improvements

Author: h908714124

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

@@ -17,43 +17,47 @@ private Polynomial(List<Fraction> coefficients) {
     @Override
     public String toString() {
         List<String> result = new ArrayList<>(coefficients.size());
-        for (int i = 0; i < coefficients.size(); i++) {
+        for (int i = coefficients.size() - 1; i >= 0; i--) {
             Fraction coefficient = coefficients.get(i);
             if (coefficient.isZero()) {
                 continue;
             }
+            String plus = (i == coefficients.size() - 1 && coefficient.compareTo(Fraction.ZERO) > 0) ? "" : "+ ";
+            String prettyCoefficient = coefficient.compareTo(Fraction.ZERO) < 0 ? "- " : plus;
             if (i == 0) {
-                result.add(coefficient.toString());
+                result.add(prettyCoefficient + coefficient.abs());
             } else {
                 String exponent = i == 1 ? "x" : "x^" + i;
                 if (coefficient.isOne()) {
-                    result.add(exponent);
+                    result.add(plus + exponent);
+                } else if (coefficient.equals(Fraction.ONE.negate())) {
+                    result.add("- " + exponent);
                 } else {
-                    result.add(coefficient + " " + "x^" + i);
+                    result.add(prettyCoefficient + coefficient.abs() + " " + "x^" + i);
                 }
             }
         }
-        return String.join(" + ", result);
+        return String.join(" ", result);
     }
 
     public static Polynomial parse(String s) {
-        String[] strings = s.split("\\w*[+-]\\w*", 0);
+        List<SignedToken> strings = split(s.trim());
         TreeMap<Integer, Fraction> cof = new TreeMap<>();
-        for (String term : strings) {
-            String[] tokens = term.split("[a-z]", 3);
+        for (SignedToken term : strings) {
+            String[] tokens = term.token.split("[a-z]", 3);
             String coefficient = tokens[0].replace("*", "").trim();
             if (coefficient.isEmpty()) {
                 coefficient = "1";
             }
             if (tokens.length == 1) {
-                cof.put(0, Fraction.parse(coefficient));
+                cof.put(0, Fraction.parse(coefficient).multiply(term.sign.factor));
             } else {
                 String rawExponent = tokens[1].replace("^", "").trim();
                 if (rawExponent.isEmpty()) {
-                    cof.put(1, Fraction.parse(coefficient));
+                    cof.put(1, Fraction.parse(coefficient).multiply(term.sign.factor));
                 } else {
                     int exponent = Integer.parseInt(rawExponent);
-                    cof.put(exponent, Fraction.parse(coefficient));
+                    cof.put(exponent, Fraction.parse(coefficient).multiply(term.sign.factor));
                 }
             }
         }
@@ -69,4 +73,37 @@ public static Polynomial parse(String s) {
         }
         return new Polynomial(result);
     }
+
+    enum Sign {
+        PLUS(1), MINUS(-1);
+        final int factor;
+
+        Sign(int factor) {
+            this.factor = factor;
+        }
+    }
+
+    record SignedToken(Sign sign, String token) {
+    }
+
+    static List<SignedToken> split(String s) {
+        List<SignedToken> result = new ArrayList<>();
+        Sign sign = Sign.PLUS;
+        int pos = -1;
+        for (int i = 0; i < s.length(); i++) {
+            if (s.charAt(i) == '-') {
+                result.add(new SignedToken(sign, s.substring(pos + 1, i).trim()));
+                sign = Sign.MINUS;
+                pos = i;
+            } else if (s.charAt(i) == '+') {
+                result.add(new SignedToken(sign, s.substring(pos + 1, i).trim()));
+                sign = Sign.PLUS;
+                pos = i;
+            }
+        }
+        if (pos < s.length() - 1) {
+            result.add(new SignedToken(sign, s.substring(pos + 1).trim()));
+        }
+        return result;
+    }
 }

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

@@ -8,4 +8,9 @@ class PolynomialTest {
     void parse() {
         System.out.println(Polynomial.parse("x^5 - x - 1"));
     }
+
+    @Test
+    void split() {
+        System.out.println(Polynomial.split("1 + 2 - 3"));
+    }
 }