TreeParser: implement LRU caching

javacc parsing is an expensive operation, so this caches the generated TreeNodes instead.

Author: babaissarkar

src/main/java/com/babai/ssplot/math/system/parser/ParserManager.java

@@ -42,7 +42,7 @@ private static void init() {
 		// Engine set via envvar
 		String engineName = System.getenv("SSPLOT_ENGINE");
 		if (engineName == null) {
-			engineName = TreeParser.NAME;
+			engineName = TreeParser.internalParserName();
 		}
 		currParser = parsers.get(engineName);
 		System.out.println("Using parsing engine: " + engineName);

src/main/java/com/babai/ssplot/math/system/parser/internal/tree/TreeParser.java

@@ -22,20 +22,36 @@
 
 package com.babai.ssplot.math.system.parser.internal.tree;
 
+import java.util.LinkedHashMap;
 import java.util.Map;
 import com.babai.ssplot.math.system.parser.Parser;
 import com.babai.ssplot.math.system.parser.internal.SSMathParser;
 
 public class TreeParser implements Parser {
-	public static final String NAME = "Internal"; 
+	private static final String NAME = "Internal";
+	private static final Map<String, TreeNode> exprCache = new LinkedHashMap<>(16, 0.75f, true) {
+		private static final int MAX_CACHE_ENTRIES = 100;
+		@Override
+		protected boolean removeEldestEntry(Map.Entry<String, TreeNode> eldest) {
+			return size() > MAX_CACHE_ENTRIES;
+		}
+	};
 
 	@Override
-	public double evaluate(String expression, Map<String, Double> variables) {
-		return parse(expression).evalAt(variables);
+	public double evaluate(String expression, Map<String, Double> variables) {		
+		// get the cached TreeNode or parse it if missing
+		TreeNode tree = exprCache.computeIfAbsent(expression, this::parse);
+
+		// evaluate the cached AST with the provided variables
+		return tree.evalAt(variables);
 	}
 
 	@Override
 	public String getName() {
+		return internalParserName();
+	}
+	
+	public static String internalParserName() {
 		return NAME;
 	}
 

src/main/java/com/babai/ssplot/math/system/solver/Solver.java

@@ -68,7 +68,7 @@ public Vector<Vector<Double>> RK4Iterate(double x0, double y0) {
 			return soln;
 		}
 
-		Evaluator2D dx_dt = (x, y) -> parser.evaluate(system.eqns()[0], Map.of("x", x, "y", y));		
+		Evaluator2D dx_dt = (x, y) -> parser.evaluate(system.eqns()[0], Map.of("x", x, "y", y));
 		Evaluator2D dy_dt = (x, y) -> parser.evaluate(system.eqns()[1], Map.of("x", x, "y", y));
 
 		double x, y;