add multiply

Author: h908714124

src/main/java/io/polypen/Monomial.java

@@ -6,6 +6,9 @@
 import java.util.List;
 
 public record Monomial(Fraction coefficient, int exponent) {
+
+    public static final Monomial ZERO = new Monomial(Fraction.ZERO, 0);
+
     public Polynomial multiply(Polynomial p) {
         List<Fraction> result = new ArrayList<>(p.degree() + exponent + 1);
         for (int i = 0; i < exponent; i++) {

src/main/java/io/polypen/Polynomial.java

@@ -8,6 +8,9 @@
 import static io.polypen.Util.isAbsoluteOne;
 
 public final class Polynomial {
+
+    public static final Polynomial ZERO = new Polynomial(List.of(Fraction.ZERO));
+
     private final List<Fraction> coefficients;
 
     Polynomial(List<Fraction> coefficients) {
@@ -18,16 +21,33 @@ public static Polynomial parse(String s) {
         return new Polynomial(Parser.parse(s));
     }
 
-    public Polynomial add(String s) {
-        List<Fraction> other = Parser.parse(s);
-        int rank = Math.max(coefficients.size(), other.size());
-        List<Fraction> r = new ArrayList<>(rank);
-        for (int i = 0; i < rank; i++) {
-            r.add(coefficient(i).add(i < other.size() ? other.get(i) : Fraction.ZERO));
+    public Polynomial add(Polynomial other) {
+        int degree = Math.max(degree(), other.degree());
+        List<Fraction> r = new ArrayList<>(degree + 1);
+        for (int i = 0; i <= degree; i++) {
+            r.add(coefficient(i).add(other.coefficient(i)));
         }
         return new Polynomial(r);
     }
 
+    public Polynomial add(String s) {
+        return add(parse(s));
+    }
+
+    public Polynomial multiply(Polynomial other) {
+        Polynomial result = ZERO;
+        for (int i = 0; i <= degree(); i++) {
+            Monomial m = monomial(i);
+            Polynomial p = m.multiply(other);
+            result = result.add(p);
+        }
+        return result;
+    }
+
+    public Polynomial multiply(String s) {
+        return multiply(parse(s));
+    }
+
     @Override
     public String toString() {
         List<String> result = new ArrayList<>(coefficients.size());
@@ -72,6 +92,13 @@ public Fraction coefficient(int i) {
         return coefficients.get(i);
     }
 
+    public Monomial monomial(int i) {
+        if (i >= coefficients.size()) {
+            return Monomial.ZERO;
+        }
+        return new Monomial(coefficients.get(i), i);
+    }
+
     public int degree() {
         return coefficients.size() - 1;
     }

src/test/java/io/polypen/PolynomialTest.java

@@ -3,29 +3,35 @@
 import org.apache.commons.numbers.fraction.Fraction;
 import org.junit.jupiter.api.Test;
 
+import static io.polypen.Polynomial.parse;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
 class PolynomialTest {
 
     @Test
     void polynomialToString() {
-        assertEquals("x^5 - x - 1", Polynomial.parse("-x + x^5 - 1").toString());
-        assertEquals("2 x^6 - 4 x^2 - 2 x", Polynomial.parse("2x^6 - 4x^2 - 2x").toString());
+        assertEquals("x^5 - x - 1", parse("-x + x^5 - 1").toString());
+        assertEquals("2 x^6 - 4 x^2 - 2 x", parse("2x^6 - 4x^2 - 2x").toString());
     }
 
     @Test
     void add() {
-        assertEquals(Polynomial.parse("2x + 3"), Polynomial.parse("x + 1").add("x + 2"));
+        assertEquals(parse("2x + 3"), parse("x + 1").add("x + 2"));
+    }
+
+    @Test
+    void multiply() {
+        assertEquals(parse("x^2 - 1"), parse("x - 1").multiply("x + 1"));
     }
 
     @Test
     void polynomialEquals() {
-        assertEquals(Polynomial.parse("x^5 - x - 1"), Polynomial.parse("-x + x^5 - 1"));
+        assertEquals(parse("x^5 - x - 1"), parse("-x + x^5 - 1"));
     }
 
     @Test
     void monomialMultiplication() {
-        assertEquals(Polynomial.parse("2x^6 - 4x^2 - 2x"),
-                new Monomial(Fraction.of(2), 1).multiply(Polynomial.parse("x^5 - 2x - 1")));
+        assertEquals(parse("2x^6 - 4x^2 - 2x"),
+                new Monomial(Fraction.of(2), 1).multiply(parse("x^5 - 2x - 1")));
     }
 }