ComputeFstat_Demod.c

Go to the documentation of this file.

//
// Copyright (C) 2012--2015 Karl Wette
// Copyright (C) 2005--2007, 2009, 2010, 2012, 2014 Reinhard Prix
// Copyright (C) 2007--2010, 2012 Bernd Machenschalk
// Copyright (C) 2007 Chris Messenger
// Copyright (C) 2006 John T. Whelan, Badri Krishnan
//
// This program is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation; either version 2 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with with program; see the file COPYING. If not, write to the
// Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
// MA  02110-1301  USA
//
 
#include <stdlib.h>
#include <stdio.h>
#include <math.h>
 
#include "ComputeFstat_internal.h"
 
#include <lal/Factorial.h>
#include <lal/LogPrintf.h>
#include <lal/SinCosLUT.h>
 
///
/// \defgroup ComputeFstat_Demod_c Module ComputeFstat_Demod.c
/// \ingroup ComputeFstat_h
/// \brief Implements the \a Demod Dirichlet kernel-based demodulation algorithm
/// for computing the \f$ \mathcal{F} \f$ -statistic \cite Williams1999 .
///
 
/// @{
 
// ========== Demod internals ==========
 
// ----- local types ----------
 
// ---------- BEGIN: Demod-specific timing model data ----------
typedef struct tagFstatTimingDemod {
  REAL4 Nsft;                      //  (average) number of SFTs per detector
  REAL4 tau0_coreLD;               // 'fundamental' timing coefficient: fully-buffered time to compute F-stat for one SFT
  REAL4 tau0_bufferLD;             // 'fundamental' timing coefficient: time to re-compute buffered quantities for one SFT
} FstatTimingDemod;
 
static const char FstatTimingDemodHelp[] =
  "%%%% ----- Demod-specific F-statistic timing model -----\n"
  "%%%% Nsft:          (average) number of SFTs per detector\n"
  "%%%% tau0_coreLD:    timing coefficient for core Demod F-stat time\n"
  "%%%% tau0_bufferLD:  timing coefficient for computation of buffered quantities\n"
  "%%%%\n"
  "%%%% Demod F-statistic timing model:\n"
  "%%%% tauF_core      =  Nsft * tau0_coreLD\n"
  "%%%% tauF_buffer    =  Nsft * tau0_bufferLD / NFbin\n"
  "%%%%"
  "";
// ---------- END: Demod-specific timing model data ----------
 
 
typedef struct {
  int ( *computefafb_func )(                    // XLALComputeFaFb_...() function for the selected Demod hotloop
    COMPLEX8 *, COMPLEX8 *, FstatAtomVector **, const SFTVector *, const PulsarSpins, const SSBtimes *, const AMCoeffs *, const UINT4 Dterms
  );
  UINT4 Dterms;                                 // Number of terms to keep in Dirichlet kernel
  MultiSFTVector *multiSFTs;                    // Input multi-detector SFTs
  REAL8 prevAlpha, prevDelta;                   // buffering: previous skyposition computed
  LIGOTimeGPS prevRefTime;                      // buffering: keep track of previous refTime for SSBtimes buffering
  MultiSSBtimes *prevMultiSSBtimes;             // buffering: previous multiSSB times, unique to skypos + SFTs
  MultiAMCoeffs *prevMultiAMcoef;               // buffering: previous AM-coeffs, unique to skypos + SFTs
 
  // ----- timing -----
  BOOLEAN collectTiming;                        // flag whether or not to collect timing information
  FstatTimingGeneric timingGeneric;             // measured generic F-statistic timing values
  FstatTimingDemod   timingDemod;               // storage for measured Demod-specific timing model
 
} DemodMethodData;
 
// ----- local prototypes ----------
 
int XLALSetupFstatDemod( void **method_data, FstatCommon *common, FstatMethodFuncs *funcs, MultiSFTVector *multiSFTs, const FstatOptionalArgs *optArgs );
 
int XLALComputeFaFb_Generic( COMPLEX8 *Fa, COMPLEX8 *Fb, FstatAtomVector **FstatAtoms, const SFTVector *sfts,
                             const PulsarSpins fkdot, const SSBtimes *tSSB, const AMCoeffs *amcoe, const UINT4 Dterms );
 
int XLALComputeFaFb_OptC( COMPLEX8 *Fa, COMPLEX8 *Fb, FstatAtomVector **FstatAtoms, const SFTVector *sfts,
                          const PulsarSpins fkdot, const SSBtimes *tSSB, const AMCoeffs *amcoe, const UINT4 Dterms );
 
#ifdef HAVE_ALTIVEC
int XLALComputeFaFb_Altivec( COMPLEX8 *Fa, COMPLEX8 *Fb, FstatAtomVector **FstatAtoms, const SFTVector *sfts,
                             const PulsarSpins fkdot, const SSBtimes *tSSB, const AMCoeffs *amcoe, const UINT4 Dterms );
#endif
 
#ifdef HAVE_SSE_COMPILER
int XLALComputeFaFb_SSE( COMPLEX8 *Fa, COMPLEX8 *Fb, FstatAtomVector **FstatAtoms, const SFTVector *sfts,
                         const PulsarSpins fkdot, const SSBtimes *tSSB, const AMCoeffs *amcoe, const UINT4 Dterms );
#endif
 
int XLALGetFstatTiming_Demod( const void *method_data, FstatTimingGeneric *timingGeneric, FstatTimingModel *timingModel );
void *XLALFstatInputTimeslice_Demod( const void *method_data, const UINT4 iStart[PULSAR_MAX_DETECTORS], const UINT4 iEnd[PULSAR_MAX_DETECTORS] );
void XLALDestroyFstatInputTimeslice_Demod( void *method_data );
 
// ----- local function definitions ----------
static int
XLALComputeFstatDemod( FstatResults *Fstats,
                       const FstatCommon *common,
                       void *method_data
                     )
{
  // Check input
  XLAL_CHECK( Fstats != NULL, XLAL_EFAULT );
  XLAL_CHECK( common != NULL, XLAL_EFAULT );
  XLAL_CHECK( method_data != NULL, XLAL_EFAULT );
 
  DemodMethodData *demod = ( DemodMethodData * ) method_data;
  // get internal timing info
  BOOLEAN collectTiming = demod->collectTiming;
  BOOLEAN BufferRecomputed = 0; // keep track if buffer was recomputed in this call or not
  REAL8 Tau_buffer = 0;
  REAL8 tic = 0, toc = 0;
 
  // Get which F-statistic quantities to compute
  const FstatQuantities whatToCompute = Fstats->whatWasComputed;
 
  // handy shortcuts
  BOOLEAN returnAtoms = ( whatToCompute & FSTATQ_ATOMS_PER_DET );
  PulsarDopplerParams thisPoint = Fstats->doppler;
  const REAL8 fStart = thisPoint.fkdot[0];
  const MultiSFTVector *multiSFTs = demod->multiSFTs;
  const MultiNoiseWeights *multiWeights = common->multiNoiseWeights;
  const MultiDetectorStateSeries *multiDetStates = common->multiDetectorStates;
 
  UINT4 numDetectors = multiSFTs->length;
  XLAL_CHECK( multiDetStates->length == numDetectors, XLAL_EINVAL );
  XLAL_CHECK( multiWeights == NULL || ( multiWeights->length == numDetectors ), XLAL_EINVAL );
 
  // initialize timing info struct
  if ( collectTiming ) {
    tic = XLALGetCPUTime();
  }
 
  MultiSSBtimes *multiSSB = NULL;
  MultiAMCoeffs *multiAMcoef = NULL;
  // ----- check if we have buffered SSB+AMcoef for current sky-position
  if ( ( demod->prevAlpha == thisPoint.Alpha ) && ( demod->prevDelta == thisPoint.Delta ) &&
       ( demod->prevMultiSSBtimes != NULL ) && ( XLALGPSDiff( &demod->prevRefTime, &thisPoint.refTime ) == 0 ) && // have SSB times for same reftime?
       ( demod->prevMultiAMcoef != NULL )
     ) {
    // if yes ==> reuse
    multiSSB    = demod->prevMultiSSBtimes;
    multiAMcoef = demod->prevMultiAMcoef;
    BufferRecomputed = 0;
  } else {
    // if not, compute SSB + AMcoef for this skyposition
    BufferRecomputed = 1;
    demod->timingGeneric.NBufferMisses ++;
 
    SkyPosition skypos;
    skypos.system = COORDINATESYSTEM_EQUATORIAL;
    skypos.longitude = thisPoint.Alpha;
    skypos.latitude  = thisPoint.Delta;
    XLAL_CHECK( ( multiSSB = XLALGetMultiSSBtimes( multiDetStates, skypos, thisPoint.refTime, common->SSBprec ) ) != NULL, XLAL_EFUNC );
    XLAL_CHECK( ( multiAMcoef = XLALComputeMultiAMCoeffs( multiDetStates, multiWeights, skypos ) ) != NULL, XLAL_EFUNC );
 
    // store these for possible later re-use in buffer
    XLALDestroyMultiSSBtimes( demod->prevMultiSSBtimes );
    demod->prevMultiSSBtimes = multiSSB;
    demod->prevRefTime = thisPoint.refTime;
    XLALDestroyMultiAMCoeffs( demod->prevMultiAMcoef );
    demod->prevMultiAMcoef = multiAMcoef;
    demod->prevAlpha = thisPoint.Alpha;
    demod->prevDelta = thisPoint.Delta;
  } // if could not reuse previously buffered quantites
 
  if ( collectTiming ) {
    toc = XLALGetCPUTime();
    Tau_buffer = ( toc - tic );
  }
 
  MultiSSBtimes *multiBinary = NULL;
  MultiSSBtimes *multiSSBTotal = NULL;
  // handle binary-orbital timing corrections, if applicable
  if ( thisPoint.asini > 0 ) {
    // compute binary time corrections to the SSB time delays and SSB time derivitive
    XLAL_CHECK( XLALAddMultiBinaryTimes( &multiBinary, multiSSB, &thisPoint ) == XLAL_SUCCESS, XLAL_EFUNC );
    multiSSBTotal = multiBinary;
  } else {
    multiSSBTotal = multiSSB;
  }
 
  // ----- compute final Fstatistic-value -----
  REAL4 Ad = multiAMcoef->Mmunu.Ad;
  REAL4 Bd = multiAMcoef->Mmunu.Bd;
  REAL4 Cd = multiAMcoef->Mmunu.Cd;
  REAL4 Ed = multiAMcoef->Mmunu.Ed;;
  REAL4 Dd_inv = 1.0 / multiAMcoef->Mmunu.Dd;
 
  // ---------- Compute F-stat for each frequency bin ----------
  for ( UINT4 k = 0; k < Fstats->numFreqBins; k++ ) {
    // Set frequency to search at
    thisPoint.fkdot[0] = fStart + k * Fstats->dFreq;
 
    COMPLEX8 Fa = 0;                 // complex amplitude Fa
    COMPLEX8 Fb = 0;                 // complex amplitude Fb
    MultiFstatAtomVector *multiFstatAtoms = NULL;     // per-IFO, per-SFT arrays of F-stat 'atoms', ie quantities required to compute F-stat
 
    // prepare return of 'FstatAtoms' if requested
    if ( returnAtoms ) {
      XLAL_CHECK( ( multiFstatAtoms = XLALMalloc( sizeof( *multiFstatAtoms ) ) ) != NULL, XLAL_ENOMEM );
      multiFstatAtoms->length = numDetectors;
      XLAL_CHECK( ( multiFstatAtoms->data = XLALMalloc( numDetectors * sizeof( *multiFstatAtoms->data ) ) ) != NULL, XLAL_ENOMEM );
    } // if returnAtoms
 
    // loop over detectors and compute all detector-specific quantities
    for ( UINT4 X = 0; X < numDetectors; X ++ ) {
      COMPLEX8 FaX, FbX;
      FstatAtomVector *FstatAtoms = NULL;
      FstatAtomVector **FstatAtoms_p = returnAtoms ? ( &FstatAtoms ) : NULL;
 
      // call XLALComputeFaFb_...() function for the user-requested hotloop variant
      XLAL_CHECK( ( demod->computefafb_func )( &FaX, &FbX, FstatAtoms_p, multiSFTs->data[X], thisPoint.fkdot,
                  multiSSBTotal->data[X], multiAMcoef->data[X], demod->Dterms ) == XLAL_SUCCESS, XLAL_EFUNC );
 
      if ( returnAtoms ) {
        multiFstatAtoms->data[X] = FstatAtoms;     // copy pointer to IFO-specific Fstat-atoms 'contents'
      }
 
      XLAL_CHECK( isfinite( creal( FaX ) ) && isfinite( cimag( FaX ) ) && isfinite( creal( FbX ) ) && isfinite( cimag( FbX ) ), XLAL_EFPOVRFLW );
 
      if ( whatToCompute & FSTATQ_FAFB_PER_DET ) {
        Fstats->FaPerDet[X][k] = FaX;
        Fstats->FbPerDet[X][k] = FbX;
      }
 
      // compute single-IFO F-stats, if requested
      if ( whatToCompute & FSTATQ_2F_PER_DET ) {
        REAL4 AdX = multiAMcoef->data[X]->A;
        REAL4 BdX = multiAMcoef->data[X]->B;
        REAL4 CdX = multiAMcoef->data[X]->C;
        REAL4 EdX = 0;
        REAL4 DdX_inv = 1.0 / multiAMcoef->data[X]->D;
 
        // compute final single-IFO F-stat
        Fstats->twoFPerDet[X][k] = compute_fstat_from_fa_fb( FaX, FbX, AdX, BdX, CdX, EdX, DdX_inv );
 
      } // if FSTATQ_2F_PER_DET
 
      /* Fa = sum_X Fa_X */
      Fa += FaX;
 
      /* Fb = sum_X Fb_X */
      Fb += FbX;
 
    } // for  X < numDetectors
 
    if ( whatToCompute & FSTATQ_2F ) {
      Fstats->twoF[k] = compute_fstat_from_fa_fb( Fa, Fb, Ad, Bd, Cd, Ed, Dd_inv );
    }
    if ( Fstats->whatWasComputed & FSTATQ_2F_CUDA ) {
      XLAL_ERROR( XLAL_EINVAL, "Not implemented for FSTATQ_2F_CUDA" );
    }
    if ( Fstats->whatWasComputed & FSTATQ_FAFB_CUDA ) {
      XLAL_ERROR( XLAL_EINVAL, "Not implemented for FSTATQ_FAFB_CUDA" );
    }
 
    // Return multi-detector Fa & Fb
    if ( whatToCompute & FSTATQ_FAFB ) {
      Fstats->Fa[k] = Fa;
      Fstats->Fb[k] = Fb;
    }
 
    // Return F-atoms per detector
    if ( whatToCompute & FSTATQ_ATOMS_PER_DET ) {
      XLALDestroyMultiFstatAtomVector( Fstats->multiFatoms[k] );
      Fstats->multiFatoms[k] = multiFstatAtoms;
    }
 
  } // for k < Fstats->numFreqBins
 
  // this needs to be free'ed, as it's currently not buffered
  XLALDestroyMultiSSBtimes( multiBinary );
 
  // Return amplitude modulation coefficients
  Fstats->Mmunu = demod->prevMultiAMcoef->Mmunu;
 
  // return per-detector antenna-pattern matrices
  for ( UINT4 X = 0; X < numDetectors; X ++ ) {
    Fstats->MmunuX[X].Ad = multiAMcoef->data[X]->A;
    Fstats->MmunuX[X].Bd = multiAMcoef->data[X]->B;
    Fstats->MmunuX[X].Cd = multiAMcoef->data[X]->C;
    Fstats->MmunuX[X].Dd = multiAMcoef->data[X]->D;
    Fstats->MmunuX[X].Ed = 0;
  }
 
  if ( collectTiming ) {
    toc = XLALGetCPUTime();
    REAL8 Tau_total = ( toc - tic );
 
    FstatTimingGeneric *tiGen = &( demod->timingGeneric );    // generic part
    FstatTimingDemod   *tiLD  = &( demod->timingDemod );      // Demod-method specific part
 
    XLAL_CHECK( numDetectors == tiGen->Ndet, XLAL_EINVAL, "Inconsistent number of detectors between XLALCreateSetup() [%d] and XLALComputeFstat() [%d]\n", tiGen->Ndet, numDetectors );
 
    // rescale all relevant timings to per-detector
    Tau_total  /= numDetectors;
    Tau_buffer /= numDetectors;
 
    // compute generic F-stat timing model contributions
    UINT4 NFbin     = Fstats->numFreqBins;
    REAL8 tauF_eff  = Tau_total / NFbin;
    REAL8 tauF_core = ( Tau_total - Tau_buffer ) / NFbin;
 
    // compute Demod timing model coefficients
    REAL8 NsftPerDet    = tiLD->Nsft;
    REAL8 tau0_coreLD   = tauF_core / NsftPerDet;
 
    // update the averaged timing-model quantities
    tiGen->NCalls ++; // keep track of number of Fstat-calls for timing
#define updateAvgF(q) tiGen->q = ((tiGen->q *(tiGen->NCalls-1) + q)/(tiGen->NCalls))
    updateAvgF( tauF_eff );
    updateAvgF( tauF_core );
    // we also average NFbin, which can be different betnween different calls to XLALComputeFstat() (contrary to Ndet)
    updateAvgF( NFbin );
 
#define updateAvgLD(q) tiLD->q = ((tiLD->q *(tiGen->NCalls-1) + q)/(tiGen->NCalls))
    updateAvgLD( tau0_coreLD );
 
    // buffer-quantities only updated if buffer was actually recomputed
    if ( BufferRecomputed ) {
      REAL8 tau0_bufferLD = Tau_buffer / NsftPerDet;
      REAL8 tauF_buffer   = Tau_buffer / NFbin;
 
      updateAvgF( tauF_buffer );
      updateAvgLD( tau0_bufferLD );
    } // if BufferRecomputed
 
  } // if collect timing
 
  return XLAL_SUCCESS;
 
} // XLALComputeFstatDemod()
 
 
static void
XLALDestroyDemodMethodData( void *method_data )
{
 
  DemodMethodData *demod = ( DemodMethodData * ) method_data;
 
  XLALDestroyMultiSFTVector( demod->multiSFTs );
  XLALDestroyMultiSSBtimes( demod->prevMultiSSBtimes );
  XLALDestroyMultiAMCoeffs( demod->prevMultiAMcoef );
  XLALFree( demod );
 
} // XLALDestroyDemodMethodData()
 
int
XLALSetupFstatDemod( void **method_data,
                     FstatCommon *common,
                     FstatMethodFuncs *funcs,
                     MultiSFTVector *multiSFTs,
                     const FstatOptionalArgs *optArgs
                   )
{
  // Check input
  XLAL_CHECK( method_data != NULL, XLAL_EFAULT );
  XLAL_CHECK( common != NULL, XLAL_EFAULT );
  XLAL_CHECK( funcs != NULL, XLAL_EFAULT );
  XLAL_CHECK( multiSFTs != NULL, XLAL_EFAULT );
  XLAL_CHECK( optArgs != NULL, XLAL_EFAULT );
 
  // Allocate method data
  DemodMethodData *demod = *method_data = XLALCalloc( 1, sizeof( *demod ) );
  XLAL_CHECK( demod != NULL, XLAL_ENOMEM );
 
  // Set method function pointers
  funcs->compute_func = XLALComputeFstatDemod;
  funcs->method_data_destroy_func = XLALDestroyDemodMethodData;
  funcs->workspace_destroy_func = NULL;
 
  // Save pointer to SFTs
  demod->multiSFTs = multiSFTs;
 
  // Save Dterms
  demod->Dterms = optArgs->Dterms;
 
  // turn on timing collection if requested
  demod->collectTiming = optArgs->collectTiming;
 
  // initialize struct for collecting timing data, store invariant 'meta' quantities about this setup
  if ( demod->collectTiming ) {
    XLAL_INIT_MEM( demod->timingGeneric );
    XLAL_INIT_MEM( demod->timingDemod );
    UINT4 numDetectors = demod->multiSFTs->length;
    UINT4 numSFTs = 0;
    for ( UINT4 X = 0; X < numDetectors; X ++ ) {
      numSFTs += demod->multiSFTs->data[X]->length;
    }
    demod->timingGeneric.Ndet = numDetectors;
    demod->timingDemod.Nsft   = 1.0 * numSFTs / numDetectors; // average number of sfts *per detector*
  } // if collectTiming
 
  // Select XLALComputeFaFb_...() function for the user-requested hotloop variant
  switch ( optArgs->FstatMethod ) {
  case  FMETHOD_DEMOD_GENERIC:
    demod->computefafb_func = XLALComputeFaFb_Generic;
    break;
  case FMETHOD_DEMOD_OPTC:
    demod->computefafb_func = XLALComputeFaFb_OptC;
    break;
#ifdef HAVE_ALTIVEC
  case FMETHOD_DEMOD_ALTIVEC:
    demod->computefafb_func = XLALComputeFaFb_Altivec;
    break;
#endif
#ifdef HAVE_SSE_COMPILER
  case FMETHOD_DEMOD_SSE:
    demod->computefafb_func = XLALComputeFaFb_SSE;
    break;
#endif
  default:
    XLAL_ERROR( XLAL_EINVAL, "Invalid Demod hotloop optArgs->FstatMethod='%d'", optArgs->FstatMethod );
    break;
  }
 
  return XLAL_SUCCESS;
 
} // XLALSetupFstatDemod()
 
 
int
XLALGetFstatTiming_Demod( const void *method_data, FstatTimingGeneric *timingGeneric, FstatTimingModel *timingModel )
{
  XLAL_CHECK( method_data != NULL, XLAL_EINVAL );
  XLAL_CHECK( timingGeneric != NULL, XLAL_EINVAL );
  XLAL_CHECK( timingModel != NULL, XLAL_EINVAL );
 
  const DemodMethodData *demod = ( const DemodMethodData * ) method_data;
  XLAL_CHECK( demod != NULL, XLAL_EINVAL );
 
  ( *timingGeneric ) = demod->timingGeneric; // struct-copy generic timing measurements
 
  const FstatTimingDemod *tiLD = &( demod->timingDemod );
 
  // return method-specific timing model values
  XLAL_INIT_MEM( ( *timingModel ) );
 
  UINT4 i = 0;
  timingModel->names[i]     = "Nsft";
  timingModel->values[i]    = tiLD->Nsft;
 
  i++;
  timingModel->names[i]     = "tau0_coreLD";
  timingModel->values[i]    = tiLD->tau0_coreLD;
 
  i++;
  timingModel->names[i]     = "tau0_bufferLD";
  timingModel->values[i]    = tiLD->tau0_bufferLD;
 
  timingModel->numVariables = i + 1;
  timingModel->help         = FstatTimingDemodHelp;
 
  return XLAL_SUCCESS;
} // XLALGetFstatTiming_Demod()
 
// Generates an FstatInput Timeslice based on minStartGPS and maxStartGPS according to XLALCWGPSinRange().
void *
XLALFstatInputTimeslice_Demod( const void *method_data,
                               const UINT4 iStart[PULSAR_MAX_DETECTORS],
                               const UINT4 iEnd[PULSAR_MAX_DETECTORS]
                             )
{
  XLAL_CHECK_NULL( method_data != NULL, XLAL_EINVAL );
  XLAL_CHECK_NULL( iStart != NULL, XLAL_EINVAL );
  XLAL_CHECK_NULL( iEnd != NULL, XLAL_EINVAL );
 
  const DemodMethodData *demod_input = ( const DemodMethodData * )method_data;
 
  // allocate memory and copy the input method_data struct
  DemodMethodData *demod_slice;
  XLAL_CHECK_NULL( ( demod_slice = XLALCalloc( 1, sizeof( *demod_input ) ) ) != NULL, XLAL_ENOMEM );
  memcpy( demod_slice, demod_input, sizeof( *demod_input ) );
 
  UINT4 numIFOs = demod_input->multiSFTs->length;
 
  // prepare MultiSFTs struct
  MultiSFTVector *multiSFTs;
  XLAL_CHECK_NULL( ( multiSFTs = XLALCalloc( 1, sizeof( *multiSFTs ) ) ) != NULL, XLAL_ENOMEM );
  XLAL_CHECK_NULL( ( multiSFTs->data = XLALCalloc( numIFOs, sizeof( *multiSFTs->data ) ) ) != NULL, XLAL_ENOMEM );
  multiSFTs->length = numIFOs;
 
  // loop over all detectors
  for ( UINT4 X = 0; X < numIFOs; X ++ ) {
    XLAL_CHECK_NULL( ( multiSFTs->data[X] = XLALCalloc( 1, sizeof( *multiSFTs->data[X] ) ) ) != NULL, XLAL_EFUNC );
 
    if ( iStart[X] > iEnd[X] ) {
      continue;       // empty slice
    }
    UINT4 sliceLength =  iEnd[X] - iStart[X] + 1; // guaranteed >= 1
    multiSFTs->data[X]->length = sliceLength;
    multiSFTs->data[X]->data   = &( demod_input->multiSFTs->data[X]->data[iStart[X]] );
 
  } // for loop over all detectors
 
  demod_slice->multiSFTs = multiSFTs;
 
  // empty all buffering quantities
  demod_slice->prevAlpha = 0;
  demod_slice->prevDelta = 0;
  XLAL_INIT_MEM( demod_slice->prevRefTime );
  demod_slice->prevMultiSSBtimes = NULL;
  demod_slice->prevMultiAMcoef = NULL;
 
  // reset timing counters
  XLAL_INIT_MEM( demod_slice->timingGeneric );
  XLAL_INIT_MEM( demod_slice->timingDemod );
 
  return demod_slice;
 
} // XLALFstatInputTimeslice_Demod()
 
///
/// Free all memory not needed by the orginal FstatInput structure
///
void
XLALDestroyFstatInputTimeslice_Demod( void *method_data )
{
  if ( !method_data ) {
    return;
  }
 
  DemodMethodData *demod = ( DemodMethodData * ) method_data;
 
  XLALDestroyMultiSSBtimes( demod->prevMultiSSBtimes );
  XLALDestroyMultiAMCoeffs( demod->prevMultiAMcoef );
 
  for ( UINT4 X = 0; X < demod->multiSFTs->length; X ++ ) {
    XLALFree( demod->multiSFTs->data[X] );
  }
 
  XLALFree( demod->multiSFTs->data );
  XLALFree( demod->multiSFTs );
  XLALFree( demod );
 
  return;
 
} // XLALDestroyFstatInputTimeslice_Demod()
 
/// @}