Add memory-efficient map for storing nested set -> digest.

Instead of using ConcurrentHashMap, we use a dead-simple open addressed hash hable with a giant byte array with 16-byte slots. We then read or write fingerprints straight into and out of the array, obviating the need to generate intermediate garbage.

Locking mechanism is a read-write lock. This should be faster than full synchronisation for read-heavy loads.

RELNOTES: None
PiperOrigin-RevId: 184019301

1 files changed, 21 insertions, 0 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/util/Fingerprint.java b/src/main/java/com/google/devtools/build/lib/util/Fingerprint.java
index c2119eeccb..ddb71b4aaf 100644
--- a/src/main/java/com/google/devtools/build/lib/util/Fingerprint.java
+++ b/src/main/java/com/google/devtools/build/lib/util/Fingerprint.java
@@ -20,6 +20,7 @@ import com.google.devtools.build.lib.vfs.PathFragment;
 import com.google.protobuf.CodedOutputStream;
 import java.io.IOException;
 import java.nio.charset.StandardCharsets;
+import java.security.DigestException;
 import java.security.DigestOutputStream;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
@@ -73,6 +74,26 @@ public final class Fingerprint {
     return md5.digest();
   }
 
+  /**
+   * Completes the hash computation by doing final operations and resets the underlying state,
+   * allowing this instance to be used again.
+   *
+   * <p>Instead of returning a digest, this method writes the digest straight into the supplied byte
+   * array, at the given offset.
+   *
+   * @see java.security.MessageDigest#digest()
+   */
+  public void digestAndReset(byte[] buf, int offset, int len) {
+    try {
+      codedOut.flush();
+      md5.digest(buf, offset, len);
+    } catch (IOException e) {
+      throw new IllegalStateException("failed to flush", e);
+    } catch (DigestException e) {
+      throw new IllegalStateException("failed to digest", e);
+    }
+  }
+
   /** Same as {@link #digestAndReset()}, except returns the digest in hex string form. */
   public String hexDigestAndReset() {
     return hexDigest(digestAndReset());