Expose a platform-specific line separator

Closes #448

Author: fzhinkin

core/api/kotlinx-io-core.api

@@ -91,6 +91,7 @@ public abstract interface annotation class kotlinx/io/InternalIoApi : java/lang/
 public final class kotlinx/io/JvmCoreKt {
 	public static final fun asSink (Ljava/io/OutputStream;)Lkotlinx/io/RawSink;
 	public static final fun asSource (Ljava/io/InputStream;)Lkotlinx/io/RawSource;
+	public static final fun getSystemLineSeparator ()Ljava/lang/String;
 }
 
 public abstract interface class kotlinx/io/RawSink : java/io/Flushable, java/lang/AutoCloseable {

core/api/kotlinx-io-core.klib.api

@@ -233,6 +233,8 @@ final val kotlinx.io.unsafe/SegmentReadContextImpl // kotlinx.io.unsafe/SegmentR
     final fun <get-SegmentReadContextImpl>(): kotlinx.io.unsafe/SegmentReadContext // kotlinx.io.unsafe/SegmentReadContextImpl.<get-SegmentReadContextImpl>|<get-SegmentReadContextImpl>(){}[0]
 final val kotlinx.io.unsafe/SegmentWriteContextImpl // kotlinx.io.unsafe/SegmentWriteContextImpl|{}SegmentWriteContextImpl[0]
     final fun <get-SegmentWriteContextImpl>(): kotlinx.io.unsafe/SegmentWriteContext // kotlinx.io.unsafe/SegmentWriteContextImpl.<get-SegmentWriteContextImpl>|<get-SegmentWriteContextImpl>(){}[0]
+final val kotlinx.io/SystemLineSeparator // kotlinx.io/SystemLineSeparator|{}SystemLineSeparator[0]
+    final fun <get-SystemLineSeparator>(): kotlin/String // kotlinx.io/SystemLineSeparator.<get-SystemLineSeparator>|<get-SystemLineSeparator>(){}[0]
 
 final fun (kotlinx.io.files/Path).kotlinx.io.files/sink(): kotlinx.io/Sink // kotlinx.io.files/sink|[email protected](){}[0]
 final fun (kotlinx.io.files/Path).kotlinx.io.files/source(): kotlinx.io/Source // kotlinx.io.files/source|[email protected](){}[0]

core/apple/src/-PlatformApple.kt

@@ -0,0 +1,13 @@
+/*
+ * Copyright 2010-2025 JetBrains s.r.o. and respective authors and developers.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the LICENCE file.
+ */
+
+package kotlinx.io
+
+/**
+ * Sequence of characters used as a line separator by the underlying platform.
+ *
+ * The value of this property is always `"\n"`.
+ */
+public actual val SystemLineSeparator: String = "\n"

core/common/src/Core.kt

@@ -43,3 +43,12 @@ private class DiscardingSink : RawSink {
   override fun flush() {}
   override fun close() {}
 }
+
+/**
+ * Sequence of characters used as a line separator by the underlying platform.
+ *
+ * The value of this property is platform-specific, but usually it is either `"\n"` or `"\r\n"`.
+ *
+ * See the documentation for a particular platform to get more information about the value of this property.
+ */
+public expect val SystemLineSeparator: String

core/common/test/LineSeparatorTest.kt

@@ -0,0 +1,21 @@
+/*
+ * Copyright 2010-2025 JetBrains s.r.o. and respective authors and developers.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the LICENCE file.
+ */
+
+package kotlinx.io
+
+import kotlinx.io.files.isWindows
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class LineSeparatorTest {
+    @Test
+    public fun testLineSeparator() {
+        if (isWindows) {
+            assertEquals("\r\n", SystemLineSeparator)
+        } else {
+            assertEquals("\n", SystemLineSeparator)
+        }
+    }
+}

core/js/src/-PlatformJs.kt

@@ -5,6 +5,8 @@
 
 package kotlinx.io
 
+import kotlinx.io.node.os
+
 public actual open class IOException : Exception {
     public actual constructor() : super()
 
@@ -31,3 +33,17 @@ internal actual fun withCaughtException(block: () -> Unit): Throwable? {
         return t
     }
 }
+
+/**
+ * Sequence of characters used as a line separator by the underlying platform.
+ *
+ * In NodeJS-compatible environments, this property derives value from [os.EOL](https://nodejs.org/api/os.html#oseol),
+ * in all other environments (like a web-browser), its value is always `"\n"`.
+ */
+public actual val SystemLineSeparator: String by lazy {
+    try {
+        os.EOL
+    } catch (_: Throwable) {
+        "\r\n"
+    }
+}

core/jvm/src/JvmCore.kt

@@ -109,3 +109,10 @@ internal val AssertionError.isAndroidGetsocknameError: Boolean
     get() {
         return cause != null && message?.contains("getsockname failed") ?: false
     }
+
+/**
+ * Sequence of characters used as a line separator by the underlying platform.
+ *
+ * Returns the same value as [System.lineSeparator].
+ */
+public actual val SystemLineSeparator: String = System.lineSeparator()

core/mingw/src/-PlatformMingw.kt

@@ -0,0 +1,13 @@
+/*
+ * Copyright 2010-2025 JetBrains s.r.o. and respective authors and developers.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the LICENCE file.
+ */
+
+package kotlinx.io
+
+/**
+ * Sequence of characters used as a line separator by the underlying platform.
+ *
+ * The value of this property is always `"\r\n"`.
+ */
+public actual val SystemLineSeparator: String = "\r\n"

core/nodeFilesystemShared/src/node/os.kt

@@ -16,6 +16,11 @@ internal external interface Os {
      * See https://nodejs.org/api/os.html#osplatform
      */
     fun platform(): String
+
+    /**
+     * See https://nodejs.org/api/os.html#oseol
+     */
+    val EOL: String
 }
 
 internal expect val os: Os

core/unix/src/-PlatformUnix.kt

@@ -0,0 +1,13 @@
+/*
+ * Copyright 2010-2025 JetBrains s.r.o. and respective authors and developers.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the LICENCE file.
+ */
+
+package kotlinx.io
+
+/**
+ * Sequence of characters used as a line separator by the underlying platform.
+ *
+ * The value of this property is always `"\n"`.
+ */
+public actual val SystemLineSeparator: String = "\n"

core/wasmJs/src/-PlatformWasmJs.kt

@@ -5,6 +5,8 @@
 
 package kotlinx.io
 
+import kotlinx.io.node.os
+
 internal class JsException(message: String) : RuntimeException(message)
 
 internal actual fun withCaughtException(block: () -> Unit): Throwable? {
@@ -23,3 +25,16 @@ private fun catchJsThrowable(block: () -> Unit): JsAny? = js("""{
     }
 }""")
 
+/**
+ * Sequence of characters used as a line separator by the underlying platform.
+ *
+ * In NodeJS-compatible environments, this property derives value from [os.EOL](https://nodejs.org/api/os.html#oseol),
+ * in all other environments (like a web-browser), its value is always `"\n"`.
+ */
+public actual val SystemLineSeparator: String by lazy {
+    try {
+        os.EOL
+    } catch (_: Throwable) {
+        "\n"
+    }
+}

core/wasmWasi/src/-PlatformWasmWasi.kt

@@ -0,0 +1,13 @@
+/*
+ * Copyright 2010-2025 JetBrains s.r.o. and respective authors and developers.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the LICENCE file.
+ */
+
+package kotlinx.io
+
+/**
+ * Sequence of characters used as a line separator by the underlying platform.
+ *
+ * The value of this property is always `"\n"`.
+ */
+public actual val SystemLineSeparator: String = "\n"