Created
January 15, 2023 14:26
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Quickly count lines in large files. 大きなファイルの行数を素早く数える。
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| import java.nio.ByteBuffer; | |
| import java.nio.file.Files; | |
| import java.nio.file.Paths; | |
| import java.nio.file.StandardOpenOption; | |
| public class CountLines { | |
| private static final int MAX_BUFFER_SIZE = 1024 * 1024; | |
| public static void main(String[] args) throws Exception { | |
| if (args.length < 1) { | |
| System.err.println("The file path is not specified."); | |
| System.exit(1); | |
| } | |
| final var filePath = Paths.get(args[0]).toAbsolutePath(); | |
| if (!Files.isRegularFile(filePath)) { | |
| System.err.println("The specified file path does not exist."); | |
| System.exit(1); | |
| } | |
| try (var channel = Files.newByteChannel(filePath, StandardOpenOption.READ)) { | |
| final var bufferSize = (int) clamp(Files.size(filePath), 1, MAX_BUFFER_SIZE); | |
| final var buf = ByteBuffer.allocateDirect(bufferSize); | |
| var result = 0L; | |
| while (channel.read(buf) != -1) { | |
| buf.flip(); | |
| while (buf.hasRemaining()) { | |
| if (buf.get() == '\n') { | |
| result++; | |
| } | |
| } | |
| buf.clear(); | |
| } | |
| System.out.println(result); | |
| } | |
| } | |
| private static long clamp(long value, long min, long max) { | |
| return Math.max(min, Math.min(max, value)); | |
| } | |
| } |
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