Qute: try to reduce the number of array allocations during rendering

- try to optimize the initial capacity of the StringBuilder used to
render the template, i.e. do not use the default or the hard-coded value
but try to precompute the value based on the template complexity and
also track the maximum length of results
- previously a hard-coded value of ~1K was always used as the initial
capacity
- also make it possible to set the initial capacity as an attribute of a template instance to override the default behavior

independent-projects/qute/core/src/main/java/io/quarkus/qute/Parser.java

@@ -1339,6 +1339,10 @@ public SectionHelper initialize(SectionInitContext context) {
     private static final BlockNode BLOCK_NODE = new BlockNode();
     static final CommentNode COMMENT_NODE = new CommentNode();
 
+    static boolean isDummyNode(TemplateNode node) {
+        return node == COMMENT_NODE || node == BLOCK_NODE;
+    }
+
     // A dummy node for section blocks, it's only used when removing standalone lines
     private static class BlockNode implements TemplateNode {
 

independent-projects/qute/core/src/main/java/io/quarkus/qute/TemplateImpl.java

@@ -37,6 +37,9 @@ class TemplateImpl implements Template {
     private final List<ParameterDeclaration> parameterDeclarations;
     private final LazyValue<Map<String, Fragment>> fragments;
 
+    // The initial capacity of the StringBuilder used to render the template
+    final Capacity capacity;
+
     TemplateImpl(EngineImpl engine, SectionNode root, String templateId, String generatedId, Optional<Variant> variant) {
         this.engine = engine;
         this.root = root;
@@ -47,6 +50,7 @@ class TemplateImpl implements Template {
         this.parameterDeclarations = ImmutableList.copyOf(root.getParameterDeclarations());
         // Use a lazily initialized map to avoid unnecessary performance costs during parsing
         this.fragments = initFragments(root);
+        this.capacity = new Capacity();
     }
 
     @Override
@@ -231,8 +235,30 @@ protected Engine engine() {
         }
 
         private CompletionStage<String> renderAsyncNoTimeout() {
-            StringBuilder builder = new StringBuilder(1028);
-            return renderData(data(), builder::append).thenApply(v -> builder.toString());
+            StringBuilder builder = new StringBuilder(getCapacity());
+            return renderData(data(), builder::append).thenApply(v -> {
+                String str = builder.toString();
+                capacity.update(str.length());
+                return str;
+            });
+        }
+
+        private int getCapacity() {
+            if (!attributes.isEmpty()) {
+                Object c = getAttribute(TemplateInstance.CAPACITY);
+                if (c != null) {
+                    if (c instanceof Number) {
+                        return ((Number) c).intValue();
+                    } else {
+                        try {
+                            return Integer.parseInt(c.toString());
+                        } catch (NumberFormatException e) {
+                            LOG.warnf("Invalid capacity value set for " + toString() + ": " + c);
+                        }
+                    }
+                }
+            }
+            return capacity.get();
         }
 
         private CompletionStage<Void> renderData(Object data, Consumer<String> consumer) {
@@ -280,6 +306,64 @@ public String toString() {
 
     }
 
+    class Capacity {
+
+        static final int LIMIT = 64 * 1024;
+
+        final int computed;
+        // intentionally not volatile; it's not a big deal if working with an outdated value
+        int max;
+
+        Capacity() {
+            this.computed = Math.min(computeCapacity(root.blocks.get(0)), LIMIT);
+        }
+
+        void update(int length) {
+            if (length > max) {
+                max = length < LIMIT ? length : LIMIT;
+            }
+        }
+
+        int get() {
+            return Math.max(max, computed);
+        }
+
+        private int computeCapacity(SectionBlock block) {
+            // This is a bit tricky because a template can contain a lot of dynamic parts
+            // Our approach is rather conservative, i.e. try not to overestimate/waste memory
+            int ret = 0;
+            for (TemplateNode node : block.nodes) {
+                if (Parser.isDummyNode(node)) {
+                    continue;
+                }
+                if (node.isText()) {
+                    ret += node.asText().getValue().length();
+                } else if (node.isExpression()) {
+                    // Reserve 10 characters per expression
+                    ret += 10;
+                } else if (node.isSection()) {
+                    SectionHelper helper = node.asSection().getHelper();
+                    if (LoopSectionHelper.class.isInstance(helper)) {
+                        // Loop secion - multiply the capacity of the main block by 10
+                        ret += 10 * computeCapacity(node.asSection().blocks.get(0));
+                    } else if (IncludeSectionHelper.class.isInstance(helper)) {
+                        // At this point we don't really know - the included template can be tiny or huge
+                        // So we just reserve 500 characters
+                        ret += 500;
+                    } else if (UserTagSectionHelper.class.isInstance(helper)) {
+                        // For user tags we don't expect large templates
+                        ret += 200;
+                    } else {
+                        for (SectionBlock b : node.asSection().blocks) {
+                            ret += computeCapacity(b);
+                        }
+                    }
+                }
+            }
+            return ret;
+        }
+    }
+
     class FragmentImpl extends TemplateImpl implements Fragment {
 
         FragmentImpl(EngineImpl engine, SectionNode root, String fragmentId, String generatedId,

independent-projects/qute/core/src/main/java/io/quarkus/qute/TemplateInstance.java

@@ -37,6 +37,11 @@ public interface TemplateInstance {
      */
     String LOCALE = "locale";
 
+    /**
+     * Attribute key - the initial capacity of the StringBuilder used to render the template.
+     */
+    String CAPACITY = "capacity";
+
     /**
      * Set the the root data object. Invocation of this method removes any data set previously by
      * {@link #data(String, Object)} and {@link #computedData(String, Function)}.
@@ -204,13 +209,23 @@ default TemplateInstance setLocale(Locale locale) {
     /**
      * Sets the variant attribute that can be used to select a specific variant of the template.
      *
-     * @param variant the variant
+     * @param variant
      * @return self
      */
     default TemplateInstance setVariant(Variant variant) {
         return setAttribute(SELECTED_VARIANT, variant);
     }
 
+    /**
+     * Sets the initial capacity of the StringBuilder used to render the template.
+     *
+     * @param capacity
+     * @return self
+     */
+    default TemplateInstance setCapacity(int capacity) {
+        return setAttribute(CAPACITY, capacity);
+    }
+
     /**
      * This component can be used to initialize a template instance, i.e. the data and attributes.
      *

independent-projects/qute/core/src/test/java/io/quarkus/qute/TemplateInstanceTest.java

@@ -4,11 +4,15 @@
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
+import java.util.List;
 import java.util.Locale;
+import java.util.Map;
 import java.util.concurrent.atomic.AtomicBoolean;
 
 import org.junit.jupiter.api.Test;
 
+import io.quarkus.qute.TemplateImpl.Capacity;
+
 public class TemplateInstanceTest {
 
     @Test
@@ -94,4 +98,26 @@ public void testVariant() {
         String render = hello.instance().setVariant(Variant.forContentType(Variant.TEXT_HTML)).render();
         assertEquals("Hello text/html!", render);
     }
+
+    @Test
+    public void testCapacity() {
+        Engine engine = Engine.builder().addDefaults().build();
+        assertCapacity(engine, "foo", 3, 3, Map.of());
+        assertCapacity(engine, "{! comment is ignored !}foo", 3, 3, Map.of());
+        assertCapacity(engine, "{foo} and bar", 10 + 8, 28, Map.of("foo", "bazzz".repeat(4)));
+        assertCapacity(engine, "{#each foo}bar{/}", 10 * 3, 3, Map.of("foo", List.of(1)));
+        assertCapacity(engine, "{#include bar /} and bar", 500 + 8, -1, Map.of());
+        // limit reached
+        assertCapacity(engine, "{#each}{foo}{/}".repeat(1000), Capacity.LIMIT, -1, Map.of());
+        assertCapacity(engine, "{foo}", 10, Capacity.LIMIT, Map.of("foo", "b".repeat(70_000)));
+    }
+
+    private void assertCapacity(Engine engine, String val, int expectedComputed, int expectedMax, Map<String, Object> data) {
+        TemplateImpl template = (TemplateImpl) engine.parse(val);
+        assertEquals(expectedComputed, template.capacity.computed);
+        if (expectedMax != -1) {
+            template.render(data);
+            assertEquals(expectedMax, template.capacity.max);
+        }
+    }
 }