% is a proper modulus operator, not reminder. #fixes 842, #fixes 843

docs/scarpet/language/Operators.md

@@ -143,6 +143,9 @@ in the output map.
 Functional forms of `-` and `/` have less intuitive multi-nary interpretation, but they might be useful in some situations.
 `x-y-z` resolves to `difference(x, y, z)`.
 
+`/` always produces a properly accurate result, fully reversible with `*` operator. To obtain a integer 'div' result, use
+`floor(x/y)`.
+
 Examples:
 
 <pre>
@@ -160,11 +163,15 @@ b = [100,63,100]; b+[10,0,10]  => [110,63,110]
 
 ### Just Operators `%`, `^`
 
-The modulo and exponent (power) operators work only if both operands are numbers
+The modulo and exponent (power) operators work only if both operands are numbers. `%` is a proper (and useful) 'modulus' operator,
+not a useless 'reminder' operator that you would expect from anything that touches Java. While typically modulus is reserved
+to integer numbers, scarpet expands them to floats with as much sense as possible.
 
 <pre>pi^pi%euler  => 1.124....
--9 % 4  => -1
-9 % -4  => 0 ¯\_(ツ)_/¯ Java
+-9 % 4  => 3
+9 % -4  => -3
+9.1 % -4.2  => -3.5
+9.1 % 4.2  => 0.7
 -3 ^ 2  => 9
 -3 ^ pi => // Error
 </pre>

src/main/java/carpet/script/language/Operators.java

@@ -80,7 +80,7 @@ public class Operators {
         expression.addFunctionalEquivalence("/", "quotient");
 
         expression.addBinaryOperator("%", precedence.get("multiplication*/%"), true, (v1, v2) ->
-                new NumericValue(NumericValue.asNumber(v1).getDouble() % NumericValue.asNumber(v2).getDouble()));
+                NumericValue.asNumber(v1).mod(NumericValue.asNumber(v2)));
         expression.addBinaryOperator("^", precedence.get("exponent^"), false, (v1, v2) ->
                 new NumericValue(java.lang.Math.pow(NumericValue.asNumber(v1).getDouble(), NumericValue.asNumber(v2).getDouble())));
 

src/main/java/carpet/script/value/NumericValue.java

@@ -313,4 +313,14 @@ public class NumericValue extends Value
     {
         return longValue!= null ||  getDouble() == (double)getLong();
     }
+
+    public Value mod(NumericValue n2)
+    {
+        if (this.longValue != null && n2.longValue != null)
+            return new NumericValue(Math.floorMod(longValue, n2.longValue));
+        double x = value;
+        double y = n2.value;
+        if (y == 0) throw new ArithmeticException("Division by zero");
+        return new NumericValue( x - Math.floor(x / y) * y);
+    }
 }