Fix computeGitObjectHash() to compute the same object has as Git

* Renamed from computeGitHash()

Issue-Ref: 4310
Issue-Url: af3#4310



Signed-off-by: Simon Barner <barner@fortiss.org>

org.fortiss.tooling.ext.quality/src/org/fortiss/tooling/ext/quality/storage/.ratings

-CSVFileWriter.java a779fce240c25aae1b72dfd11d27b74b0b21d599 RED
+CSVFileWriter.java 0cae6203edc159cec0d3f68c61f4b04a3c61eecd RED
 ModelQualityStorageManager.java 8293f17743bdc85e2595eae99b978ed868bd029b RED

org.fortiss.tooling.ext.quality/src/org/fortiss/tooling/ext/quality/storage/CSVFileWriter.java

@@ -15,18 +15,14 @@
 +--------------------------------------------------------------------------*/
 package org.fortiss.tooling.ext.quality.storage;
 
-import static java.lang.Integer.toHexString;
-
 import java.io.BufferedReader;
 import java.io.BufferedWriter;
 import java.io.File;
 import java.io.FileReader;
 import java.io.FileWriter;
 import java.io.IOException;
-import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Path;
-import java.nio.file.Paths;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
 import java.time.LocalDateTime;
@@ -102,7 +98,7 @@ public class CSVFileWriter {
 		// Add topLevelElement.doSave(new NullProgressMonitor());
 		Path modelFile = new File(ModelQualityStorageManager.MODEL_PROJECT_DIR,
 				topLevelElement.getSaveableName()).toPath();
-		String projectHash = computeGitHash(modelFile);
+		String projectHash = computeGitObjectHash(modelFile);
 
 		List<String> allKeys = new ArrayList<>();
 		boolean createNewIndex = true;
@@ -216,27 +212,39 @@ public class CSVFileWriter {
 	}
 
 	/**
-	 * Function to generate the hash for the model in order to later on match versions of the file
-	 * project with saved metrics (assuming both are under version control using Git).
+	 * Return the Git object hash for the contents of the given file:
+	 * <p>
+	 * {@code SHA-1("blob <size>\0"+<contents>).asHexString()}.
 	 */
-	private static String computeGitHash(Path filePath)
+	private static String computeGitObjectHash(Path filePath)
 			throws NoSuchAlgorithmException, IOException {
-		String fileAsString = new String(Files.readAllBytes(Paths.get(filePath.toString())));
-		MessageDigest digest = MessageDigest.getInstance("SHA-1");
-		byte[] hash = digest.digest(fileAsString.getBytes(StandardCharsets.UTF_8));
-		return bytesToHex(hash);
-	}
 
-	/** Switching from bytes to Hex notation. */
-	private static String bytesToHex(byte[] hash) {
-		StringBuilder hexString = new StringBuilder(2 * hash.length);
-		for(byte b : hash) {
-			String hex = toHexString(0xff & b);
-			if(hex.length() == 1) {
-				hexString.append('0');
+		try(BufferedReader reader = new BufferedReader(new FileReader(filePath.toString()))) {
+			// Read file line-wise with UNIX line endings
+			StringBuilder inputBuilder = new StringBuilder();
+			do {
+				String line = reader.readLine();
+				if(line == null) {
+					break;
+				}
+				inputBuilder.append(line);
+				inputBuilder.append("\n");
+			} while(true);
+
+			// Construct pre-image (input to hash function) according to Git specification
+			String fileContents = inputBuilder.toString();
+			int n = fileContents.length();
+			String preImage = "blob " + n + "\0" + fileContents;
+
+			// Compute hash and convert it to a hex string
+			MessageDigest sha1 = MessageDigest.getInstance("SHA-1");
+			byte[] digest = sha1.digest(preImage.getBytes());
+
+			StringBuilder resultBuilder = new StringBuilder();
+			for(byte b : digest) {
+				resultBuilder.append(String.format("%02x", b & 0xff));
 			}
-			hexString.append(hex);
+			return resultBuilder.toString();
 		}
-		return hexString.toString();
 	}
 }