Add LineEnding.PRESERVE policy (#2304)

Author: nedtwigg

CHANGES.md

@@ -10,6 +10,8 @@ This document is intended for Spotless developers.
 We adhere to the [keepachangelog](https://keepachangelog.com/en/1.0.0/) format (starting after version `1.27.0`).
 
 ## [Unreleased]
+### Added
+* Support for line ending policy `PRESERVE` which just takes the first line ending of every given file as setting (no matter if `\n`, `\r\n` or `\r`) ([#2304](https://github.com/diffplug/spotless/pull/2304))
 ### Fixed
 * `ktlint` steps now read from the `string` instead of the `file` so they don't clobber earlier steps. (fixes [#1599](https://github.com/diffplug/spotless/issues/1599))
 

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2023 DiffPlug
+ * Copyright 2016-2024 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -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;
 
@@ -49,7 +53,9 @@ public Policy createPolicy() {
     /** {@code \n} */
     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 +1,50 @@
+/*
+ * Copyright 2024 DiffPlug
+ *
+ * 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));
+		}
+	}
+}

plugin-gradle/CHANGES.md

@@ -3,6 +3,8 @@
 We adhere to the [keepachangelog](https://keepachangelog.com/en/1.0.0/) format (starting after version `3.27.0`).
 
 ## [Unreleased]
+### Added
+* Support for line ending policy `PRESERVE` which just takes the first line ending of every given file as setting (no matter if `\n`, `\r\n` or `\r`) ([#2304](https://github.com/diffplug/spotless/pull/2304))
 ### Fixed
 * `ktlint` steps now read from the `string` instead of the `file` so they don't clobber earlier steps. (fixes [#1599](https://github.com/diffplug/spotless/issues/1599))
 

plugin-gradle/README.md

@@ -1577,7 +1577,13 @@ spotless {
     encoding 'Cp1252' // except java, which will be Cp1252
 ```
 
-Line endings can also be set globally or per-format using the `lineEndings` property.  Spotless supports four line ending modes: `UNIX`, `WINDOWS`, `MAC_CLASSIC`, `PLATFORM_NATIVE`, `GIT_ATTRIBUTES`, and `GIT_ATTRIBUTES_FAST_ALLSAME`.  The default value is `GIT_ATTRIBUTES_FAST_ALLSAME`, and *we highly recommend that you* ***do not change*** *this value*.  Git has opinions about line endings, and if Spotless and git disagree, then you're going to have a bad time. `FAST_ALLSAME` just means that Spotless can assume that every file being formatted has the same line endings ([more info](https://github.com/diffplug/spotless/pull/1838)).
+Line endings can also be set globally or per-format using the `lineEndings` property.  Spotless supports
+
+- constant modes (`UNIX`, `WINDOWS`, `MAC_CLASSIC`)
+- simple modes (`PLATFORM_NATIVE`, `PRESERVE`)
+- and git-aware modes (`GIT_ATTRIBUTES`, `GIT_ATTRIBUTES_FAST_ALLSAME`)
+
+The default value is `GIT_ATTRIBUTES_FAST_ALLSAME`, and *we highly recommend that you* ***do not change*** *this value*.  Git has opinions about line endings, and if Spotless and git disagree, then you're going to have a bad time. `FAST_ALLSAME` just means that Spotless can assume that every file being formatted has the same line endings ([more info](https://github.com/diffplug/spotless/pull/1838)).
 
 You can easily set the line endings of different files using [a `.gitattributes` file](https://help.github.com/articles/dealing-with-line-endings/).  Here's an example `.gitattributes` which sets all files to unix newlines: `* text eol=lf`.
 

plugin-maven/CHANGES.md

@@ -3,6 +3,8 @@
 We adhere to the [keepachangelog](https://keepachangelog.com/en/1.0.0/) format (starting after version `1.27.0`).
 
 ## [Unreleased]
+### Added
+* Support for line ending policy `PRESERVE` which just takes the first line ending of every given file as setting (no matter if `\n`, `\r\n` or `\r`) ([#2304](https://github.com/diffplug/spotless/pull/2304))
 ### Fixed
 * `ktlint` steps now read from the `string` instead of the `file` so they don't clobber earlier steps. (fixes [#1599](https://github.com/diffplug/spotless/issues/1599))
 

plugin-maven/README.md

@@ -1784,9 +1784,15 @@ Spotless uses UTF-8 by default, but you can use [any encoding which Java support
 </configuration>
 ```
 
-Line endings can also be set globally or per-format using the `lineEndings` property.  Spotless supports four line ending modes: `UNIX`, `WINDOWS`, `MAC_CLASSIC`, `PLATFORM_NATIVE`, `GIT_ATTRIBUTES`, and `GIT_ATTRIBUTES_FAST_ALLSAME`.  The default value is `GIT_ATTRIBUTES_FAST_ALLSAME`, and *we highly recommend that you* ***do not change*** *this value*.  Git has opinions about line endings, and if Spotless and git disagree, then you're going to have a bad time. `FAST_ALLSAME` just means that Spotless can assume that every file being formatted has the same line endings ([more info](https://github.com/diffplug/spotless/pull/1838)).
+Line endings can also be set globally or per-format using the `lineEndings` property.  Spotless supports
 
-You can easily set the line endings of different files using [a `.gitattributes` file](https://help.github.com/articles/dealing-with-line-endings/).  Here's an example `.gitattributes` which sets all files to unix newlines: `* text eol=lf`.
+- constant modes (`UNIX`, `WINDOWS`, `MAC_CLASSIC`)
+- simple modes (`PLATFORM_NATIVE`, `PRESERVE`)
+- and git-aware modes (`GIT_ATTRIBUTES`, `GIT_ATTRIBUTES_FAST_ALLSAME`)
+
+The default value is `GIT_ATTRIBUTES_FAST_ALLSAME`, and *we highly recommend that you* ***do not change*** *this value*.  Git has opinions about line endings, and if Spotless and git disagree, then you're going to have a bad time. `FAST_ALLSAME` just means that Spotless can assume that every file being formatted has the same line endings ([more info](https://github.com/diffplug/spotless/pull/1838)).
+
+You can easily set the line endings of different files using [a `.gitattributes` file](https://help.github.com/articles/dealing-with-line-endings/). Here's an example `.gitattributes` which sets all files to unix newlines: `* text eol=lf`.
 
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