Support exporting and re-importing ClassNameTrie content to an extern…

…al resource (#3692)

* The trie data is stored using the same "modified-UTF8" format used by the JDK:
  https://docs.oracle.com/javase/8/docs/api/java/io/DataInput.html#modified-utf-8

* Don't alias '/' to '.' while building trie, only do that when querying

internal-api/src/main/java/datadog/trace/util/ClassNameTrie.java

@@ -1,5 +1,7 @@
 package datadog.trace.util;
 
+import java.io.DataInput;
+import java.io.DataOutput;
 import java.io.File;
 import java.io.IOException;
 import java.nio.charset.StandardCharsets;
@@ -103,6 +105,8 @@ public final class ClassNameTrie {
   /** Constant to account for the fact that the last branch doesn't have a jump offset/id. */
   private static final int NO_END_JUMP = 1;
 
+  private static final int FILE_MAGIC = 0xDD097213;
+
   /** The compressed trie. */
   private final char[] trieData;
 
@@ -196,10 +200,10 @@ public static int apply(char[] data, int[] longJumps, String key) {
   public static class Builder {
     private static final Pattern MAPPING_LINE = Pattern.compile("^\\s*(?:([0-9]+)\\s+)?([^\\s#]+)");
 
-    private char[] trieData = new char[8192];
-    private int trieLength = 0;
-    private int[] longJumps = new int[16];
-    private int longJumpCount = 0;
+    private char[] trieData;
+    private int trieLength;
+    private int[] longJumps;
+    private int longJumpCount;
 
     public boolean isEmpty() {
       return trieLength == 0;
@@ -211,9 +215,74 @@ public int apply(String key) {
     }
 
     public ClassNameTrie buildTrie() {
-      return new ClassNameTrie(
-          Arrays.copyOfRange(trieData, 0, trieLength),
-          Arrays.copyOfRange(longJumps, 0, longJumpCount));
+      // avoid unnecessary allocation when compaction isn't required
+      if (null != trieData && trieData.length > trieLength) {
+        trieData = Arrays.copyOfRange(trieData, 0, trieLength);
+      }
+      if (null != longJumps && longJumps.length > longJumpCount) {
+        longJumps = Arrays.copyOfRange(longJumps, 0, longJumpCount);
+      }
+      return new ClassNameTrie(trieData, longJumps);
+    }
+
+    /** Writes trie content to an external resource. */
+    public void writeTo(DataOutput out) throws IOException {
+      out.writeInt(FILE_MAGIC);
+      out.writeInt(trieLength);
+      for (int i = 0; i < trieLength; i++) {
+        char c = trieData[i];
+        if (c >= 0x0001 && c <= 0x007F) {
+          // write 7-bit non-zero char as 1 byte
+          out.writeByte(c);
+        } else if (c > 0x07FF) {
+          // write 16-bit char as 3 bytes (4+6+6)
+          out.writeByte(0xE0 | ((c >> 12) & 0x0F));
+          out.writeByte(0x80 | ((c >> 6) & 0x3F));
+          out.writeByte(0x80 | (c & 0x3F));
+        } else {
+          // write 11-bit char as 2 bytes (5+6)
+          out.writeByte(0xC0 | ((c >> 6) & 0x1F));
+          out.writeByte(0x80 | (c & 0x3F));
+        }
+      }
+      out.writeInt(longJumpCount);
+      for (int i = 0; i < longJumpCount; i++) {
+        out.writeInt(longJumps[i]);
+      }
+    }
+
+    /** Reads trie content from an external resource. */
+    public void readFrom(DataInput in) throws IOException {
+      int magic = in.readInt();
+      if (magic != FILE_MAGIC) {
+        throw new IOException("Unexpected file magic " + magic);
+      }
+      trieLength = in.readInt();
+      trieData = new char[trieLength];
+      for (int i = 0; i < trieLength; i++) {
+        byte b = in.readByte();
+        char c;
+        if ((b & 0x80) == 0) {
+          // read 7-bit non-zero char as 1 byte
+          c = (char) b;
+        } else if ((b & 0xE0) == 0xE0) {
+          // read 16-bit char as 3 bytes (4+6+6)
+          c = (char) (((b & 0x0F) << 12) | ((in.readByte() & 0x3F) << 6) | (in.readByte() & 0x3F));
+        } else {
+          // read 11-bit char as 2 bytes (5+6)
+          c = (char) (((b & 0x1F) << 6) | (in.readByte() & 0x3F));
+        }
+        trieData[i] = c;
+      }
+      longJumpCount = in.readInt();
+      if (longJumpCount > 0) {
+        longJumps = new int[longJumpCount];
+        for (int i = 0; i < longJumpCount; i++) {
+          longJumps[i] = in.readInt();
+        }
+      } else {
+        longJumps = null;
+      }
     }
 
     /** Reads a class-name mapping file into the current builder */
@@ -252,6 +321,7 @@ public void put(String className, int number) {
       if (trieLength == 0) {
         int keyLength = key.length();
         trieLength = (keyLength > 1 ? 3 : 2) + keyLength;
+        trieData = new char[8192]; // create table on first mapping
         trieData[0] = (char) 1;
         trieData[1] = key.charAt(0);
         if (keyLength > 1) {
@@ -280,8 +350,11 @@ private char setJump(int jump) {
       if (jump < LONG_JUMP_MARKER) {
         return (char) jump; // jump is small enough to fit into the trie
       }
-      if (longJumpCount == longJumps.length) {
+      if (longJumpCount == 0) {
+        longJumps = new int[16]; // create table on first long-jump
+      } else if (longJumpCount == longJumps.length) {
         int[] oldJumps = longJumps;
+        // expand table by 50% to fit additional long-jumps
         longJumps = new int[longJumpCount + (longJumpCount >> 1)];
         System.arraycopy(oldJumps, 0, longJumps, 0, longJumpCount);
       }
@@ -319,8 +392,7 @@ private void insertMapping(String key, char valueToInsert) {
         char branchCount = trieData[dataIndex++];
 
         // trie is ordered, so we can use binary search to pick the right branch
-        int branchIndex =
-            Arrays.binarySearch(trieData, dataIndex, dataIndex + branchCount, c == '/' ? '.' : c);
+        int branchIndex = Arrays.binarySearch(trieData, dataIndex, dataIndex + branchCount, c);
 
         if (branchIndex < 0) {
           jumpOffset =
@@ -391,7 +463,7 @@ private void insertMapping(String key, char valueToInsert) {
           int segmentEnd = dataIndex + segmentLength;
           while (keyIndex < keyLength && dataIndex < segmentEnd) {
             c = key.charAt(keyIndex);
-            if ((c == '/' ? '.' : c) != trieData[dataIndex]) {
+            if (c != trieData[dataIndex]) {
               break;
             }
             keyIndex++;

internal-api/src/test/groovy/datadog/trace/util/ClassNameTrieTest.groovy

@@ -178,4 +178,28 @@ class ClassNameTrieTest extends DDSpecification {
       assert trie.apply(it.key) == (0x1000 | it.value)
     })
   }
+
+  def 'trie content can be exported and re-imported'() {
+    setup:
+    def mapping = (0..128).collectEntries({
+      [UUID.randomUUID().toString().replace('-', '.'), it]
+    }) as TreeMap<String, Integer>
+    when:
+    def exporter = new ClassNameTrie.Builder()
+    // initial values
+    mapping.each { className, number ->
+      exporter.put(className, number)
+    }
+    // export
+    def sink = new ByteArrayOutputStream()
+    exporter.writeTo(new DataOutputStream(sink))
+    // re-import
+    def importer = new ClassNameTrie.Builder()
+    def source = new ByteArrayInputStream(sink.toByteArray())
+    importer.readFrom(new DataInputStream(source))
+    then:
+    mapping.each({
+      assert importer.apply(it.key) == it.value
+    })
+  }
 }