Implement fraction comparison using cross multiplication

java-ejercicios/src/main/java/mathbook/AppMathBook.java

@@ -1,33 +1,76 @@
 package mathbook;
 
+import java.math.BigInteger;
+
+/**
+ * Utility class to compare fractions using cross multiplication.
+ */
 public class AppMathBook {
 
+    /**
+     * Entry point. If four arguments are provided they are interpreted as two
+     * fractions in the form {@code num1 den1 num2 den2}. Otherwise a set of
+     * predefined examples is executed.
+     */
     public static void main(String[] args) {
-        System.out.println("001: " + checkMayorMenorIgual(2d/3d,1d/4d));
-        System.out.println("002: " + checkMayorMenorIgual(3d/5d,7d/8d));
-        System.out.println("003: " + checkMayorMenorIgual(-1d/6d,-1d/2d));
-        System.out.println("004: " + checkMayorMenorIgual(7d/9d,21d/27d));
-        System.out.println("005: " + checkMayorMenorIgual(11d/4d,12d/5d));
-        System.out.println("006: " + checkMayorMenorIgual(6d/4d,18d/12d));
-
-        System.out.println("007: " + checkMayorMenorIgual(-7d/7d,0d));
-        System.out.println("008: " + checkMayorMenorIgual(-5d/10d,13d/26d));
-        System.out.println("009: " + checkMayorMenorIgual(5d/2d,1d));
-        System.out.println("010: " + checkMayorMenorIgual(17d/6d,3d));
-        System.out.println("011: " + checkMayorMenorIgual(-3d,-39/13d));
-        System.out.println("012: " + checkMayorMenorIgual(4d/3d,4d/9d));
+        if (args.length == 4) {
+            Fraction a = new Fraction(Long.parseLong(args[0]), Long.parseLong(args[1]));
+            Fraction b = new Fraction(Long.parseLong(args[2]), Long.parseLong(args[3]));
+            System.out.println(checkMayorMenorIgual(a, b));
+            return;
+        }
+
+        System.out.println("001: " + checkMayorMenorIgual(new Fraction(2, 3), new Fraction(1, 4)));
+        System.out.println("002: " + checkMayorMenorIgual(new Fraction(3, 5), new Fraction(7, 8)));
+        System.out.println("003: " + checkMayorMenorIgual(new Fraction(-1, 6), new Fraction(-1, 2)));
+        System.out.println("004: " + checkMayorMenorIgual(new Fraction(7, 9), new Fraction(21, 27)));
+        System.out.println("005: " + checkMayorMenorIgual(new Fraction(11, 4), new Fraction(12, 5)));
+        System.out.println("006: " + checkMayorMenorIgual(new Fraction(6, 4), new Fraction(18, 12)));
+
+        System.out.println("007: " + checkMayorMenorIgual(new Fraction(-7, 7), new Fraction(0, 1)));
+        System.out.println("008: " + checkMayorMenorIgual(new Fraction(-5, 10), new Fraction(13, 26)));
+        System.out.println("009: " + checkMayorMenorIgual(new Fraction(5, 2), new Fraction(1, 1)));
+        System.out.println("010: " + checkMayorMenorIgual(new Fraction(17, 6), new Fraction(3, 1)));
+        System.out.println("011: " + checkMayorMenorIgual(new Fraction(-3, 1), new Fraction(-39, 13)));
+        System.out.println("012: " + checkMayorMenorIgual(new Fraction(4, 3), new Fraction(4, 9)));
+
+    }
 
+    private static String checkMayorMenorIgual(Fraction a, Fraction b) {
+        return switch (a.compareTo(b)) {
+            case -1 -> "a: " + a + " ((<)) b: " + b;
+            case 0 -> "a: " + a + " ((=)) b: " + b;
+            case 1 -> "a: " + a + " ((>)) b: " + b;
+            default -> "ERROR"; // This should never occur
+        };
     }
 
-    private static String checkMayorMenorIgual(double a, double b) {
-        if(a < b){
-            return "a: "+a+" ((<)) b: "+b;
-        }else if(a == b){
-            return "a: "+a+" ((=)) b: "+b;
-        } else if(a > b){
-            return "a: "+a+" ((>)) b: "+b;
-        } else {
-            return "ERROR";
+    /**
+     * Simple immutable fraction record with a comparison method based on cross
+     * multiplication.
+     */
+    public record Fraction(long numerator, long denominator) implements Comparable<Fraction> {
+
+        public Fraction {
+            if (denominator == 0) {
+                throw new IllegalArgumentException("Denominator cannot be zero");
+            }
+            if (denominator < 0) { // normalize sign
+                numerator = -numerator;
+                denominator = -denominator;
+            }
+        }
+
+        @Override
+        public int compareTo(Fraction other) {
+            BigInteger left = BigInteger.valueOf(numerator).multiply(BigInteger.valueOf(other.denominator));
+            BigInteger right = BigInteger.valueOf(other.numerator).multiply(BigInteger.valueOf(denominator));
+            return left.compareTo(right);
+        }
+
+        @Override
+        public String toString() {
+            return numerator + "/" + denominator;
         }
     }