Add the variable database.

core/src/main/kotlin/org/nwapw/abacus/Abacus.kt

@@ -48,6 +48,10 @@ class Abacus(val configuration: Configuration) {
      * The promotion manager used to convert between number implementations.
      */
     val promotionManager = PromotionManager(this)
+    /**
+     * The database of variable definitions.
+     */
+    val variableDatabase = VariableDatabase(this)
     /**
      * The number implementation used by default.
      */
@@ -63,6 +67,7 @@ class Abacus(val configuration: Configuration) {
         pluginManager.addListener(tokenizer)
         pluginManager.addListener(parser)
         pluginManager.addListener(promotionManager)
+        pluginManager.addListener(variableDatabase)
     }
 
     /**

core/src/main/kotlin/org/nwapw/abacus/variables/VariableDatabase.kt

@@ -0,0 +1,79 @@
+package org.nwapw.abacus.variables
+
+import org.nwapw.abacus.Abacus
+import org.nwapw.abacus.number.NumberInterface
+import org.nwapw.abacus.plugin.PluginListener
+import org.nwapw.abacus.plugin.PluginManager
+import javax.swing.tree.TreeNode
+
+/**
+ * A database for variables and definition.
+ *
+ * The variable database is used to keep track of
+ * variables and definitions throughout the calculator.
+ *
+ * @property abacus the Abacus instance.
+ */
+class VariableDatabase(val abacus: Abacus): PluginListener {
+
+    /**
+     * The variables that are stored in the database.
+     */
+    private val variables = mutableMapOf<String, NumberInterface>()
+    /**
+     * The definitions that are stored in the database.
+     */
+    private val definitions = mutableMapOf<String, TreeNode>()
+
+    /**
+     * Stores a variable in the database.
+     *
+     * @param name the name of the variable.
+     * @param value the new value of the variable.
+     */
+    fun storeVariable(name: String, value: NumberInterface) {
+        variables[name] = value
+    }
+
+    /**
+     * Stores a definition in the database
+     *
+     * @param name the name of the definition.
+     * @param value the new value of the definition.
+     */
+    fun storeDefinition(name: String, value: TreeNode) {
+        definitions[name] = value
+    }
+
+    /**
+     * Gets the value of the variable, or 0 if
+     * it is not defined.
+     *
+     * @param name the name of the variable.
+     * @return the value of the variable.
+     */
+    fun getVariableValue(name: String): NumberInterface {
+        return variables[name] ?:
+                abacus.numberImplementation.instanceForString("0")
+    }
+
+    /**
+     * Gets the definition.
+     *
+     * @param name the name of the definition.
+     * @return the value of the definition, or null if one doesn't exist.
+     */
+    fun getDefinition(name: String): TreeNode? {
+        return definitions[name]
+    }
+
+    override fun onLoad(manager: PluginManager?) {
+
+    }
+
+    override fun onUnload(manager: PluginManager?) {
+        variables.clear()
+        definitions.clear()
+    }
+
+}