Add comments and make minor modifications to the Graph class.

fx/src/main/kotlin/org/nwapw/abacus/fx/graphing/Graph.kt

@@ -12,30 +12,66 @@ import org.nwapw.abacus.number.NumberInterface
 import org.nwapw.abacus.plugin.StandardPlugin
 import org.nwapw.abacus.tree.TreeNode
 
+/**
+ * A class that holds information about a graph.
+ *
+ * Graph that uses an instance of [Abacus] to generate graphs of an
+ * [expression]. Continuous graphing on large expressions is expensive,
+ * and therefore the method of graphing is discrete. The graph class
+ * uses the [pointExpression] to generate inputs until an input is outside
+ * the function's domain. In both the expressions, the [inputVariable] and
+ * [pointInputVariable] are used, respectively, in order to either pass in
+ * the x-coordinate or the number of the input value being generated.
+ *
+ * @property abacus the abacus instance to use to evaluate expressions.
+ * @property domain the domain used in computation.
+ * @property range the range used for displaying the output.
+ * @property inputVariable the variable which is substituted for the input x-coordinate.
+ * @property pointInputVariable the variable which is substituted for the number of the input point being generated.
+ */
 class Graph(val abacus: Abacus,
-            expression: String, pointExpression: String,
             var domain: ClosedRange<NumberInterface>, var range: ClosedRange<NumberInterface>,
             var inputVariable: String = "x", var pointInputVariable: String = "n") {
 
+    /**
+     * Property used for storing the parsed version of the [expression]
+     */
     private var expressionTree: TreeNode? = null
+    /**
+     * Property used for storing the parsed version of the [pointExpression]
+     */
     private var pointExpressionTree: TreeNode? = null
 
-    var expression: String = ""
+    /**
+     * The expression being graphed.
+     */
+    var expression: String? = null
         set(value) {
-            expressionTree = abacus.parseString(value)
             field = value
+            expressionTree = abacus.parseString(value ?: return)
         }
-    var pointExpression: String = ""
+    /**
+     * The expression being used to generate points.
+     */
+    var pointExpression: String? = null
         set(value) {
-            pointExpressionTree = abacus.parseString(value)
             field = value
+            pointExpressionTree = abacus.parseString(value ?: return)
         }
 
-    init {
-        this.expression = expression
-        this.pointExpression = pointExpression
-    }
-
+    /**
+     * Evaluates a parsed expression [tree] with the given input [values] of indeterminate type.
+     * This is is an asynchronous operation using coroutines, and for every input
+     * value a new sub-context is created. The [contextModifier] function is executed
+     * on each new sub-context, and is expected to use the input value to modify the context
+     * state so that the evaluation of the expression produces a correct result.
+     *
+     * @param T the type of input values.
+     * @param tree the tree to evaluate.
+     * @param values the values to plug into the expression.
+     * @param contextModifier the function that plugs values into the context.
+     * @return the list of outputs.
+     */
     fun <T> evaluateWith(tree: TreeNode, values: List<T>,
                          contextModifier: MutableEvaluationContext.(T) -> Unit) = runBlocking {
         values.map {
@@ -45,10 +81,23 @@ class Graph(val abacus: Abacus,
         }.map { it.await() }
     }
 
+    /**
+     * Extension function that calls [evaluateWith] with the current list.
+     * @param T the type of the values in the list.
+     * @param tree the tree node to evaluate.
+     * @param contextModifier the function that plugs values into the context.
+     * @return the list of outputs.
+     */
     fun <T> List<T>.evaluateWith(tree: TreeNode,
                                  contextModifier: MutableEvaluationContext.(T) -> Unit ) =
             evaluateWith(tree, this, contextModifier)
 
+    /**
+     * Uses the [pointExpression] and [pointInputVariable] to generate
+     * a set of points as inputs for the actual expression. These points are generated
+     * as long as they are within the [domain].
+     * @return the list of generated input values.
+     */
     fun generateInputs(): List<NumberInterface> =
             generateSequence(1) {
                 it + 1
@@ -59,8 +108,13 @@ class Graph(val abacus: Abacus,
                 abacus.evaluateTreeWithContext(pointExpressionTree!!, context).value
             }.takeWhile { it in domain }.toList()
 
-    fun generateOutputs(inputs: List<NumberInterface>): List<NumberInterface> =
+    /**
+     * Uses the [expression] and [inputVariable] to generate
+     * a set of outputs from the given set of [inputs].
+     * @return the list of generated points.
+     */
+    fun generateOutputs(inputs: List<NumberInterface>): List<Pair<NumberInterface, NumberInterface>> =
             inputs.evaluateWith(expressionTree!!) {
                 setVariable(inputVariable, it)
-            }.map { it.value }
+            }.mapIndexed { index, (value) -> inputs[index] to value }
 }