CommitGraphWriter: write changed-path filters

Add support for writing the BIDX and BDAT chunks of the commit graph
file, as described in man gitformat-commit-graph(5). The ability to read
such chunks will be added in a subsequent commit.

This work is based on earlier work by Kyle Zhao
(Ib863782af209f26381e3ca0a2c119b99e84b679c).

Change-Id: Ic18e6f0eeec7da1e1ff31751aabda5e6952dbe6e
Signed-off-by: kylezhao <kylezhao@tencent.com>
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>

1 files changed, 83 insertions, 0 deletions

diff --git a/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/debug/ReadChangedPathFilter.java b/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/debug/ReadChangedPathFilter.java
new file mode 100644
index 0000000000..6927de84e4
--- /dev/null
+++ b/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/debug/ReadChangedPathFilter.java
@@ -0,0 +1,83 @@
+/*
+ * Copyright (C) 2023, Google LLC
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Distribution License v. 1.0 which is available at
+ * https://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+package org.eclipse.jgit.pgm.debug;
+
+import java.io.FileInputStream;
+import java.util.HashSet;
+
+import org.eclipse.jgit.pgm.Command;
+import org.eclipse.jgit.pgm.TextBuiltin;
+import org.eclipse.jgit.util.NB;
+import org.kohsuke.args4j.Argument;
+
+/**
+ * Prints the contents of the BDAT chunk from commit-graph file.
+ * <p>
+ * This is a debugging tool for changed path filter development.
+ */
+@Command
+class ReadChangedPathFilter extends TextBuiltin {
+
+	static final int CHUNK_ID_OID_FANOUT = 0x4f494446; /* "OIDF" */
+
+	static final int CHUNK_ID_BLOOM_FILTER_INDEX = 0x42494458; /* "BIDX" */
+
+	static final int CHUNK_ID_BLOOM_FILTER_DATA = 0x42444154; /* "BDAT" */
+
+	@Argument(index = 0)
+	private String input;
+
+	static HashSet<String> changedPathStrings(byte[] data) {
+		int oidf_offset = -1;
+		int bidx_offset = -1;
+		int bdat_offset = -1;
+		for (int i = 8; i < data.length - 4; i += 12) {
+			switch (NB.decodeInt32(data, i)) {
+			case CHUNK_ID_OID_FANOUT:
+				oidf_offset = (int) NB.decodeInt64(data, i + 4);
+				break;
+			case CHUNK_ID_BLOOM_FILTER_INDEX:
+				bidx_offset = (int) NB.decodeInt64(data, i + 4);
+				break;
+			case CHUNK_ID_BLOOM_FILTER_DATA:
+				bdat_offset = (int) NB.decodeInt64(data, i + 4);
+				break;
+			}
+		}
+		bdat_offset += 12; // skip version, hash count, bits per entry
+		int commit_count = NB.decodeInt32(data, oidf_offset + 255 * 4);
+		int[] changed_path_length_cumuls = new int[commit_count];
+		for (int i = 0; i < commit_count; i++) {
+			changed_path_length_cumuls[i] = NB.decodeInt32(data,
+					bidx_offset + i * 4);
+		}
+		HashSet<String> changed_paths = new HashSet<>();
+		for (int i = 0; i < commit_count; i++) {
+			int prior_cumul = i == 0 ? 0 : changed_path_length_cumuls[i - 1];
+			String changed_path = "";
+			for (int j = prior_cumul; j < changed_path_length_cumuls[i]; j++) {
+				changed_path += data[bdat_offset + j] + ",";
+			}
+			changed_paths.add(changed_path);
+		}
+		return changed_paths;
+	}
+
+	/** {@inheritDoc} */
+	@Override
+	protected void run() throws Exception {
+		try (FileInputStream in = new FileInputStream(input)
+				) {
+				byte[] data = in.readAllBytes();
+				outw.println(changedPathStrings(data).toString());
+		}
+	}
+}