Split the project into separate modules.

core/src/main/kotlin/org/nwapw/abacus/function/Documentation.kt

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+package org.nwapw.abacus.function
+
+/**
+ * A data class used for storing information about a function.
+ *
+ * The Documentation class holds the information necessary to display the information
+ * about a function to the user.
+ *
+ * @param codeName the name of the function as it occurs in code.
+ * @param name the name of the function in English.
+ * @param description the short description of this function.
+ * @param longDescription the full description of this function.
+ * @param type the things this documentation maps to.
+ */
+data class Documentation(val codeName: String, val name: String,
+                         val description: String, val longDescription: String,
+                         val type: DocumentationType) {
+
+    fun matches(other: String): Boolean {
+        return codeName.toLowerCase().contains(other.toLowerCase()) ||
+                name.toLowerCase().contains(other.toLowerCase()) ||
+                description.toLowerCase().contains(other.toLowerCase()) ||
+                longDescription.toLowerCase().contains(other.toLowerCase())
+    }
+
+}

core/src/main/kotlin/org/nwapw/abacus/function/Operator.kt

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+package org.nwapw.abacus.function
+
+/**
+ * A single operator that can be used by Abacus.
+ *
+ * This is a data class that holds the information about a single operator, such as a plus or minus.
+ *
+ * @param associativity the associativity of this operator, used for order of operations;.
+ * @param type the type of this operator, used for parsing (infix / prefix / postfix and binary / unary)
+ * @param precedence the precedence of this operator, used for order of operations.
+ * @param function the function this operator applies to its arguments.
+ */
+data class Operator(val associativity: OperatorAssociativity, val type: OperatorType,
+                    val precedence: Int, val function: Function)

core/src/main/kotlin/org/nwapw/abacus/fx/HistoryModel.kt

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+package org.nwapw.abacus.fx
+
+import javafx.beans.property.SimpleStringProperty
+
+/**
+ * A model representing an input / output in the calculator.
+ *
+ * The HistoryModel class stores a record of a single user-provided input,
+ * its parsed form as it was interpreted by the calculator, and the output
+ * that was provided by the calculator. These are represented as properties
+ * to allow easy access by JavaFX cells.
+ *
+ * @param input the user input
+ * @param parsed the parsed version of the input.
+ * @param output the output string.
+ */
+class HistoryModel(input: String, parsed: String, output: String){
+
+    /**
+     * The property that holds the input.
+     */
+    val inputProperty = SimpleStringProperty(input)
+    /**
+     * The property that holds the parsed input.
+     */
+    val parsedProperty = SimpleStringProperty(parsed)
+    /**
+     * The property that holds the output.
+     */
+    val outputProperty = SimpleStringProperty(output)
+
+}

core/src/main/kotlin/org/nwapw/abacus/fx/ToggleablePlugin.kt

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+package org.nwapw.abacus.fx
+
+import javafx.beans.property.SimpleBooleanProperty
+
+/**
+ * A model representing a plugin that can be disabled or enabled.
+ *
+ * ToggleablePlugin is a model that is used to present to the user the option
+ * of disabling / enabling plugins. The class name in this plugin is stored if
+ * its "enabledPropery" is false, essentially blacklisting the plugin.
+ *
+ * @param className the name of the class that this model concerns.
+ * @param enabled whether or not the model should start enabled.
+ */
+class ToggleablePlugin (val className: String, enabled: Boolean) {
+
+    /**
+     * The property used to interact with JavaFX components.
+     */
+    val enabledProperty = SimpleBooleanProperty(enabled)
+
+    /**
+     * Checks whether this plugin is currently enabled or not.
+     *
+     * @return true if it is enabled, false otherwise.
+     */
+    fun isEnabled(): Boolean {
+        return enabledProperty.value
+    }
+
+}

core/src/main/kotlin/org/nwapw/abacus/tree/BinaryNode.kt

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+package org.nwapw.abacus.tree
+
+/**
+ * A tree node that holds a binary operation.
+ *
+ * This node represents any binary operation, such as binary infix or binary postfix. The only
+ * currently implemented into Abacus is binary infix, but that has more to do with the parser than
+ * this class, which doesn't care about the order that its operation and nodes were found in text.
+ *
+ * @param operation the operation this node performs on its children.
+ * @param left the left node.
+ * @param right the right node.
+ */
+data class BinaryNode(val operation: String, val left: TreeNode? = null, val right: TreeNode?) : TreeNode() {
+
+    override fun <T : Any> reduce(reducer: Reducer<T>): T? {
+        val leftReduce = left?.reduce(reducer) ?: return null
+        val rightReduce = right?.reduce(reducer) ?: return null
+        return reducer.reduceNode(this, leftReduce, rightReduce)
+    }
+
+    override fun toString(): String {
+        return "(" + (left?.toString() ?: "null") + operation + (right?.toString() ?: "null") + ")"
+    }
+
+}

core/src/main/kotlin/org/nwapw/abacus/tree/FunctionNode.kt

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+package org.nwapw.abacus.tree
+
+/**
+ * A tree node that holds a function call.
+ *
+ * The function call node can hold any number of children, and passes the to the appropriate reducer,
+ * but that is its sole purpose.
+ *
+ * @param function the function string.
+ */
+data class FunctionNode(val function: String) : TreeNode() {
+
+    /**
+     * List of function parameters added to this node.
+     */
+    val children: MutableList<TreeNode> = mutableListOf()
+
+    override fun <T : Any> reduce(reducer: Reducer<T>): T? {
+        val children = Array<Any>(children.size, { children[it].reduce(reducer) ?: return null; })
+        return reducer.reduceNode(this, *children)
+    }
+
+    override fun toString(): String {
+        val buffer = StringBuffer()
+        buffer.append(function)
+        buffer.append('(')
+        for (i in 0 until children.size) {
+            buffer.append(children[i].toString())
+            buffer.append(if (i == children.size - 1) ")" else ",")
+        }
+        return buffer.toString()
+    }
+
+    /**
+     * Appends a child to this node's list of children.
+     *
+     * @node the node to append.
+     */
+    fun appendChild(node: TreeNode){
+        children.add(node)
+    }
+
+    /**
+     * Prepends a child to this node's list of children.
+     *
+     * @node the node to prepend.
+     */
+    fun prependChild(node: TreeNode){
+        children.add(0, node)
+    }
+
+}

core/src/main/kotlin/org/nwapw/abacus/tree/NumberNode.kt

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+package org.nwapw.abacus.tree
+
+import org.nwapw.abacus.number.NumberInterface
+
+/**
+ * A tree node that holds a single number value.
+ *
+ * This is a tree node that holds a single NumberInterface, which represents any number,
+ * and is not defined during compile time.
+ *
+ * @number the number value of this node.
+ */
+data class NumberNode(val number: NumberInterface) : TreeNode() {
+
+    override fun <T : Any> reduce(reducer: Reducer<T>): T? {
+        return reducer.reduceNode(this)
+    }
+
+    override fun toString(): String {
+        return number.toString()
+    }
+
+}

core/src/main/kotlin/org/nwapw/abacus/tree/Reducer.kt

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+package org.nwapw.abacus.tree
+
+/**
+ * Reducer interface that takes a tree and returns a single value.
+ *
+ * The reducer walks the tree, visiting the children first, converting them into
+ * a value, and then attempts to reduce the parent. Eventually, the single final value is returned.
+ */
+interface Reducer <out T> {
+
+    /**
+     * Reduces the given tree node, given its already reduced children.
+     *
+     * @param treeNode the tree node to reduce.
+     * @param children the list of children, of type T.
+     */
+    fun reduceNode(treeNode: TreeNode, vararg children: Any) : T?
+
+}

core/src/main/kotlin/org/nwapw/abacus/tree/TreeNode.kt

@@ -0,0 +1,10 @@
+package org.nwapw.abacus.tree
+
+/**
+ * A tree node.
+ */
+abstract class TreeNode {
+
+    abstract fun <T: Any> reduce(reducer: Reducer<T>) : T?
+
+}

core/src/main/kotlin/org/nwapw/abacus/tree/UnaryNode.kt

@@ -0,0 +1,23 @@
+package org.nwapw.abacus.tree
+
+/**
+ * A tree node that holds a unary operation.
+ *
+ * This node holds a single operator applied to a single parameter, and does not care
+ * whether the operation was found before or after the parameter in the text.
+ *
+ * @param operation the operation applied to the given node.
+ * @param applyTo the node to which the operation will be applied.
+ */
+data class UnaryNode(val operation: String, val applyTo: TreeNode? = null) : TreeNode() {
+
+    override fun <T : Any> reduce(reducer: Reducer<T>): T? {
+        val reducedChild = applyTo?.reduce(reducer) ?: return null
+        return reducer.reduceNode(this, reducedChild)
+    }
+
+    override fun toString(): String {
+        return "(" + (applyTo?.toString() ?: "null") + ")" + operation
+    }
+
+}