Detect and preserve line separators

Update `Formatter` so that line endings are now detected based on
the contents of the file. This mirrors the behavior of the Eclipse
plugin when used in the IDE, specifically the `nextDelimiterInfo`
method in `org.eclipse.jface.text.DefaultLineTracker`.

See gh-340

spring-javaformat-eclipse/io.spring.javaformat.eclipse.tests/src/io/spring/javaformat/eclipse/projectsettings/ProjectSettingsFilesTests.java

@@ -95,7 +95,8 @@ void applyToProjectWithFileMergesToDotSettings() throws Exception {
 		}).given(projectFile).setContents((InputStream) any(), anyInt(), any());
 		files.applyToProject(project, monitor);
 		verify(projectFile).setContents((InputStream) any(), eq(1), eq(monitor));
-		assertThat(out.toString(StandardCharsets.UTF_8)).isEqualTo("a=b\ny=z\n");
+		assertThat(out.toString(StandardCharsets.UTF_8))
+				.isEqualToNormalizingNewlines("a=b\ny=z\n");
 	}
 
 	private ProjectSettingsFile createPrefsFile() throws IOException {

spring-javaformat-gradle/spring-javaformat-gradle-plugin/src/test/java/io/spring/javaformat/gradle/CheckTaskTests.java

@@ -23,7 +23,7 @@
 import java.nio.file.StandardCopyOption;
 import java.nio.file.StandardOpenOption;
 import java.util.Arrays;
-import java.util.Collections;
+import java.util.List;
 import java.util.stream.Stream;
 
 import org.gradle.testkit.runner.BuildResult;
@@ -72,8 +72,8 @@ void whenFirstInvocationSucceedsAndSourceIsModifiedThenSecondInvocationSucceeds(
 		GradleBuild gradleBuild = this.gradleBuild.source(this.temp);
 		BuildResult result = gradleBuild.build("check");
 		assertThat(result.task(":checkFormatMain").getOutcome()).isEqualTo(TaskOutcome.SUCCESS);
-		Files.write(new File(this.temp, "src/main/java/simple/Simple.java").toPath(),
-				Collections.singletonList("// A change to the file"), StandardOpenOption.APPEND);
+		appendToFileNormalizingNewlines(new File(this.temp, "src/main/java/simple/Simple.java").toPath(),
+				"// A change to the file");
 		result = gradleBuild.build("--debug", "check");
 		assertThat(result.task(":checkFormatMain").getOutcome()).isEqualTo(TaskOutcome.SUCCESS);
 	}
@@ -146,4 +146,14 @@ private void copyFolder(Path source, Path target) throws IOException {
 		}
 	}
 
+	/**
+	 * Uses a read/modify/truncate approach to append a line to a file.
+	 * This avoids issues where the standard append option results in mixed line-endings.
+	 */
+	private void appendToFileNormalizingNewlines(Path sourceFilePath, String lineToAppend) throws IOException {
+		List<String> lines = Files.readAllLines(sourceFilePath);
+		lines.add(lineToAppend);
+		Files.write(sourceFilePath, lines, StandardOpenOption.TRUNCATE_EXISTING);
+	}
+
 }

spring-javaformat-maven/spring-javaformat-maven-plugin/src/it/.gitattributes

@@ -0,0 +1,2 @@
+# Test resources that need a predictable eol
+apply*/src/main/java/simple/Simple.java eol=lf

spring-javaformat-maven/spring-javaformat-maven-plugin/src/test/java/io/spring/format/maven/VerifyApply.java

@@ -30,7 +30,7 @@
  */
 public class VerifyApply {
 
-	private static final String LF = System.lineSeparator();
+	private static final String LF = "\n";
 
 	private static final String JAVA_FILE = "src/main/java/simple/Simple.java";
 

spring-javaformat/spring-javaformat-formatter-tests/src/test/resources/.gitattributes

@@ -0,0 +1,4 @@
+# Test resources that need specific eol
+**/correct-crlf.txt eol=crlf
+**/correct-cr.txt   eol=cr
+**/correct-lf.txt   eol=lf

spring-javaformat/spring-javaformat-formatter-tests/src/test/resources/expected/correct-cr.txt

@@ -0,0 +1 @@
+package correct;public class CorrectCr {	public static void main(String[] args) throws Exception {		// FIXME	}}

spring-javaformat/spring-javaformat-formatter-tests/src/test/resources/expected/correct-crlf.txt

@@ -0,0 +1,9 @@
+package correct;
+
+public class CorrectCrlf {
+
+	public static void main(String[] args) throws Exception {
+		// FIXME
+	}
+
+}

spring-javaformat/spring-javaformat-formatter-tests/src/test/resources/expected/correct-lf.txt

@@ -0,0 +1,9 @@
+package correct;
+
+public class CorrectLf {
+
+	public static void main(String[] args) throws Exception {
+		// FIXME
+	}
+
+}

spring-javaformat/spring-javaformat-formatter-tests/src/test/resources/source/correct-cr.txt

@@ -0,0 +1 @@
+package correct;public class CorrectCr {	public static void main(String[] args) throws Exception {		// FIXME	}}

spring-javaformat/spring-javaformat-formatter-tests/src/test/resources/source/correct-crlf.txt

@@ -0,0 +1,9 @@
+package correct;
+
+public class CorrectCrlf {
+
+	public static void main(String[] args) throws Exception {
+		// FIXME
+	}
+
+}

spring-javaformat/spring-javaformat-formatter-tests/src/test/resources/source/correct-lf.txt

@@ -0,0 +1,9 @@
+package correct;
+
+public class CorrectLf {
+
+	public static void main(String[] args) throws Exception {
+		// FIXME
+	}
+
+}

spring-javaformat/spring-javaformat-formatter/src/main/java/io/spring/javaformat/formatter/Formatter.java

@@ -17,6 +17,8 @@
 package io.spring.javaformat.formatter;
 
 import java.util.Map;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
 
 import org.eclipse.jface.text.IRegion;
 import org.eclipse.text.edits.TextEdit;
@@ -56,6 +58,11 @@ public class Formatter {
 	 */
 	private static final int DEFAULT_INDENTATION_LEVEL = 0;
 
+	/**
+	 * Pattern that matches all line separators into named-capturing group "sep".
+	 */
+	private static final Pattern LINE_SEPARATOR_PATTERN = Pattern.compile("(?<sep>(\r\n|\r|\n))");
+
 	/**
 	 * The default line separator.
 	 */
@@ -123,6 +130,9 @@ public TextEdit format(String source, int offset, int length, String lineSeparat
 
 	public TextEdit format(int kind, String source, int offset, int length, int indentationLevel,
 			String lineSeparator) {
+		if (lineSeparator == null) {
+			lineSeparator = detectLineSeparator(source);
+		}
 		return this.delegate.format(kind, source, offset, length, indentationLevel, lineSeparator);
 	}
 
@@ -148,6 +158,9 @@ public TextEdit format(String source, IRegion[] regions, String lineSeparator) {
 	}
 
 	public TextEdit format(int kind, String source, IRegion[] regions, int indentationLevel, String lineSeparator) {
+		if (lineSeparator == null) {
+			lineSeparator = detectLineSeparator(source);
+		}
 		return this.delegate.format(kind, source, regions, indentationLevel, lineSeparator);
 	}
 
@@ -159,4 +172,17 @@ public void setOptions(Map<String, String> options) {
 		this.delegate.setOptions(options);
 	}
 
+	private String detectLineSeparator(String contents) {
+		Matcher matcher = LINE_SEPARATOR_PATTERN.matcher(contents);
+		if (!matcher.find()) {
+			return DEFAULT_LINE_SEPARATOR;
+		}
+		String firstMatch = matcher.group("sep");
+		while (matcher.find()) {
+			if (!matcher.group("sep").equals(firstMatch)) {
+				return DEFAULT_LINE_SEPARATOR;
+			}
+		}
+		return firstMatch;
+	}
 }