How to call Hilbert curve encode C routines - c

Question

I was trying to run a Hilbert curve code written in C, which I found here
http://www.tddft.org/svn/octopus/trunk/src/grid/hilbert.c
The code runs, but the results I get form the output are not correct. I made a simple driver routine, that takes 3 values as arguments from the command line and passes them to a Hilbert curve encode, decode routines.
More precisely, I can't decode back the original coordinates (x,y,z).
One of my problems was to understand what the variable nbits is doing. I assume it is the size of the encoded Hilbert value. To check this I tried to modify the original definition of one of the functions from
void InttoTranspose(const int dim, const long long int h, int * x)
to
void InttoTranspose(const int dim, const long long int h, int * x, int* size)
Where I assign to *size the bit count variable ifbit. Anyway, this all didn't work. Therefore I would like to ask for your help.
The modified code is here:
#include<stdio.h>
//#include <config.h>
#include <assert.h>
/* This code is based on the implementation of the Hilbert curves
presented in:
J. Skilling, Programming the Hilbert curve, AIP Conf. Proc. 707, 381 (2004); http://dx.doi.org/10.1063/1.1751381
*/
*/ The int* size was included later in an attempt to find the proper size /*
/* void InttoTranspose(const int dim, const long long int h, int * x)*/
void InttoTranspose(const int dim, const long long int h, int * x, int* size){
/* the code uses some funny way of storing the bits */
int idir, ibit, ifbit;
for(idir = 0; idir < dim; idir++) x[idir] = 0;
ifbit = 0;
for(ibit = 0; ibit < 21; ibit++){
for(idir = dim - 1; idir >= 0; idir--){
x[idir] += (((h>>ifbit)&1)<<ibit);
ifbit++;
}
}
*size=ifbit; // I think that this should be nbits
}
void TransposetoAxes(int* X, int b, int n ){ /* position, #bits, dimension */
int N = 2 << (b-1), P, Q, t;
int i;
/* Gray decode by H ^ (H/2) */
t = X[n-1] >> 1;
for(i = n - 1; i > 0; i--) X[i] ^= X[i-1];
X[0] ^= t;
/* Undo excess work */
for( Q = 2; Q != N; Q <<= 1 ) {
P = Q - 1;
for( i = n-1; i >= 0 ; i-- ){
if( X[i] & Q ) {
X[0] ^= P; /* invert */
} else{
t = (X[0]^X[i]) & P; X[0] ^= t; X[i] ^= t; /* exchange */
}
}
}
}
//void FC_FUNC_(hilbert_index_to_point, HILBERT_INDEX_TO_POINT)(const int * dim, const int * nbits, const long long int * index, int * point){
// InttoTranspose(*dim, *index, point);
// TransposetoAxes(point, *nbits, *dim);
//}
int main(int argc,char* argv[]){
long long int hilbert;
int i=0,x[2],m;
while(argc--){
if(m=atoi(*argv++)) x[i++]=m, printf("--> %5d %5d\n",m,argc);
}
printf("x= %5d y= %5d z= %5d \n",x[0],x[1],x[2]);
InttoTranspose(3, hilbert, x,&m);
printf("hilbert encoded --> %llu size -->%d \n",hilbert,m);
TransposetoAxes(x,m, 3 );
printf("x= %5d y= %5d z= %5d \n",x[0],x[1],x[2]);
return 0;
}