Move the source files into a new default directory.

src/main/java/org/nwapw/abacus/function/Function.java

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+package org.nwapw.abacus.function;
+
+import org.nwapw.abacus.number.NaiveNumber;
+import org.nwapw.abacus.number.NumberInterface;
+import org.nwapw.abacus.number.PreciseNumber;
+
+import java.util.HashMap;
+
+/**
+ * A function that operates on one or more
+ * inputs and returns a single number.
+ */
+public abstract class Function {
+
+    /**
+     * A map to correctly promote different number implementations to each other.
+     */
+    private static final HashMap<Class<? extends NumberInterface>, Integer> priorityMap =
+            new HashMap<Class<? extends NumberInterface>, Integer>() {{
+                put(NaiveNumber.class, 0);
+                put(PreciseNumber.class, 1);
+            }};
+
+    /**
+     * Checks whether the given params will work for the given function.
+     * @param params the given params
+     * @return true if the params can be used with this function.
+     */
+    protected abstract boolean matchesParams(NumberInterface[] params);
+
+    /**
+     * Internal apply implementation, which already receives appropriately promoted
+     * parameters that have bee run through matchesParams
+     * @param params the promoted parameters.
+     * @return the return value of the function.
+     */
+    protected abstract NumberInterface applyInternal(NumberInterface[] params);
+
+    /**
+     * Function to check, promote arguments and run the function.
+     * @param params the raw input parameters.
+     * @return the return value of the function, or null if an error occurred.
+     */
+    public NumberInterface apply(NumberInterface...params) {
+        if(!matchesParams(params)) return null;
+        return applyInternal(params);
+    }
+
+}

src/main/java/org/nwapw/abacus/function/Operator.java

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+package org.nwapw.abacus.function;
+
+/**
+ * A class that represents a single infix operator.
+ */
+public class Operator {
+
+    /**
+     * The associativity of the operator.
+     */
+    private OperatorAssociativity associativity;
+    /**
+     * The type of this operator.
+     */
+    private OperatorType type;
+    /**
+     * The precedence of the operator.
+     */
+    private int precedence;
+    /**
+     * The function that is called by this operator.
+     */
+    private Function function;
+
+    /**
+     * Creates a new operator with the given parameters.
+     * @param associativity the associativity of the operator.
+     * @param precedence the precedence of the operator.
+     * @param function the function that the operator calls.
+     */
+    public Operator(OperatorAssociativity associativity, OperatorType operatorType, int precedence, Function function){
+        this.associativity = associativity;
+        this.type = operatorType;
+        this.precedence = precedence;
+        this.function = function;
+    }
+
+    /**
+     * Gets the operator's associativity.
+     * @return the associativity.
+     */
+    public OperatorAssociativity getAssociativity() {
+        return associativity;
+    }
+
+    /**
+     * Gets the operator's type.
+     * @return the type.
+     */
+    public OperatorType getType() {
+        return type;
+    }
+
+    /**
+     * Gets the operator's precedence.
+     * @return the precedence.
+     */
+    public int getPrecedence() {
+        return precedence;
+    }
+
+    /**
+     * Gets the operator's function.
+     * @return the function.
+     */
+    public Function getFunction() {
+        return function;
+    }
+
+}

src/main/java/org/nwapw/abacus/function/OperatorAssociativity.java

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+package org.nwapw.abacus.function;
+
+/**
+ * Enum to represent the associativity of an operator.
+ */
+public enum OperatorAssociativity {
+    LEFT, RIGHT
+}

src/main/java/org/nwapw/abacus/function/OperatorType.java

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+package org.nwapw.abacus.function;
+
+/**
+ * The type of an operator, describing how it should behave.
+ */
+public enum OperatorType {
+    BINARY_INFIX, UNARY_POSTFIX
+}