refactor: Enhance docs, code, add tests in `LinearDiophantineEquation…

src/main/java/com/thealgorithms/maths/LinearDiophantineEquationsSolver.java

@@ -2,20 +2,77 @@
 
 import java.util.Objects;
 
+/**
+ * A solver for linear Diophantine equations of the form ax + by = c.
+ * <p>
+ * A linear Diophantine equation is an equation in which only integer solutions
+ * are allowed.
+ * This solver uses the Extended Euclidean Algorithm to find integer solutions
+ * (x, y)
+ * for equations of the form ax + by = c, where a, b, and c are integers.
+ * </p>
+ * <p>
+ * The equation has solutions if and only if gcd(a, b) divides c.
+ * If solutions exist, this solver finds one particular solution.
+ * </p>
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Diophantine_equation">Diophantine
+ *      Equation</a>
+ * @see <a href=
+ *      "https://en.wikipedia.org/wiki/Extended_Euclidean_algorithm">Extended
+ *      Euclidean Algorithm</a>
+ */
 public final class LinearDiophantineEquationsSolver {
     private LinearDiophantineEquationsSolver() {
     }
 
+    /**
+     * Demonstrates the solver with a sample equation: 3x + 4y = 7.
+     *
+     * @param args command line arguments (not used)
+     */
     public static void main(String[] args) {
         // 3x + 4y = 7
         final var toSolve = new Equation(3, 4, 7);
         System.out.println(findAnySolution(toSolve));
     }
 
+    /**
+     * Finds any integer solution to the linear Diophantine equation ax + by = c.
+     * <p>
+     * The method returns one of three types of solutions:
+     * <ul>
+     * <li>A specific solution (x, y) if solutions exist</li>
+     * <li>{@link Solution#NO_SOLUTION} if no integer solutions exist</li>
+     * <li>{@link Solution#INFINITE_SOLUTIONS} if the equation is 0x + 0y = 0</li>
+     * </ul>
+     * </p>
+     *
+     * @param equation the linear Diophantine equation to solve
+     * @return a Solution object containing the result
+     * @throws NullPointerException if equation is null
+     */
     public static Solution findAnySolution(final Equation equation) {
         if (equation.a() == 0 && equation.b() == 0 && equation.c() == 0) {
             return Solution.INFINITE_SOLUTIONS;
         }
+        if (equation.a() == 0 && equation.b() == 0) {
+            return Solution.NO_SOLUTION;
+        }
+        if (equation.a() == 0) {
+            if (equation.c() % equation.b() == 0) {
+                return new Solution(0, equation.c() / equation.b());
+            } else {
+                return Solution.NO_SOLUTION;
+            }
+        }
+        if (equation.b() == 0) {
+            if (equation.c() % equation.a() == 0) {
+                return new Solution(equation.c() / equation.a(), 0);
+            } else {
+                return Solution.NO_SOLUTION;
+            }
+        }
         final var stub = new GcdSolutionWrapper(0, new Solution(0, 0));
         final var gcdSolution = gcd(equation.a(), equation.b(), stub);
         if (equation.c() % gcdSolution.getGcd() != 0) {
@@ -29,43 +86,100 @@ public static Solution findAnySolution(final Equation equation) {
         return toReturn;
     }
 
+    /**
+     * Computes the GCD of two integers using the Extended Euclidean Algorithm.
+     * <p>
+     * This method also finds coefficients x and y such that ax + by = gcd(a, b).
+     * The coefficients are stored in the 'previous' wrapper object.
+     * </p>
+     *
+     * @param a        the first integer
+     * @param b        the second integer
+     * @param previous a wrapper to store the solution coefficients
+     * @return a GcdSolutionWrapper containing the GCD and coefficients
+     */
     private static GcdSolutionWrapper gcd(final int a, final int b, final GcdSolutionWrapper previous) {
         if (b == 0) {
             return new GcdSolutionWrapper(a, new Solution(1, 0));
         }
         // stub wrapper becomes the `previous` of the next recursive call
         final var stubWrapper = new GcdSolutionWrapper(0, new Solution(0, 0));
-        final var next = /* recursive call */ gcd(b, a % b, stubWrapper);
+        final var next = gcd(b, a % b, stubWrapper);
         previous.getSolution().setX(next.getSolution().getY());
         previous.getSolution().setY(next.getSolution().getX() - (a / b) * (next.getSolution().getY()));
         previous.setGcd(next.getGcd());
         return new GcdSolutionWrapper(next.getGcd(), previous.getSolution());
     }
 
+    /**
+     * Represents a solution (x, y) to a linear Diophantine equation.
+     * <p>
+     * Special instances:
+     * <ul>
+     * <li>{@link #NO_SOLUTION} - indicates no integer solutions exist</li>
+     * <li>{@link #INFINITE_SOLUTIONS} - indicates infinitely many solutions
+     * exist</li>
+     * </ul>
+     * </p>
+     */
     public static final class Solution {
 
+        /**
+         * Singleton instance representing the case where no solution exists.
+         */
         public static final Solution NO_SOLUTION = new Solution(Integer.MAX_VALUE, Integer.MAX_VALUE);
+
+        /**
+         * Singleton instance representing the case where infinite solutions exist.
+         */
         public static final Solution INFINITE_SOLUTIONS = new Solution(Integer.MIN_VALUE, Integer.MIN_VALUE);
+
         private int x;
         private int y;
 
+        /**
+         * Constructs a solution with the given x and y values.
+         *
+         * @param x the x coordinate of the solution
+         * @param y the y coordinate of the solution
+         */
         public Solution(int x, int y) {
             this.x = x;
             this.y = y;
         }
 
+        /**
+         * Gets the x value of this solution.
+         *
+         * @return the x value
+         */
         public int getX() {
             return x;
         }
 
+        /**
+         * Gets the y value of this solution.
+         *
+         * @return the y value
+         */
         public int getY() {
             return y;
         }
 
+        /**
+         * Sets the x value of this solution.
+         *
+         * @param x the new x value
+         */
         public void setX(int x) {
             this.x = x;
         }
 
+        /**
+         * Sets the y value of this solution.
+         *
+         * @param y the new y value
+         */
         public void setY(int y) {
             this.y = y;
         }
@@ -95,14 +209,35 @@ public String toString() {
         }
     }
 
+    /**
+     * Represents a linear Diophantine equation of the form ax + by = c.
+     *
+     * @param a the coefficient of x
+     * @param b the coefficient of y
+     * @param c the constant term
+     */
     public record Equation(int a, int b, int c) {
     }
 
+    /**
+     * A wrapper class that holds both the GCD and the solution coefficients
+     * from the Extended Euclidean Algorithm.
+     * <p>
+     * This class is used internally to pass results between recursive calls
+     * of the GCD computation.
+     * </p>
+     */
     public static final class GcdSolutionWrapper {
 
         private int gcd;
         private Solution solution;
 
+        /**
+         * Constructs a GcdSolutionWrapper with the given GCD and solution.
+         *
+         * @param gcd      the greatest common divisor
+         * @param solution the solution coefficients
+         */
         public GcdSolutionWrapper(int gcd, Solution solution) {
             this.gcd = gcd;
             this.solution = solution;
@@ -120,18 +255,38 @@ public boolean equals(Object obj) {
             return (this.gcd == that.gcd && Objects.equals(this.solution, that.solution));
         }
 
+        /**
+         * Gets the GCD value.
+         *
+         * @return the GCD
+         */
         public int getGcd() {
             return gcd;
         }
 
+        /**
+         * Sets the GCD value.
+         *
+         * @param gcd the new GCD value
+         */
         public void setGcd(int gcd) {
             this.gcd = gcd;
         }
 
+        /**
+         * Gets the solution coefficients.
+         *
+         * @return the solution
+         */
         public Solution getSolution() {
             return solution;
         }
 
+        /**
+         * Sets the solution coefficients.
+         *
+         * @param solution the new solution
+         */
         public void setSolution(Solution solution) {
             this.solution = solution;
         }