monomial multiplication

Author: h908714124

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

@@ -0,0 +1,19 @@
+package io.polypen;
+
+import org.apache.commons.numbers.fraction.Fraction;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public record Monomial(Fraction coefficient, int exponent) {
+    public Polynomial multiply(Polynomial p) {
+        List<Fraction> result = new ArrayList<>(p.degree() + exponent + 1);
+        for (int i = 0; i < exponent; i++) {
+            result.add(Fraction.ZERO);
+        }
+        for (int i = 0; i <= p.degree(); i++) {
+            result.add(coefficient.multiply(p.coefficient(i)));
+        }
+        return new Polynomial(result);
+    }
+}

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

@@ -10,7 +10,7 @@
 public final class Polynomial {
     private final List<Fraction> coefficients;
 
-    private Polynomial(List<Fraction> coefficients) {
+    Polynomial(List<Fraction> coefficients) {
         this.coefficients = coefficients;
     }
 
@@ -23,7 +23,7 @@ public Polynomial add(String s) {
         int rank = Math.max(coefficients.size(), other.size());
         List<Fraction> r = new ArrayList<>(rank);
         for (int i = 0; i < rank; i++) {
-            r.add(get(i).add(i < other.size() ? other.get(i) : Fraction.ZERO));
+            r.add(coefficient(i).add(i < other.size() ? other.get(i) : Fraction.ZERO));
         }
         return new Polynomial(r);
     }
@@ -41,11 +41,11 @@ public String toString() {
             if (i == 0) {
                 result.add(prettySign + coefficient.abs());
             } else {
-                String exponent = i == 1 ? "x" : "x^" + i;
+                String factor = i == 1 ? "x" : "x^" + i;
                 if (isAbsoluteOne(coefficient)) {
-                    result.add(prettySign + exponent);
+                    result.add(prettySign + factor);
                 } else {
-                    result.add(prettySign + coefficient.abs() + " " + "x^" + i);
+                    result.add(prettySign + coefficient.abs() + " " + factor);
                 }
             }
         }
@@ -65,10 +65,14 @@ public int hashCode() {
         return coefficients.hashCode();
     }
 
-    private Fraction get(int i) {
+    public Fraction coefficient(int i) {
         if (i >= coefficients.size()) {
             return Fraction.ZERO;
         }
         return coefficients.get(i);
     }
+
+    public int degree() {
+        return coefficients.size() - 1;
+    }
 }

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

@@ -1,5 +1,6 @@
 package io.polypen;
 
+import org.apache.commons.numbers.fraction.Fraction;
 import org.junit.jupiter.api.Test;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
@@ -9,6 +10,7 @@ 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());
     }
 
     @Test
@@ -20,4 +22,10 @@ void add() {
     void polynomialEquals() {
         assertEquals(Polynomial.parse("x^5 - x - 1"), Polynomial.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")));
+    }
 }