IO-639: Fixed ReversedLinesFileReader does not read first line if it's empty (#829)

Author: KishorMore-2005

src/main/java/org/apache/commons/io/input/ReversedLinesFileReader.java

@@ -212,6 +212,13 @@ private String readLine() { //NOPMD Bug in PMD
             final boolean isLastFilePart = partNumber == 1;
 
             int i = currentLastBytePos;
+
+            if (i == -1 && isLastFilePart && leftOver != null) {
+                line = new String(leftOver, charset);
+                leftOver = null;
+                return line;
+            }
+
             while (i > -1) {
 
                 if (!isLastFilePart && i < avoidNewlineSplitBufferSize) {
@@ -234,6 +241,10 @@ private String readLine() { //NOPMD Bug in PMD
                     line = new String(lineData, charset);
 
                     currentLastBytePos = i - newLineMatchByteCount;
+
+                    if (isLastFilePart && currentLastBytePos == -1 && i == 0) {
+                        leftOver = new byte[0];
+                    }
                     break; // found line
                 }
 
@@ -242,14 +253,22 @@ private String readLine() { //NOPMD Bug in PMD
 
                 // end of file part handling
                 if (i < 0) {
-                    createLeftOver();
+                    if (isLastFilePart) {
+                        final int lineLengthBytes = currentLastBytePos + 1;
+                        if (lineLengthBytes > 0) {
+                            final byte[] lineData = Arrays.copyOf(data, lineLengthBytes);
+                            line = new String(lineData, charset);
+                        }
+                        currentLastBytePos = -1;
+                    } else {
+                        createLeftOver();
+                    }
                     break; // end of file part
                 }
             }
 
-            // last file part handling
-            if (isLastFilePart && leftOver != null) {
-                // there will be partNumber line break anymore, this is the first line of the file
+            // there will be partNumber line break anymore, this is the first line of the file
+            if (line == null && isLastFilePart && leftOver != null) {
                 line = new String(leftOver, charset);
                 leftOver = null;
             }

src/test/java/org/apache/commons/io/input/ReversedLinesFileReaderSimpleTest.java

@@ -30,7 +30,6 @@
 
 import org.apache.commons.io.IOUtils;
 import org.apache.commons.io.TestResources;
-import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.MethodSource;
@@ -42,10 +41,10 @@ class ReversedLinesFileReaderSimpleTest {
      */
     @ParameterizedTest
     @MethodSource("org.apache.commons.io.input.ReversedLinesFileReaderParamBlockSizeTest#blockSizes")
-    @Disabled
     void testEmptyFirstLine(final int blockSize) throws Exception {
         final File testFileEmptyFirstLine = TestResources.getFile("/empty-first-line.bin");
-        try (ReversedLinesFileReader reversedLinesFileReader = new ReversedLinesFileReader(testFileEmptyFirstLine, 10, StandardCharsets.US_ASCII.name())) {
+        try (ReversedLinesFileReader reversedLinesFileReader = new ReversedLinesFileReader(testFileEmptyFirstLine,
+                blockSize, StandardCharsets.US_ASCII.name())) {
             assertEqualsAndNoLineBreaks("test2", reversedLinesFileReader.readLine());
             assertEqualsAndNoLineBreaks("", reversedLinesFileReader.readLine());
             assertEqualsAndNoLineBreaks("test1", reversedLinesFileReader.readLine());