#357 Provide a copy constructor / function

docs/index.md

@@ -92,6 +92,19 @@ EvaluationValue result = expression
 System.out.println(result.getNumberValue()); // prints 6.00
 ```
 
+### Expression can be copied and evaluated with a different set of values:
+
+```java
+Expression expression = new Expression("a + b").with("a", 1).and("b", 2);
+Expression copiedExpression = expression.copy().with("a", 3).and("b", 4);
+
+EvaluationValue result = expression.evaluate();
+EvaluationValue copiedResult = copiedExpression.evaluate();
+
+System.out.println(result.getNumberValue()); // prints 3
+System.out.println(copiedResult.getNumberValue()); // prints 7
+```
+
 ### Values can be passed in a map
 
 Instead of specifying the variable values one  by one, they can be set by defining a map with names and values and then

src/main/java/com/ezylang/evalex/Expression.java

@@ -66,6 +66,17 @@ public Expression(String expressionString, ExpressionConfiguration configuration
     this.constants.putAll(configuration.getDefaultConstants());
   }
 
+  /**
+   * Creates a copy with the same expression string, configuration, dataAccessor and syntax tree
+   * from an existing expression. The existing expression will be parsed to populate the syntax tree
+   *
+   * @param expression An existing expression.
+   * @throws ParseException If there were problems while parsing the existing expression.
+   */
+  public Expression(Expression expression) throws ParseException {
+    this(expression.getExpressionString(), expression.getConfiguration());
+    this.abstractSyntaxTree = expression.getAbstractSyntaxTree();
+  }
   /**
    * Evaluates the expression by parsing it (if not done before) and the evaluating it.
    *
@@ -311,6 +322,16 @@ public Expression withValues(Map<String, ?> values) {
     return this;
   }
 
+  /**
+   * Return a copy of the expression using the copy constructor {@link Expression(Expression)}.
+   *
+   * @return The copied Expression instance.
+   * @throws ParseException If there were problems while parsing the existing expression.
+   */
+  public Expression copy() throws ParseException {
+    return new Expression(this);
+  }
+
   /**
    * Create an AST representation for an expression string. The node can then be used as a
    * sub-expression. Subexpressions are not cached.

src/test/java/com/ezylang/evalex/ExpressionTest.java

@@ -192,4 +192,16 @@ void testGetUndefinedVariables() throws ParseException {
     Expression expression = new Expression("a+A+b+B+c+C+E+e+PI+x").with("x", 1);
     assertThat(expression.getUndefinedVariables()).containsExactlyInAnyOrder("a", "b", "c");
   }
+
+  @Test
+  void testCopy() throws ParseException, EvaluationException {
+    Expression expression = new Expression("a + b").with("a", 1).and("b", 2);
+    Expression copiedExpression = expression.copy().with("a", 3).and("b", 4);
+
+    EvaluationValue result = expression.evaluate();
+    EvaluationValue copiedResult = copiedExpression.evaluate();
+
+    assertThat(result.getStringValue()).isEqualTo("3");
+    assertThat(copiedResult.getStringValue()).isEqualTo("7");
+  }
 }