Add LineEnding.PRESERVE policy

WIP

Author: kwin

lib/src/main/java/com/diffplug/spotless/LineEnding.java

@@ -16,8 +16,12 @@
 package com.diffplug.spotless;
 
 import java.io.File;
+import java.io.FileReader;
+import java.io.IOException;
+import java.io.Reader;
 import java.io.Serializable;
 import java.lang.reflect.Method;
+import java.nio.charset.StandardCharsets;
 import java.util.Objects;
 import java.util.function.Supplier;
 
@@ -47,9 +51,11 @@ public Policy createPolicy() {
 	/** {@code \r\n} */
 	WINDOWS,
     /** {@code \n} */
-    UNIX,
+    UNIX,   
     /** {@code \r} */
-    MAC_CLASSIC;
+    MAC_CLASSIC,
+    /** preserve the line ending of the first line (no matter which format) */
+    PRESERVE;
 	// @formatter:on
 
 	/** Returns a {@link Policy} appropriate for files which are contained within the given rootFolder. */
@@ -81,6 +87,7 @@ public Policy createPolicy() {
 		case WINDOWS:			return WINDOWS_POLICY;
 		case UNIX:				return UNIX_POLICY;
 		case MAC_CLASSIC:		return MAC_CLASSIC_POLICY;
+		case PRESERVE:			return PRESERVE_POLICY;
 		default:	throw new UnsupportedOperationException(this + " is a path-specific line ending.");
 		}
 	}
@@ -100,9 +107,50 @@ public String getEndingFor(File file) {
 		}
 	}
 
+	static class PreserveLineEndingPolicy extends NoLambda.EqualityBasedOnSerialization implements Policy {
+        private static final long serialVersionUID = 2L;
+
+        @Override
+        public String getEndingFor(File file) {
+            // assume US-ASCII encoding (only line ending characters need to be decoded anyways)
+            try (Reader reader = new FileReader(file, StandardCharsets.US_ASCII)) {
+                return getEndingFor(reader);
+            } catch (IOException e) {
+                throw new IllegalArgumentException("Could not determine line ending of file: " + file, e);
+            }
+        }
+
+        static String getEndingFor(Reader reader) throws IOException {
+            char previousCharacter = 0;
+            char currentCharacter = 0;
+            int readResult;
+            while ((readResult = reader.read()) != -1) {
+                currentCharacter = (char)readResult;
+                if (currentCharacter == '\n') {
+                    if (previousCharacter == '\r') {
+                        return WINDOWS.str();
+                    } else {
+                        return UNIX.str();
+                    }
+                } else {
+                    if (previousCharacter == '\r') {
+                        return MAC_CLASSIC.str();
+                    }
+                }
+                previousCharacter = currentCharacter;
+            }
+            if (previousCharacter == '\r') {
+                return MAC_CLASSIC.str();
+            }
+            // assume UNIX line endings if no line ending was found
+            return UNIX.str();
+        }
+	}
+
 	private static final Policy WINDOWS_POLICY = new ConstantLineEndingPolicy(WINDOWS.str());
 	private static final Policy UNIX_POLICY = new ConstantLineEndingPolicy(UNIX.str());
     private static final Policy MAC_CLASSIC_POLICY = new ConstantLineEndingPolicy(MAC_CLASSIC.str());
+    private static final Policy PRESERVE_POLICY = new PreserveLineEndingPolicy();
 	private static final String _platformNative = System.getProperty("line.separator");
 	private static final Policy _platformNativePolicy = new ConstantLineEndingPolicy(_platformNative);
 	private static final boolean nativeIsWin = _platformNative.equals(WINDOWS.str());

lib/src/test/java/com/diffplug/spotless/LineEndingTest.java

@@ -0,0 +1,48 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.diffplug.spotless;
+
+import java.io.IOException;
+import java.io.Reader;
+import java.io.StringReader;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+class LineEndingTest {
+
+    @Test
+    void testGetEndingFor() throws IOException {
+        assertLineEnding("\r", "\r");
+        assertLineEnding("\r", "Test\r");
+        assertLineEnding("\r", "Test\rTest2\n");
+        
+        assertLineEnding("\n", "Test");
+        
+        assertLineEnding("\r\n", "\r\n");
+        assertLineEnding("\r\n", "Test\r\n");
+        assertLineEnding("\r\n", "Test\r\nTest2\n");
+
+        assertLineEnding("\n", "\n");
+        assertLineEnding("\n", "Test\n");
+        assertLineEnding("\n", "Test\nTest2\r");
+        assertLineEnding("\n", "\n\t");
+    }
+    
+    static void assertLineEnding(String ending, String input) throws IOException {
+        try (Reader reader = new StringReader(input)) {
+            Assertions.assertEquals(ending, LineEnding.PreserveLineEndingPolicy.getEndingFor(reader));
+        }
+    }
+}