Optimize deferred render to do a single input scan and use a single output buffer

Author: sugmanue

smithy-utils/src/main/java/software/amazon/smithy/utils/AbstractCodeWriter.java

@@ -11,6 +11,7 @@
 import java.util.Deque;
 import java.util.HashMap;
 import java.util.Iterator;
+import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
@@ -621,7 +622,7 @@ public abstract class AbstractCodeWriter<T extends AbstractCodeWriter<T>> {
     private boolean trailingNewline = true;
     private int trimBlankLines = -1;
     private boolean enableStackTraceComments;
-    private final Map<String, Supplier<String>> deferredValues = new HashMap<>();
+    private final Map<String, Supplier<String>> deferredValues = new LinkedHashMap<>();
     private int deferredCounter = 0;
 
     /**
@@ -789,8 +790,33 @@ public String toString() {
 
         // Resolve any deferred values.
         if (!deferredValues.isEmpty()) {
+            // Find all sentinel positions, then resolve in a single linear pass.
+            List<int[]> positions = new ArrayList<>();
+            int idx = 0;
             for (Map.Entry<String, Supplier<String>> entry : deferredValues.entrySet()) {
-                result = result.replace(entry.getKey(), entry.getValue().get());
+                String key = entry.getKey();
+                int fromIndex = 0;
+                while ((fromIndex = result.indexOf(key, fromIndex)) != -1) {
+                    // Store [position, index-into-entries-list, key-length]
+                    positions.add(new int[]{fromIndex, idx, key.length()});
+                    fromIndex += key.length();
+                }
+                idx++;
+            }
+            if (!positions.isEmpty()) {
+                positions.sort((a, b) -> Integer.compare(a[0], b[0]));
+                // Build list of suppliers indexed by insertion order for O(1) lookup.
+                @SuppressWarnings("unchecked")
+                Supplier<String>[] suppliers = deferredValues.values().toArray(new Supplier[0]);
+                StringBuilder resolved = new StringBuilder(result.length());
+                int pos = 0;
+                for (int[] entry : positions) {
+                    resolved.append(result, pos, entry[0]);
+                    resolved.append(suppliers[entry[1]].get());
+                    pos = entry[0] + entry[2];
+                }
+                resolved.append(result, pos, result.length());
+                result = resolved.toString();
             }
         }