Day19.java

import java.util.ArrayList;
import java.util.List;

public class Day19 {

	public static String replace(String where, String what, String with, int pos) {
		return where.substring(0, pos) + with + where.substring(pos + what.length());
	}

	public static void main(String[] args) {
		List<String[]> transforms = FileIO.getFileLinesSplit(".\\InputDay19.txt", " => ");

		String medicine = "ORnPBPMgArCaCaCaSiThCaCaSiThCaCaPBSiRnFArRnFArCaCaSiThCaCaSiThCaCaCaCaCaCaSiRnFYFArSiRnMgArCaSiRnPTiTiBFYPBFArSiRnCaSiRnTiRnFArSiAlArPTiBPTiRnCaSiAlArCaPTiTiBPMgYFArPTiRnFArSiRnCaCaFArRnCaFArCaSiRnSiRnMgArFYCaSiRnMgArCaCaSiThPRnFArPBCaSiRnMgArCaCaSiThCaSiRnTiMgArFArSiThSiThCaCaSiRnMgArCaCaSiRnFArTiBPTiRnCaSiAlArCaPTiRnFArPBPBCaCaSiThCaPBSiThPRnFArSiThCaSiThCaSiThCaPTiBSiRnFYFArCaCaPRnFArPBCaCaPBSiRnTiRnFArCaPRnFArSiRnCaCaCaSiThCaRnCaFArYCaSiRnFArBCaCaCaSiThFArPBFArCaSiRnFArRnCaCaCaFArSiRnFArTiRnPMgArF";
		
		// part 1
		
		List<String> molecules = new ArrayList<>();
		
		for(String[] trans : transforms) {
			int pos = 0;
			while((pos = medicine.indexOf(trans[0], pos)) >= 0) {
				molecules.add(replace(medicine,trans[0],trans[1],pos));
				pos += trans[0].length();
			}
		}
		
		long count = molecules.stream()
				.distinct()
				.count();
		
		System.out.println("Part 1: Total unique molecules = " + count);
		
		// part 2
		
		int count2 = 0;
		while (!medicine.equals("e")) {
			for(String[] trans : transforms) {
				if(medicine.contains(trans[1])) {
					medicine = replace(medicine, trans[1], trans[0], medicine.indexOf(trans[1]));
					System.out.println(medicine);
					
					count2++;
				}
			}
		}

		System.out.println("Part 2: Total unique molecules = " + count2);

	}

}

InputDay19.txt

Al => ThF
Al => ThRnFAr
B => BCa
B => TiB
B => TiRnFAr
Ca => CaCa
Ca => PB
Ca => PRnFAr
Ca => SiRnFYFAr
Ca => SiRnMgAr
Ca => SiTh
F => CaF
F => PMg
F => SiAl
H => CRnAlAr
H => CRnFYFYFAr
H => CRnFYMgAr
H => CRnMgYFAr
H => HCa
H => NRnFYFAr
H => NRnMgAr
H => NTh
H => OB
H => ORnFAr
Mg => BF
Mg => TiMg
N => CRnFAr
N => HSi
O => CRnFYFAr
O => CRnMgAr
O => HP
O => NRnFAr
O => OTi
P => CaP
P => PTi
P => SiRnFAr
Si => CaSi
Th => ThCa
Ti => BP
Ti => TiTi
e => HF
e => NAl
e => OMg