+  private static void test() {
+    final ColorContrastRatio algImpl = new ColorContrastRatio();
+
+    final Color black = Color.BLACK;
+    final double blackLuminance = algImpl.getRelativeLuminance(black);
+    assert blackLuminance == 0 : "Test 1 Failed - Incorrect relative luminance.";
+
+    final Color white = Color.WHITE;
+    final double whiteLuminance = algImpl.getRelativeLuminance(white);
+    assert whiteLuminance == 1 : "Test 2 Failed - Incorrect relative luminance.";
+
+    final double highestColorRatio = algImpl.getContrastRatio(black, white);
+    assert highestColorRatio == 21 : "Test 3 Failed - Incorrect contrast ratio.";
+
+    final Color foreground = new Color(23, 103, 154);
+    final double foregroundLuminance = algImpl.getRelativeLuminance(foreground);
+    assert foregroundLuminance == 0.12215748057375966
+        : "Test 4 Failed - Incorrect relative luminance.";
+
+    final Color background = new Color(226, 229, 248);
+    final double backgroundLuminance = algImpl.getRelativeLuminance(background);
+    assert backgroundLuminance == 0.7898468477881603
+        : "Test 5 Failed - Incorrect relative luminance.";
+
+    final double contrastRatio = algImpl.getContrastRatio(foreground, background);
+    assert contrastRatio == 4.878363954846178 : "Test 6 Failed - Incorrect contrast ratio.";
+  }
+
+  public static void main(String args[]) {
+    test();
+  }
+}