introduce prependBOM option

README.md

@@ -268,6 +268,7 @@ val writer = csvWriter {
 | nullCode | `(empty string)` | Character used when a written field is null value. |
 | lineTerminator | `\r\n` | Character used as line terminator. |
 | outputLastLineTerminator | `true` | Output line break at the end of file or not. |
+| prependBOM | `false` | Output BOM (Byte Order Mark) at the beginning of file or not. |
 | quote.char  | `"` | Character to quote each fields. |
 | quote.mode  | `CANONICAL` | Quote mode. <br />- `CANONICAL`: Not quote normally, but quote special characters (quoteChar, delimiter, line feed). This is [the specification of CSV](https://tools.ietf.org/html/rfc4180#section-2).<br />- `ALL`: Quote all fields.<br />- `NON_NUMERIC`: Quote non-numeric fields. (ex. 1,"a",2.3) |
 

src/commonMain/kotlin/com/github/doyaaaaaken/kotlincsv/client/BufferedLineReader.kt

@@ -1,13 +1,15 @@
 package com.github.doyaaaaaken.kotlincsv.client
 
+import com.github.doyaaaaaken.kotlincsv.util.Const
+
 /**
  * buffered reader which can read line with line terminator
  */
 internal class BufferedLineReader(
     private val br: Reader
 ) {
     companion object {
-        private const val BOM = '\uFEFF'
+        private const val BOM = Const.BOM
     }
 
     private fun StringBuilder.isEmptyLine(): Boolean =

src/commonMain/kotlin/com/github/doyaaaaaken/kotlincsv/dsl/context/CsvWriterContext.kt

@@ -57,6 +57,12 @@ interface ICsvWriterContext {
      */
     val outputLastLineTerminator: Boolean
 
+    /**
+     * Output BOM (Byte Order Mark) at the beginning of file or not.
+     * See https://github.com/doyaaaaaken/kotlin-csv/issues/84
+     */
+    val prependBOM: Boolean
+
     /**
      * Options about quotes of each fields
      */
@@ -75,6 +81,7 @@ class CsvWriterContext : ICsvWriterContext {
     override var nullCode: String = ""
     override var lineTerminator: String = "\r\n"
     override var outputLastLineTerminator = true
+    override var prependBOM = false
     override val quote: CsvWriteQuoteContext = CsvWriteQuoteContext()
 
     fun quote(init: CsvWriteQuoteContext.() -> Unit) {

src/commonMain/kotlin/com/github/doyaaaaaken/kotlincsv/parser/ParseStateMachine.kt

@@ -1,6 +1,7 @@
 package com.github.doyaaaaaken.kotlincsv.parser
 
 import com.github.doyaaaaaken.kotlincsv.util.CSVParseFormatException
+import com.github.doyaaaaaken.kotlincsv.util.Const
 
 /**
  * @author doyaaaaaaken
@@ -11,7 +12,7 @@ internal class ParseStateMachine(
     private val escapeChar: Char
 ) {
 
-    private val BOM = '\uFEFF'
+    private val BOM = Const.BOM
 
     private var state = ParseState.START
 

src/commonMain/kotlin/com/github/doyaaaaaken/kotlincsv/util/Const.kt

@@ -6,5 +6,7 @@ package com.github.doyaaaaaken.kotlincsv.util
  * @author doyaaaaaken
  */
 internal object Const {
-    val defaultCharset = "UTF-8"
+    const val defaultCharset = "UTF-8"
+
+    const val BOM = '\uFEFF'
 }

src/jvmMain/kotlin/com/github/doyaaaaaken/kotlincsv/client/CsvFileWriter.kt

@@ -2,6 +2,7 @@ package com.github.doyaaaaaken.kotlincsv.client
 
 import com.github.doyaaaaaken.kotlincsv.dsl.context.CsvWriterContext
 import com.github.doyaaaaaken.kotlincsv.dsl.context.WriteQuoteMode
+import com.github.doyaaaaaken.kotlincsv.util.Const
 import java.io.Closeable
 import java.io.Flushable
 import java.io.IOException
@@ -85,6 +86,10 @@ class CsvFileWriter internal constructor(
     }
 
     private fun writeNext(row: List<Any?>) {
+        if (!hasWroteInitialChar && ctx.prependBOM) {
+            writer.print(Const.BOM)
+        }
+
         val rowStr = row.joinToString(ctx.delimiter.toString()) { field ->
             if (field == null) {
                 ctx.nullCode

src/jvmTest/kotlin/com/github/doyaaaaaken/kotlincsv/client/CsvWriterTest.kt

@@ -32,6 +32,8 @@ class CsvWriterTest : WordSpec({
                 delimiter = '\t'
                 nullCode = "NULL"
                 lineTerminator = "\n"
+                outputLastLineTerminator = false
+                prependBOM = true
                 quote {
                     char = '\''
                     mode = WriteQuoteMode.ALL
@@ -43,6 +45,8 @@ class CsvWriterTest : WordSpec({
                 writer.delimiter shouldBe '\t'
                 writer.nullCode shouldBe "NULL"
                 writer.lineTerminator shouldBe "\n"
+                writer.outputLastLineTerminator shouldBe false
+                writer.prependBOM shouldBe true
                 writer.quote.char = '\''
                 writer.quote.mode = WriteQuoteMode.ALL
             }
@@ -272,6 +276,18 @@ class CsvWriterTest : WordSpec({
             val actual = readTestFile()
             actual shouldBe expected
         }
+        "write simple csv with prepending BOM" {
+            val row1 = listOf("a", "b")
+            val row2 = listOf("c", "d")
+            val expected = "\uFEFFa,b\r\nc,d\r\n"
+            csvWriter {
+                prependBOM = true
+            }.open(File(testFileName)) {
+                writeRows(listOf(row1, row2))
+            }
+            val actual = readTestFile()
+            actual shouldBe expected
+        }
         "write simple csv with disabled last line terminator multiple writes" {
             val row1 = listOf("a", "b")
             val row2 = listOf("c", "d")

src/jvmTest/kotlin/com/github/doyaaaaaken/kotlincsv/dsl/CsvWriterDslTest.kt

@@ -22,6 +22,7 @@ class CsvWriterDslTest : StringSpec({
             nullCode = "NULL"
             lineTerminator = "\n"
             outputLastLineTerminator = false
+            prependBOM = true
             quote {
                 char = '\''
                 mode = WriteQuoteMode.ALL
@@ -33,6 +34,7 @@ class CsvWriterDslTest : StringSpec({
             writer.nullCode shouldBe "NULL"
             writer.lineTerminator shouldBe "\n"
             writer.outputLastLineTerminator shouldBe false
+            writer.prependBOM shouldBe true
             writer.quote.char shouldBe '\''
             writer.quote.mode = WriteQuoteMode.ALL
         }