Add the ability for plugins to access variables, and add the operators.

core/src/main/java/org/nwapw/abacus/plugin/Plugin.java

@@ -2,6 +2,7 @@ package org.nwapw.abacus.plugin;
 
 import org.nwapw.abacus.function.*;
 import org.nwapw.abacus.number.NumberInterface;
+import org.nwapw.abacus.variables.VariableDatabase;
 
 /**
  * A plugin class that can be externally implemented and loaded via the
@@ -219,4 +220,12 @@ public abstract class Plugin {
      */
     public abstract void onDisable();
 
+    /**
+     * Get the variable database.
+     * @return the variable database.
+     */
+    public final VariableDatabase getVariableDatabase(){
+        return manager.getVariableDatabase();
+    }
+
 }

core/src/main/java/org/nwapw/abacus/plugin/PluginManager.java

@@ -3,6 +3,7 @@ package org.nwapw.abacus.plugin;
 import org.nwapw.abacus.Abacus;
 import org.nwapw.abacus.function.*;
 import org.nwapw.abacus.number.NumberInterface;
+import org.nwapw.abacus.variables.VariableDatabase;
 
 import java.lang.reflect.InvocationTargetException;
 import java.util.HashMap;
@@ -421,4 +422,12 @@ public class PluginManager {
     public Set<Class<?>> getLoadedPluginClasses() {
         return loadedPluginClasses;
     }
+
+    /**
+     * Gets the variable database.
+     * @return the database.
+     */
+    public VariableDatabase getVariableDatabase(){
+        return abacus.getVariableDatabase();
+    }
 }

core/src/main/java/org/nwapw/abacus/plugin/StandardPlugin.java

@@ -1,9 +1,14 @@
 package org.nwapw.abacus.plugin;
 
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 import org.nwapw.abacus.function.*;
 import org.nwapw.abacus.number.NaiveNumber;
 import org.nwapw.abacus.number.NumberInterface;
 import org.nwapw.abacus.number.PreciseNumber;
+import org.nwapw.abacus.tree.Reducer;
+import org.nwapw.abacus.tree.TreeNode;
+import org.nwapw.abacus.tree.VariableNode;
 
 import java.util.ArrayList;
 import java.util.HashMap;
@@ -14,6 +19,40 @@ import java.util.HashMap;
  */
 public class StandardPlugin extends Plugin {
 
+    /**
+     * The set operator.
+     */
+    public final TreeValueOperator opSet = new TreeValueOperator(OperatorAssociativity.LEFT, OperatorType.BINARY_INFIX, 0) {
+        @Override
+        public boolean matchesParams(NumberImplementation implementation, TreeNode[] params) {
+            return params.length == 2 && params[0] instanceof VariableNode;
+        }
+
+        @Override
+        public NumberInterface applyWithReducerInternal(NumberImplementation implementation, Reducer<? extends NumberInterface> reducer, TreeNode[] params) {
+            String assignTo = ((VariableNode) params[0]).getVariable();
+            NumberInterface value = params[1].reduce(reducer);
+            getVariableDatabase().getVariables().put(assignTo, value);
+            return value;
+        }
+    };
+    /**
+     * The define operator.
+     */
+    public final TreeValueOperator opDefine = new TreeValueOperator(OperatorAssociativity.LEFT, OperatorType.BINARY_INFIX, 0) {
+        @Override
+        public boolean matchesParams(NumberImplementation implementation, TreeNode[] params) {
+            return params.length == 2 && params[0] instanceof VariableNode;
+        }
+
+        @Nullable
+        @Override
+        public NumberInterface applyWithReducerInternal(NumberImplementation implementation, Reducer<? extends NumberInterface> reducer, TreeNode[] params) {
+            String assignTo = ((VariableNode) params[0]).getVariable();
+            getVariableDatabase().getDefinitions().put(assignTo, params[1]);
+            return params[1].reduce(reducer);
+        }
+    };
     /**
      * The addition operator, +
      */
@@ -712,6 +751,9 @@ public class StandardPlugin extends Plugin {
         registerOperator("^", OP_CARET);
         registerOperator("!", OP_FACTORIAL);
 
+        registerTreeValueOperator("=", opSet);
+        registerTreeValueOperator(":=", opDefine);
+
         registerOperator("nPr", OP_NPR);
         registerOperator("nCr", OP_NCR);