Quickly count lines in large files. 大きなファイルの行数を素早く数える。

CountLines.java

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));
    }
}