import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.nio.ByteOrder;
import java.nio.charset.StandardCharsets;

// # CSV编码规则
// - ','间隔列;"\r\n"间隔行(最后一行可能没有换行符结尾)
// - 文本含有','或'\r',或以'"'开头,则整个内容两边加'"'
// - 两边加'"'的情况下,里面的每个'"'改成两个
// # 简单&高性能的实现,需要严格按以上规则编码,需要JDK9以上
public final class FastCsv {
	private static final VarHandle longLeVH = MethodHandles.byteArrayViewVarHandle(long[].class, ByteOrder.LITTLE_ENDIAN);

	private FastCsv() {
	}

	public interface Handler {
		void onCell(int pos, int len); // len<0: next line
	}

	public static void parseCsv(final byte[] csv, final Handler handler) { // csv maybe modified
		parseCsv(csv, 0, csv.length, handler);
	}

	public static void parseCsv(final byte[] csv, int pos, final int len, final Handler handler) { // csv maybe modified
		final int end = pos + len;
		if (len >= 3 && csv[pos] == -17 && csv[pos + 1] == -69 && csv[pos + 2] == -65) // check UTF-8 BOM
			pos += 3; // \xef\xbb\xbf
		for (; ; ) {
			if (pos >= end) {
				handler.onCell(end, -1);
				return;
			}
			int b = csv[pos];
			if (b == ',') {
				handler.onCell(pos++, 0); // ,
				if (pos >= end) {
					handler.onCell(pos, 0);
					handler.onCell(end, -1);
					return;
				}
			} else if (b == '\r') {
				handler.onCell(pos, 0);
				handler.onCell(pos, -1);
				if ((pos += 2) >= end) // \r\n
					return;
			} else if (b == '"') {
				int e = scanQuot(csv, ++pos, end), p = e, q;
				while ((q = e + 1) < end && (b = csv[q]) == '"') {
					final int n = (e = scanQuot(csv, q + 1, end)) - q;
					System.arraycopy(csv, q, csv, p, n);
					p += n;
				}
				handler.onCell(pos, p - pos);
				if (b == ',')
					pos = e + 2; // ",
				else { // '\r' | <EOF>
					handler.onCell(e + 1, -1);
					if ((pos = e + 3) >= end) // "\r\n | "<EOF>
						return;
				}
			} else {
				final int e = scanCellEnd(csv, pos + 1, end);
				handler.onCell(pos, e - pos);
				if (e < end && csv[e] == ',')
					pos = e + 1; // ,
				else { // '\r' | <EOF>
					handler.onCell(e, -1);
					if ((pos = e + 2) >= end) // \r\n
						return;
				}
			}
		}
	}

	private static int scanQuot(final byte[] csv, int pos, final int end) {
		for (; pos + 8 <= end; pos += 8) {
			long p = (long)longLeVH.get(csv, pos) ^ 0x22222222_22222222L; // '"'
			p = (p - 0x01010101_01010101L) & ~p & 0x80808080_80808080L;
			if (p != 0)
				return pos + (Long.numberOfTrailingZeros(p) >> 3);
		}
		for (; ; pos++) {
			if (pos >= end || csv[pos] == '"')
				return pos;
		}
	}

	private static int scanCellEnd(final byte[] csv, int pos, final int end) {
		for (; pos + 8 <= end; pos += 8) {
			final long b8 = (long)longLeVH.get(csv, pos);
			long p0 = b8 ^ 0x2c2c2c2c_2c2c2c2cL; // ','
			long p1 = b8 ^ 0x0d0d0d0d_0d0d0d0dL; // '\r'
			p0 = (p0 - 0x01010101_01010101L) & ~p0 & 0x80808080_80808080L;
			p1 = (p1 - 0x01010101_01010101L) & ~p1 & 0x80808080_80808080L;
			if ((p0 |= p1) != 0)
				return pos + (Long.numberOfTrailingZeros(p0) >> 3);
		}
		for (int b; ; pos++) {
			if (pos >= end || (b = csv[pos]) == ',' || b == '\r')
				return pos;
		}
	}

	static void main() {
		/*	----------------------------
			|ab|a,b|"ab"|a"b| ab |"a,b"|
			----------------------------
			|a|
			|b|
			----------------------------
			||||||ab|
			----------------------------
			|ab"|||||a"b|
			----------------------------*/
		final byte[] csv = """
				ab,"a,b",""\"ab""\",a"b, ab ,""\"a,b""\"\r
				"a
				b",,,,,\r
				,,,,,"ab"\r
				ab",,,,,"a""b\"""".getBytes(StandardCharsets.UTF_8);
		System.out.println("---");
		System.out.println(new String(csv, StandardCharsets.UTF_8));
		System.out.println("---");
		final StringBuilder sb = new StringBuilder();
		parseCsv(csv, (pos, len) -> {
			if (len < 0)
				sb.append("<EOL>\n");
			else
				sb.append('[').append(new String(csv, pos, len, StandardCharsets.UTF_8)).append(']');
		});
		final String out = sb.toString();
		System.out.println(out);
		System.out.println("===");
		System.out.println("check: " + out.equals("""
				[ab][a,b]["ab"][a"b][ ab ]["a,b"]<EOL>
				[a
				b][][][][][]<EOL>
				[][][][][][ab]<EOL>
				[ab"][][][][][a"b]<EOL>
				"""));
	}
}