/*
 * Copyright (C) 2023 Laurent CLOUET
 * Author Laurent CLOUET <laurent.clouet@nopnop.net>
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; version 2
 * of the License.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 */

package com.sheepit.client;

import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.security.DigestInputStream;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Objects;

/**
 * Provides facilities to get an MD5 checksum and cache the results thereof
 */
public class Md5 {
	/**
	 * Hashmap of all files uniqueMD5keys (last modification time + file)
	 * and their checksums that have been calculated so far
	 */
	private static final Map<UniqueMD5Key, String> cache = new HashMap<>();
	private static final int cacheCleanTrigger = 64; //After how many cache hits to check and clean the cache
	private static int cacheHits = 0;
	

	
	/**
	 * Removes a file from cache
	 * @param path Path to the file
	 */
	public static void remove(String path) {
		UniqueMD5Key key = new UniqueMD5Key(path);
		cache.remove(key); //Doesn't handle entries that the files moved or had their lastModified changed
	}
	
	/**
	 * Gets a MD5 checksum either from the cache or computes it and puts it into the cache
	 *
	 * @param path Path of the file to get the MD5 checksum for
	 * @return the string The MD5 checksum
	 */
	public String get(String path) {
		UniqueMD5Key key = new UniqueMD5Key(path);
		if (cache.containsKey(key) == false) {
			generate(path);
		}
		if (cacheHits >= cacheCleanTrigger){
			cacheCheck();
			cacheHits = 0;
		} else {
			cacheHits++;
		}
		return cache.get(key);
	}
	
	/**
	 * Checks cache for files that don't exist anymore according to cached values and removes them
	 * Catches entries tha fall through the cracks
	 */
	private void cacheCheck(){
		Iterator<Map.Entry<UniqueMD5Key, String>> it = cache.entrySet().iterator();
		while (it.hasNext()) {
			UniqueMD5Key itKey = (UniqueMD5Key)it.next();
			if (itKey.exists() == false) {
				it.remove(); //Iterator instead of foreach because we remove items
			}
		}
	}
	
	/**
	 * Calculates the md5 checksum for a given file
	 * @param path Path to the file to calculate the checksum for
	 */
	private void generate(String path) {
		UniqueMD5Key key = new UniqueMD5Key(path);
		try {
			MessageDigest md = MessageDigest.getInstance("MD5");
			InputStream is = Files.newInputStream(Paths.get(path));
			DigestInputStream dis = new DigestInputStream(is, md);
			byte[] buffer = new byte[8192];
			while (dis.read(buffer) > 0)
				; // process the entire file
			String checksum = Utils.convertBinaryToHex(md.digest());
			dis.close();
			is.close();
			cache.put(key, checksum);
		}
		catch (NoSuchAlgorithmException | IOException e) {
			cache.put(key, "");
		}
	}
}

/**
 * Used as key for the hashmap that is the cache
 * while also making it accessible to check if key is still valid
 */
class UniqueMD5Key {
	private final long lastModified;
	private final File file;
	
	/**
	 * Returns a UniqueMD5Key for a given filepath
	 * Automatically fills in lastModified time
	 * @param filepath The filepath to the file
	 */
	public UniqueMD5Key(String filepath) {
		this.file = new File(filepath);
		this.lastModified = this.file.lastModified();
	}
	/**
	 * @see File#exists()
	 * also takes lastModified into account.
	 */
	public boolean exists(){
		return file.exists() && lastModified == this.file.lastModified();
	}
	
	/**
	 * @see Object#toString()
	 */
	@Override public String toString() {
		return "UniqueMD5Key{" + "lastModified=" + lastModified + ", file=" + file + '}';
	}
	
	/**
	 * @see Object#equals(Object)
	 */
	@Override public boolean equals(Object o) {
		if (this == o)
			return true;
		if (o == null || getClass() != o.getClass())
			return false;
		UniqueMD5Key that = (UniqueMD5Key) o;
		return lastModified == that.lastModified && Objects.equals(file, that.file);
	}
	
	/**
	 * @see Object#hashCode()
	 */
	@Override public int hashCode() {
		return Objects.hash(lastModified, file);
	}
}