Enhance docs, add more tests in AffineConverter (#5915)

src/main/java/com/thealgorithms/conversions/AffineConverter.java

@@ -1,23 +1,64 @@
 package com.thealgorithms.conversions;
 
+/**
+ * A utility class to perform affine transformations of the form:
+ * y = slope * x + intercept.
+ *
+ * This class supports inversion and composition of affine transformations.
+ * It is immutable, meaning each instance represents a fixed transformation.
+ */
 public final class AffineConverter {
     private final double slope;
     private final double intercept;
+
+    /**
+     * Constructs an AffineConverter with the given slope and intercept.
+     *
+     * @param inSlope The slope of the affine transformation.
+     * @param inIntercept The intercept (constant term) of the affine transformation.
+     * @throws IllegalArgumentException if either parameter is NaN.
+     */
     public AffineConverter(final double inSlope, final double inIntercept) {
+        if (Double.isNaN(inSlope) || Double.isNaN(inIntercept)) {
+            throw new IllegalArgumentException("Slope and intercept must be valid numbers.");
+        }
         slope = inSlope;
         intercept = inIntercept;
     }
 
+    /**
+     * Converts the given input value using the affine transformation:
+     * result = slope * inValue + intercept.
+     *
+     * @param inValue The input value to convert.
+     * @return The transformed value.
+     */
     public double convert(final double inValue) {
         return slope * inValue + intercept;
     }
 
+    /**
+     * Returns a new AffineConverter representing the inverse of the current transformation.
+     * The inverse of y = slope * x + intercept is x = (y - intercept) / slope.
+     *
+     * @return A new AffineConverter representing the inverse transformation.
+     * @throws AssertionError if the slope is zero, as the inverse would be undefined.
+     */
     public AffineConverter invert() {
-        assert slope != 0.0;
+        assert slope != 0.0 : "Slope cannot be zero for inversion.";
         return new AffineConverter(1.0 / slope, -intercept / slope);
     }
 
+    /**
+     * Composes this affine transformation with another, returning a new AffineConverter.
+     * If this transformation is f(x) and the other is g(x), the result is f(g(x)).
+     *
+     * @param other Another AffineConverter to compose with.
+     * @return A new AffineConverter representing the composition of the two transformations.
+     */
     public AffineConverter compose(final AffineConverter other) {
-        return new AffineConverter(slope * other.slope, slope * other.intercept + intercept);
+        double newSlope = slope * other.slope;
+        double newIntercept = slope * other.intercept + intercept;
+        return new AffineConverter(newSlope, newIntercept);
     }
 }