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Issue 517 - Fix combin() function to a) increase upper limit and b) keep it from continually recomputing the same values in recursion

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git-svn-id: http://google-refine.googlecode.com/svn/trunk@2459 7d457c2a-affb-35e4-300a-418c747d4874
This commit is contained in:
Tom Morris 2012-03-08 22:53:21 +00:00
parent 28ff2295fd
commit 18b780bebe
1 changed files with 23 additions and 10 deletions
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33
main/src/com/google/refine/expr/functions/math/Combin.java
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@ -59,18 +59,31 @@ public class Combin implements Function {
return Combin.combination(((Number) args[0]).intValue(), ((Number) args[1]).intValue());
}
public static long combination(long n, long k){
if (k > n) {
throw new IllegalArgumentException ("the number of elements, n, should be larger than the number of combinations, k");
/*
* Compute binomial coefficient using dynamic programming which takes
* advantage of Pascal's identity as described in:
* http://introcs.cs.princeton.edu/java/96optimization/
*/
public static long combination(int n, int k) {
long[][] binomial = new long[n+1][k+1];
for (int j = 1; j <= k; j++) {
binomial[0][j] = 0;
}
if (n < 1) {
throw new IllegalArgumentException ("the number of elements, n, cannot be less than 1");
for (int i = 0; i <= n; i++) {
binomial[i][0] = 1;
}
// TODO: This needs to use a more efficient Binomial Coefficient algorithm
long nFact = FactN.factorial(n, 1);
long rFact = FactN.factorial(k, 1);
long nminusrFact = FactN.factorial(n - k, 1);
return nFact / (rFact * nminusrFact);
for (int i = 1; i <= n; i++) {
for (int j = 1; j <= k; j++) {
binomial[i][j] = binomial[i-1][j-1] + binomial[i-1][j];
if (binomial[i][j] > Long.MAX_VALUE || binomial[i][j] < 0) {
throw new RuntimeException("Range limit exceeded");
}
}
}
return binomial[n][k];
}
@Override