com.scudata.dm.cursor.SubCursor Maven / Gradle / Ivy

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SPL(Structured Process Language) A programming language specially for structured data computing.

There is a newer version: 20240823

package com.scudata.dm.cursor;

import com.scudata.dm.Context;
import com.scudata.dm.Sequence;

/**
 * ???α꣬???ڴӸ????α???ȡָ????????
 * @author RunQian
 *
 */
public class SubCursor extends ICursor {
	private ICursor cursor; // Դ?α?
	private int count; // ??ȡ??¼??
	private int total; // ??Ҫȡ???ܼ?¼??
	
	/**
	 * ???????α?
	 * @param cursor Դ?α?
	 * @param total Ҫȡ?????ݵ???????
	 */
	public SubCursor(ICursor cursor, int total) {
		this.cursor = cursor;
		this.total = total;
		setDataStruct(cursor.getDataStruct());
	}
	
	// ???м???ʱ??Ҫ?ı???????
	// ?̳???????õ??˱???ʽ????Ҫ?????????????½???????ʽ
	public void resetContext(Context ctx) {
		if (this.ctx != ctx) {
			cursor.resetContext(ctx);
			super.resetContext(ctx);
		}
	}

	/**
	 * ??ȡָ???????????ݷ???
	 * @param n ????
	 * @return Sequence
	 */
	protected Sequence get(int n) {
		if (cursor == null || n < 1) return null;

		if (count + n > total) {
			n = total - count;
			if (n == 0) return null;
		}
		
		Sequence seq = cursor.fetch(n);
		if (seq != null) {
			count += seq.length();
		}
		
		return seq;
	}

	/**
	 * ????ָ????????????
	 * @param n ????
	 * @return long ʵ????????????
	 */
	protected long skipOver(long n) {
		if (cursor == null || n < 1) return 0;

		if (count > total - n) {
			n = total - count;
			if (n == 0) return 0;
		}
		
		n = cursor.skip(n);
		count += n;
		return n;
	}

	/**
	 * ?ر??α?
	 */
	public synchronized void close() {
		super.close();
		if (cursor != null) {
			cursor.close();
			cursor = null;
		}
	}
}