One to many mappings and combination generation

Question

Please help folks,
I want to perform one to many mapping i.e a single row of data will be taken from a single table Table 'A' into Hashtable A and it has to be matched with 'n' number of rows from another table Table 'B' taken into Hashtable B. The no. of rows selected from B can be 1/2.... upto n. For performing this mapping iam using a Combination Generator Code which will Key nos. to be selected for checking whether they can be mapped and then further processing is done on these key nos. returned by the combination generator. The code for combination generator is given below.
The issue is say if there are about 50 rows in table B then no of combinations possible are 2 ^ 50, which is just too much for the PC to handle i.e the code takes about 4-5 days of processing time. 256 MB RAM, P4. The mapping has to be done in 30 minutes!!!! Ne ideas folks????
// Systematically generate combinations.
import java.math.BigInteger;
public class CombinationGenerator {
private int[] a;
private int n;
private int r;
private BigInteger numLeft;
private BigInteger total;
// Constructor
public static void main(String[] args)
String[] elements = {"a", "b", "c", "d", "e", "f", "g"};
int[] indices;
CombinationGenerator x = new CombinationGenerator (elements.length, 3);
StringBuffer combination;
while (x.hasMore ()) {
combination = new StringBuffer ();
indices = x.getNext ();
for (int i = 0; i < indices.length; i++) {
combination.append (elements[indices[i]]);
System.out.println (combination.toString ());
public CombinationGenerator (int n, int r) {
if (r > n) {
throw new IllegalArgumentException ();
if (n < 1) {
throw new IllegalArgumentException ();
this.n = n;
this.r = r;
a = new int[r];
BigInteger nFact = getFactorial (n);
BigInteger rFact = getFactorial (r);
BigInteger nminusrFact = getFactorial (n - r);
total = nFact.divide (rFact.multiply (nminusrFact));
reset ();
// Reset
public void reset () {
for (int i = 0; i < a.length; i++) {
a[i] = i;
numLeft = new BigInteger (total.toString ());
// Return number of combinations not yet generated
public BigInteger getNumLeft () {
return numLeft;
// Are there more combinations?
public boolean hasMore () {
return numLeft.compareTo (BigInteger.ZERO) == 1;
// Return total number of combinations
public BigInteger getTotal () {
return total;
// Compute factorial
private static BigInteger getFactorial (int n) {
BigInteger fact = BigInteger.ONE;
for (int i = n; i > 1; i--) {
fact = fact.multiply (new BigInteger (Integer.toString (i)));
return fact;
// Generate next combination (algorithm from Rosen p. 286)
public int[] getNext () {
if (numLeft.equals (total)) {
numLeft = numLeft.subtract (BigInteger.ONE);
return a;
int i = r - 1;
while (a[i] == n - r + i) {
i--;
System.out.println("inside while");
a[i] = a[i] + 1;
for (int j = i + 1; j < r; j++) {
a[j] = a[i] + j - i;
numLeft = numLeft.subtract (BigInteger.ONE);
return a;

Answer 1

Next time you post, see the formatting guidelines [url http://forum.java.sun.com/features.jsp#Formatting]here. //--------------------------------------
// Systematically generate combinations.
import java.math.BigInteger;
public class CombinationGenerator {
private int[] a;
private int n;
private int r;
private BigInteger numLeft;
private BigInteger total;
// Constructor
public static void main(String[] args)
String[] elements = {"a", "b", "c", "d", "e", "f", "g"};
int[] indices;
CombinationGenerator x = new CombinationGenerator (elements.length, 3);
StringBuffer combination;
while (x.hasMore ()) {
combination = new StringBuffer ();
indices = x.getNext ();
for (int i = 0; i < indices.length; i++) {
combination.append (elements[indices]);
System.out.println (combination.toString ());
public CombinationGenerator (int n, int r) {
if (r > n) {
throw new IllegalArgumentException ();
if (n < 1) {
throw new IllegalArgumentException ();
this.n = n;
this.r = r;
a = new int[r];
BigInteger nFact = getFactorial (n);
BigInteger rFact = getFactorial (r);
BigInteger nminusrFact = getFactorial (n - r);
total = nFact.divide (rFact.multiply (nminusrFact));
reset ();
// Reset
public void reset () {
for (int i = 0; i < a.length; i++) {
a = i;
numLeft = new BigInteger (total.toString ());
// Return number of combinations not yet generated
public BigInteger getNumLeft () {
return numLeft;
// Are there more combinations?
public boolean hasMore () {
return numLeft.compareTo (BigInteger.ZERO) == 1;
// Return total number of combinations
public BigInteger getTotal () {
return total;
// Compute factorial
private static BigInteger getFactorial (int n) {
BigInteger fact = BigInteger.ONE;
for (int i = n; i > 1; i--) {
fact = fact.multiply (new BigInteger (Integer.toString (i)));
return fact;
// Generate next combination (algorithm from Rosen p. 286)
public int[] getNext () {
if (numLeft.equals (total)) {
numLeft = numLeft.subtract (BigInteger.ONE);
return a;
int i = r - 1;
while (a == n - r + i) {
i--;
System.out.println("inside while");
a = a + 1;
for (int j = i + 1; j < r; j++) {
a[j] = a + j - i;
numLeft = numLeft.subtract (BigInteger.ONE);
return a;
}