Advent of code 2019 day 16

package aoc;

import java.io.IOException;
import java.nio.charset.Charset;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Date;

public class AOC16xOPT {
	private static final String INPUT = "59764635797473718052486376718142408346357676818478503599633670059885748195966091103097769012608550645686932996546030476521264521211192035231303791868456877717957482002303790897587593845163033589025995509264282936119874431944634114034231860653524971772670684133884675724918425789232716494769777580613065860450960426147822968107966020797566015799032373298777368974345143861776639554900206816815180398947497976797052359051851907518938864559670396616664893641990595511306542705720282494028966984911349389079744726360038030937356245125498836945495984280140199805250151145858084911362487953389949062108285035318964376799823425466027816115616249496434133896";

	

	public static void main(String[] args) throws IOException {
		partOne();
		partTwo();
	}
	
	
	
	/** PART ONE STUFF **/ 

	static int[] PATTERN = {0,1,0,-1};
	
	
	public static String faseAlgorithmGood(String x)
	{
		var chars = x.toCharArray();
		StringBuffer res = new StringBuffer();
		for (int charIdxBeingGenerated = 0; charIdxBeingGenerated < chars.length; charIdxBeingGenerated++) 
		{ 
			
			long acc = 0;
			for (int charIdx = charIdxBeingGenerated /* skip initial zeroes */; charIdx < chars.length; charIdx++) 
			{
				int n = chars[charIdx]-48;
				int val = getMultiplier(charIdxBeingGenerated, charIdx);
				
				
				
				if ((val == 0)) /* skip internal zero sequences*/
				{
					int skip = (charIdxBeingGenerated); 
					// System.out.println("Skipping "+skip);
					charIdx +=skip;
					continue;
				}
				acc = acc + n * val;
			}
			res.append( Math.abs(acc) % 10 );
		}
		return res.toString();
	}

	private static int getMultiplier(int charIdxBeingGenerated, int charIdx) 
	{
		int repetition = charIdxBeingGenerated+1;
		int idx = (charIdx+1) / repetition;
		int val = PATTERN[idx % 4];
		return val;
	}
	
	

	private static void partOne() {
		String input = INPUT;
		
		for (int i = 0; i < 100; i++) {
			input = faseAlgorithmGood(input);
			System.out.println(input);
		}
	}	

	/*** PART TWO STUFF **/
	
	private static String faseAlgorithmBad(String inp)
	{
		StringBuffer b = new StringBuffer();
		var chars = inp.toCharArray();
		
		int acc = 0;		
		for (int i = 0; i < chars.length; i++) {
			int n = chars[chars.length-i-1]-48;
			acc += n;
			b.append(acc % 10);
		}
		b.reverse();
		return b.toString();
		
	}

	
	private static void partTwo() throws IOException {
		String input = generateLongInput();
		
		for (int i = 0; i < 100; i++) {
			input = faseAlgorithmBad(input);
		}
		System.out.println(input.substring(5976463,5976463+8));
	}

	private static String generateLongInput() throws IOException {

		// generate the long 10000x input and store it on a file to debug quickly :P
		Path p = Paths.get("longinput.txt");
		String input = "";
		if(!Files.exists(p))
		{
			String a = INPUT;
			for (int i = 0; i < 10000; i++) {
				input=input+a;
			}
			System.out.println("Input generated "+input.length());
			
			Files.writeString(p, input, Charset.forName("ASCII"));
			System.out.println("File written");
		}
		else
		{
			input = Files.readString(p, Charset.forName("ASCII"));
			System.out.println(new Date()+ "File read");
		}
		return input;
	}
	
}