/******************************************************************************

 *

 * Project:  OpenGIS Simple Features Reference Implementation

 * Purpose:  Implementation of simple SQL WHERE style attributes queries

 *           for OGRFeatures.

 * Author:   Frank Warmerdam, warmerdam@pobox.com

 *

 ******************************************************************************

 * Copyright (c) 2001, Frank Warmerdam <warmerdam@pobox.com>

 * Copyright (c) 2008-2014, Even Rouault <even dot rouault at spatialys.com>

 *

 * SPDX-License-Identifier: MIT

 ****************************************************************************/

#include "cpl_port.h"

#include "ogr_feature.h"

#include "ogr_swq.h"

#include <cstddef>

#include <algorithm>

#include "cpl_conv.h"

#include "cpl_error.h"

#include "cpl_string.h"

#include "ogr_attrind.h"

#include "ogr_core.h"

#include "ogr_p.h"

#include "ogrsf_frmts.h"

//! @cond Doxygen_Suppress

/************************************************************************/

/*     Support for special attributes (feature query and selection)     */

/************************************************************************/

extern const swq_field_type SpecialFieldTypes[SPECIAL_FIELD_COUNT];

const char *const SpecialFieldNames[SPECIAL_FIELD_COUNT] = {

    "FID", "OGR_GEOMETRY", "OGR_STYLE", "OGR_GEOM_WKT", "OGR_GEOM_AREA"};

const swq_field_type SpecialFieldTypes[SPECIAL_FIELD_COUNT] = {

    SWQ_INTEGER, SWQ_STRING, SWQ_STRING, SWQ_STRING, SWQ_FLOAT};

/************************************************************************/

/*                          OGRFeatureQuery()                           */

/************************************************************************/

OGRFeatureQuery::OGRFeatureQuery()

    : poTargetDefn(nullptr), pSWQExpr(nullptr),

      m_psContext(new swq_evaluation_context())

{

}

/************************************************************************/

/*                          ~OGRFeatureQuery()                          */

/************************************************************************/

OGRFeatureQuery::~OGRFeatureQuery()

{

    delete m_psContext;

    delete static_cast<swq_expr_node *>(pSWQExpr);

}

/************************************************************************/

/*                              Compile()                               */

/************************************************************************/

OGRErr

OGRFeatureQuery::Compile(const OGRLayer *poLayer, const char *pszExpression,

                         int bCheck,

                         swq_custom_func_registrar *poCustomFuncRegistrar)

{

    if (poLayer->TestCapability(OLCStringsAsUTF8))

        m_psContext->bUTF8Strings = true;

    return Compile(poLayer, poLayer->GetLayerDefn(), pszExpression, bCheck,

                   poCustomFuncRegistrar);

}

/************************************************************************/

/*                              Compile()                               */

/************************************************************************/

OGRErr

OGRFeatureQuery::Compile(const OGRFeatureDefn *poDefn,

                         const char *pszExpression, int bCheck,

                         swq_custom_func_registrar *poCustomFuncRegistrar)

{

    return Compile(nullptr, poDefn, pszExpression, bCheck,

                   poCustomFuncRegistrar);

}

/************************************************************************/

/*                              Compile()                               */

/************************************************************************/

OGRErr

OGRFeatureQuery::Compile(const OGRLayer *poLayer, const OGRFeatureDefn *poDefn,

                         const char *pszExpression, int bCheck,

                         swq_custom_func_registrar *poCustomFuncRegistrar)

{

    // Clear any existing expression.

    if (pSWQExpr != nullptr)

    {

        delete static_cast<swq_expr_node *>(pSWQExpr);

        pSWQExpr = nullptr;

    }

    const char *pszFIDColumn = nullptr;

    bool bMustAddFID = false;

    if (poLayer != nullptr)

    {

        pszFIDColumn = const_cast<OGRLayer *>(poLayer)->GetFIDColumn();

        if (pszFIDColumn != nullptr)

        {

            if (!EQUAL(pszFIDColumn, "") && !EQUAL(pszFIDColumn, "FID"))

            {

                bMustAddFID = true;

            }

        }

    }

    // Build list of fields.

    const int nFieldCount = poDefn->GetFieldCount() + SPECIAL_FIELD_COUNT +

                            poDefn->GetGeomFieldCount() + (bMustAddFID ? 1 : 0);

    char **papszFieldNames =

        static_cast<char **>(CPLMalloc(sizeof(char *) * nFieldCount));

    swq_field_type *paeFieldTypes = static_cast<swq_field_type *>(

        CPLMalloc(sizeof(swq_field_type) * nFieldCount));

    for (int iField = 0; iField < poDefn->GetFieldCount(); iField++)

    {

        const OGRFieldDefn *poField = poDefn->GetFieldDefn(iField);

        if (!poField)

        {

            CPLAssert(0);

            break;

        }

        papszFieldNames[iField] = const_cast<char *>(poField->GetNameRef());

        switch (poField->GetType())

        {

            case OFTInteger:

            {

                if (poField->GetSubType() == OFSTBoolean)

                    paeFieldTypes[iField] = SWQ_BOOLEAN;

                else

                    paeFieldTypes[iField] = SWQ_INTEGER;

                break;

            }

            case OFTInteger64:

            {

                if (poField->GetSubType() == OFSTBoolean)

                    paeFieldTypes[iField] = SWQ_BOOLEAN;

                else

                    paeFieldTypes[iField] = SWQ_INTEGER64;

                break;

            }

            case OFTReal:

                paeFieldTypes[iField] = SWQ_FLOAT;

                break;

            case OFTString:

                paeFieldTypes[iField] = SWQ_STRING;

                break;

            case OFTDate:

            case OFTTime:

            case OFTDateTime:

                paeFieldTypes[iField] = SWQ_TIMESTAMP;

                break;

            default:

                paeFieldTypes[iField] = SWQ_OTHER;

                break;

        }

    }

    int iField = 0;

    while (iField < SPECIAL_FIELD_COUNT)

    {

        papszFieldNames[poDefn->GetFieldCount() + iField] =

            const_cast<char *>(SpecialFieldNames[iField]);

        paeFieldTypes[poDefn->GetFieldCount() + iField] =

            (iField == SPF_FID) ? SWQ_INTEGER64 : SpecialFieldTypes[iField];

        ++iField;

    }

    for (iField = 0; iField < poDefn->GetGeomFieldCount(); iField++)

    {

        const OGRGeomFieldDefn *poField = poDefn->GetGeomFieldDefn(iField);

        const int iDstField =

            poDefn->GetFieldCount() + SPECIAL_FIELD_COUNT + iField;

        papszFieldNames[iDstField] = const_cast<char *>(poField->GetNameRef());

        if (*papszFieldNames[iDstField] == '\0')

            papszFieldNames[iDstField] =

                const_cast<char *>(OGR_GEOMETRY_DEFAULT_NON_EMPTY_NAME);

        paeFieldTypes[iDstField] = SWQ_GEOMETRY;

    }

    if (bMustAddFID)

    {

        papszFieldNames[nFieldCount - 1] = const_cast<char *>(pszFIDColumn);

        paeFieldTypes[nFieldCount - 1] =

            (poLayer != nullptr &&

             const_cast<OGRLayer *>(poLayer)->GetMetadataItem(OLMD_FID64) !=

                 nullptr &&

             EQUAL(const_cast<OGRLayer *>(poLayer)->GetMetadataItem(OLMD_FID64),

                   "YES"))

                ? SWQ_INTEGER64

                : SWQ_INTEGER;

    }

    // Try to parse.

    poTargetDefn = poDefn;

    const CPLErr eCPLErr = swq_expr_compile(

        pszExpression, nFieldCount, papszFieldNames, paeFieldTypes, bCheck,

        poCustomFuncRegistrar, reinterpret_cast<swq_expr_node **>(&pSWQExpr));

    OGRErr eErr = OGRERR_NONE;

    if (eCPLErr != CE_None)

    {

        eErr = OGRERR_CORRUPT_DATA;

        pSWQExpr = nullptr;

    }

    CPLFree(papszFieldNames);

    CPLFree(paeFieldTypes);

    return eErr;

}

/************************************************************************/

/*                   OGRFeatureFetcherFixFieldIndex()                   */

/************************************************************************/

static int OGRFeatureFetcherFixFieldIndex(const OGRFeatureDefn *poFDefn,

                                          int nIdx)

{

    /* Nastry trick: if we inserted the FID column as an extra column, it is */

    /* after regular fields, special fields and geometry fields */

    if (nIdx == poFDefn->GetFieldCount() + SPECIAL_FIELD_COUNT +

                    poFDefn->GetGeomFieldCount())

    {

        return poFDefn->GetFieldCount() + SPF_FID;

    }

    return nIdx;

}

/************************************************************************/

/*                         OGRFeatureFetcher()                          */

/************************************************************************/

static swq_expr_node *OGRFeatureFetcher(swq_expr_node *op,

                                        const void *pFeatureIn)

{

    const OGRFeature *poFeature = static_cast<const OGRFeature *>(pFeatureIn);

    if (op->field_type == SWQ_GEOMETRY)

    {

        const int iField = op->field_index -

                           (poFeature->GetFieldCount() + SPECIAL_FIELD_COUNT);

        swq_expr_node *poRetNode =

            new swq_expr_node(poFeature->GetGeomFieldRef(iField));

        return poRetNode;

    }

    const int idx = OGRFeatureFetcherFixFieldIndex(poFeature->GetDefnRef(),

                                                   op->field_index);

    swq_expr_node *poRetNode = nullptr;

    switch (op->field_type)

    {

        case SWQ_INTEGER:

        case SWQ_BOOLEAN:

            poRetNode = new swq_expr_node(poFeature->GetFieldAsInteger(idx));

            break;

        case SWQ_INTEGER64:

            poRetNode = new swq_expr_node(poFeature->GetFieldAsInteger64(idx));

            break;

        case SWQ_FLOAT:

            poRetNode = new swq_expr_node(poFeature->GetFieldAsDouble(idx));

            break;

        case SWQ_TIMESTAMP:

            poRetNode = new swq_expr_node(poFeature->GetFieldAsString(idx));

            poRetNode->MarkAsTimestamp();

            break;

        default:

            poRetNode = new swq_expr_node(poFeature->GetFieldAsString(idx));

            break;

    }

    poRetNode->is_null = !(poFeature->IsFieldSetAndNotNull(idx));

    return poRetNode;

}

/************************************************************************/

/*                              Evaluate()                              */

/************************************************************************/

int OGRFeatureQuery::Evaluate(const OGRFeature *poFeature)

{

    if (pSWQExpr == nullptr)

        return FALSE;

    swq_expr_node *poResult = static_cast<swq_expr_node *>(pSWQExpr)->Evaluate(

        OGRFeatureFetcher, poFeature, *m_psContext);

    if (poResult == nullptr)

        return FALSE;

    bool bLogicalResult = false;

    if (poResult->field_type == SWQ_INTEGER ||

        poResult->field_type == SWQ_INTEGER64 ||

        poResult->field_type == SWQ_BOOLEAN)

        bLogicalResult = CPL_TO_BOOL(static_cast<int>(poResult->int_value));

    delete poResult;

    return bLogicalResult;

}

/************************************************************************/

/*                            CanUseIndex()                             */

/************************************************************************/

int OGRFeatureQuery::CanUseIndex(OGRLayer *poLayer)

{

    swq_expr_node *psExpr = static_cast<swq_expr_node *>(pSWQExpr);

    // Do we have an index on the targeted layer?

    if (poLayer->GetIndex() == nullptr)

        return FALSE;

    return CanUseIndex(psExpr, poLayer);

}

int OGRFeatureQuery::CanUseIndex(const swq_expr_node *psExpr, OGRLayer *poLayer)

{

    // Does the expression meet our requirements?

    if (psExpr == nullptr || psExpr->eNodeType != SNT_OPERATION)

        return FALSE;

    if ((psExpr->nOperation == SWQ_OR || psExpr->nOperation == SWQ_AND) &&

        psExpr->nSubExprCount == 2)

    {

        return CanUseIndex(psExpr->papoSubExpr[0], poLayer) &&

               CanUseIndex(psExpr->papoSubExpr[1], poLayer);

    }

    if (!(psExpr->nOperation == SWQ_EQ || psExpr->nOperation == SWQ_IN) ||

        psExpr->nSubExprCount < 2)

        return FALSE;

    swq_expr_node *poColumn = psExpr->papoSubExpr[0];

    swq_expr_node *poValue = psExpr->papoSubExpr[1];

    if (poColumn->eNodeType != SNT_COLUMN || poValue->eNodeType != SNT_CONSTANT)

        return FALSE;

    OGRAttrIndex *poIndex =

        poLayer->GetIndex()->GetFieldIndex(OGRFeatureFetcherFixFieldIndex(

            poLayer->GetLayerDefn(), poColumn->field_index));

    if (poIndex == nullptr)

        return FALSE;

    // Have an index.

    return TRUE;

}

/************************************************************************/

/*                       EvaluateAgainstIndices()                       */

/*                                                                      */

/*      Attempt to return a list of FIDs matching the given             */

/*      attribute query conditions utilizing attribute indices.         */

/*      Returns NULL if the result cannot be computed from the          */

/*      available indices, or an "OGRNullFID" terminated list of        */

/*      FIDs if it can.                                                 */

/*                                                                      */

/*      For now we only support equality tests on a single indexed      */

/*      attribute field.  Eventually we should make this support        */

/*      multi-part queries with ranges.                                 */

/************************************************************************/

GIntBig *OGRFeatureQuery::EvaluateAgainstIndices(OGRLayer *poLayer,

                                                 OGRErr *peErr)

{

    swq_expr_node *psExpr = static_cast<swq_expr_node *>(pSWQExpr);

    if (peErr != nullptr)

        *peErr = OGRERR_NONE;

    // Do we have an index on the targeted layer?

    if (poLayer->GetIndex() == nullptr)

        return nullptr;

    GIntBig nFIDCount = 0;

    return EvaluateAgainstIndices(psExpr, poLayer, nFIDCount);

}

// The input arrays must be sorted.

static GIntBig *OGRORGIntBigArray(GIntBig panFIDList1[], GIntBig nFIDCount1,

                                  GIntBig panFIDList2[], GIntBig nFIDCount2,

                                  GIntBig &nFIDCount)

{

    const GIntBig nMaxCount = nFIDCount1 + nFIDCount2;

    GIntBig *panFIDList = static_cast<GIntBig *>(

        CPLMalloc(static_cast<size_t>(nMaxCount + 1) * sizeof(GIntBig)));

    nFIDCount = 0;

    for (GIntBig i1 = 0, i2 = 0; i1 < nFIDCount1 || i2 < nFIDCount2;)

    {

        if (i1 < nFIDCount1 && i2 < nFIDCount2)

        {

            const GIntBig nVal1 = panFIDList1[i1];

            const GIntBig nVal2 = panFIDList2[i2];

            if (nVal1 < nVal2)

            {

                if (i1 + 1 < nFIDCount1 && panFIDList1[i1 + 1] <= nVal2)

                {

                    panFIDList[nFIDCount++] = nVal1;

                    i1++;

                }

                else

                {

                    panFIDList[nFIDCount++] = nVal1;

                    panFIDList[nFIDCount++] = nVal2;

                    i1++;

                    i2++;

                }

            }

            else if (nVal1 == nVal2)

            {

                panFIDList[nFIDCount++] = nVal1;

                i1++;

                i2++;

            }

            else

            {

                if (i2 + 1 < nFIDCount2 && panFIDList2[i2 + 1] <= nVal1)

                {

                    panFIDList[nFIDCount++] = nVal2;

                    i2++;

                }

                else

                {

                    panFIDList[nFIDCount++] = nVal2;

                    panFIDList[nFIDCount++] = nVal1;

                    i1++;

                    i2++;

                }

            }

        }

        else if (i1 < nFIDCount1)

        {

            const GIntBig nVal1 = panFIDList1[i1];

            panFIDList[nFIDCount++] = nVal1;

            i1++;

        }

        else if (i2 < nFIDCount2)

        {

            const GIntBig nVal2 = panFIDList2[i2];

            panFIDList[nFIDCount++] = nVal2;

            i2++;

        }

    }

    panFIDList[nFIDCount] = OGRNullFID;

    return panFIDList;

}

// The input arrays must be sorted.

static GIntBig *OGRANDGIntBigArray(GIntBig panFIDList1[], GIntBig nFIDCount1,

                                   GIntBig panFIDList2[], GIntBig nFIDCount2,

                                   GIntBig &nFIDCount)

{

    GIntBig nMaxCount = std::max(nFIDCount1, nFIDCount2);

    GIntBig *panFIDList = static_cast<GIntBig *>(

        CPLMalloc(static_cast<size_t>(nMaxCount + 1) * sizeof(GIntBig)));

    nFIDCount = 0;

    for (GIntBig i1 = 0, i2 = 0; i1 < nFIDCount1 && i2 < nFIDCount2;)

    {

        const GIntBig nVal1 = panFIDList1[i1];

        const GIntBig nVal2 = panFIDList2[i2];

        if (nVal1 < nVal2)

        {

            if (i1 + 1 < nFIDCount1 && panFIDList1[i1 + 1] <= nVal2)

            {

                i1++;

            }

            else

            {

                i1++;

                i2++;

            }

        }

        else if (nVal1 == nVal2)

        {

            panFIDList[nFIDCount++] = nVal1;

            i1++;

            i2++;

        }

        else

        {

            if (i2 + 1 < nFIDCount2 && panFIDList2[i2 + 1] <= nVal1)

            {

                i2++;

            }

            else

            {

                i1++;

                i2++;

            }

        }

    }

    panFIDList[nFIDCount] = OGRNullFID;

    return panFIDList;

}

GIntBig *OGRFeatureQuery::EvaluateAgainstIndices(const swq_expr_node *psExpr,

                                                 OGRLayer *poLayer,

                                                 GIntBig &nFIDCount)

{

    // Does the expression meet our requirements?

    if (psExpr == nullptr || psExpr->eNodeType != SNT_OPERATION)

        return nullptr;

    if ((psExpr->nOperation == SWQ_OR || psExpr->nOperation == SWQ_AND) &&

        psExpr->nSubExprCount == 2)

    {

        GIntBig nFIDCount1 = 0;

        GIntBig nFIDCount2 = 0;

        GIntBig *panFIDList1 =

            EvaluateAgainstIndices(psExpr->papoSubExpr[0], poLayer, nFIDCount1);

        GIntBig *panFIDList2 =

            panFIDList1 == nullptr

                ? nullptr

                : EvaluateAgainstIndices(psExpr->papoSubExpr[1], poLayer,

                                         nFIDCount2);

        GIntBig *panFIDList = nullptr;

        if (panFIDList1 != nullptr && panFIDList2 != nullptr)

        {

            if (psExpr->nOperation == SWQ_OR)

                panFIDList =

                    OGRORGIntBigArray(panFIDList1, nFIDCount1, panFIDList2,

                                      nFIDCount2, nFIDCount);

            else if (psExpr->nOperation == SWQ_AND)

                panFIDList =

                    OGRANDGIntBigArray(panFIDList1, nFIDCount1, panFIDList2,

                                       nFIDCount2, nFIDCount);

        }

        CPLFree(panFIDList1);

        CPLFree(panFIDList2);

        return panFIDList;

    }

    if (!(psExpr->nOperation == SWQ_EQ || psExpr->nOperation == SWQ_IN) ||

        psExpr->nSubExprCount < 2)

        return nullptr;

    const swq_expr_node *poColumn = psExpr->papoSubExpr[0];

    const swq_expr_node *poValue = psExpr->papoSubExpr[1];

    if (poColumn->eNodeType != SNT_COLUMN || poValue->eNodeType != SNT_CONSTANT)

        return nullptr;

    const int nIdx = OGRFeatureFetcherFixFieldIndex(poLayer->GetLayerDefn(),

                                                    poColumn->field_index);

    OGRAttrIndex *poIndex = poLayer->GetIndex()->GetFieldIndex(nIdx);

    if (poIndex == nullptr)

        return nullptr;

    // Have an index, now we need to query it.

    OGRField sValue;

    const OGRFieldDefn *poFieldDefn =

        poLayer->GetLayerDefn()->GetFieldDefn(nIdx);

    // Handle the case of an IN operation.

    if (psExpr->nOperation == SWQ_IN)

    {

        int nLength = 0;

        GIntBig *panFIDs = nullptr;

        nFIDCount = 0;

        for (int iIN = 1; iIN < psExpr->nSubExprCount; iIN++)

        {

            switch (poFieldDefn->GetType())

            {

                case OFTInteger:

                    if (psExpr->papoSubExpr[iIN]->field_type == SWQ_FLOAT)

                        sValue.Integer = static_cast<int>(

                            psExpr->papoSubExpr[iIN]->float_value);

                    else

                        sValue.Integer = static_cast<int>(

                            psExpr->papoSubExpr[iIN]->int_value);

                    break;

                case OFTInteger64:

                    if (psExpr->papoSubExpr[iIN]->field_type == SWQ_FLOAT)

                        sValue.Integer64 = static_cast<GIntBig>(

                            psExpr->papoSubExpr[iIN]->float_value);

                    else

                        sValue.Integer64 = psExpr->papoSubExpr[iIN]->int_value;

                    break;

                case OFTReal:

                    sValue.Real = psExpr->papoSubExpr[iIN]->float_value;

                    break;

                case OFTString:

                    sValue.String = psExpr->papoSubExpr[iIN]->string_value;

                    break;

                default:

                    CPLAssert(false);

                    return nullptr;

            }

            int nFIDCount32 = static_cast<int>(nFIDCount);

            panFIDs = poIndex->GetAllMatches(&sValue, panFIDs, &nFIDCount32,

                                             &nLength);

            nFIDCount = nFIDCount32;

        }

        if (nFIDCount > 1)

        {

            // The returned FIDs are expected to be in sorted order.

            std::sort(panFIDs, panFIDs + nFIDCount);

        }

        return panFIDs;

    }

    // Handle equality test.

    switch (poFieldDefn->GetType())

    {

        case OFTInteger:

            if (poValue->field_type == SWQ_FLOAT)

                sValue.Integer = static_cast<int>(poValue->float_value);

            else

                sValue.Integer = static_cast<int>(poValue->int_value);

            break;

        case OFTInteger64:

            if (poValue->field_type == SWQ_FLOAT)

                sValue.Integer64 = static_cast<GIntBig>(poValue->float_value);

            else

                sValue.Integer64 = poValue->int_value;

            break;

        case OFTReal:

            sValue.Real = poValue->float_value;

            break;

        case OFTString:

            sValue.String = poValue->string_value;

            break;

        default:

            CPLAssert(false);

            return nullptr;

    }

    int nLength = 0;

    int nFIDCount32 = 0;

    GIntBig *panFIDs =

        poIndex->GetAllMatches(&sValue, nullptr, &nFIDCount32, &nLength);

    nFIDCount = nFIDCount32;

    if (nFIDCount > 1)

    {

        // The returned FIDs are expected to be sorted.

        std::sort(panFIDs, panFIDs + nFIDCount);

    }

    return panFIDs;

}

/************************************************************************/

/*                         OGRFieldCollector()                          */

/*                                                                      */

/*      Helper function for recursing through tree to satisfy           */

/*      GetUsedFields().                                                */

/************************************************************************/

char **OGRFeatureQuery::FieldCollector(void *pBareOp, char **papszList)

{

    swq_expr_node *op = static_cast<swq_expr_node *>(pBareOp);

    // References to tables other than the primarily are currently unsupported.

    // Error out.

    if (op->eNodeType == SNT_COLUMN)

    {

        if (op->table_index != 0)

        {

            CSLDestroy(papszList);

            return nullptr;

        }

        // Add the field name into our list if it is not already there.

        const char *pszFieldName = nullptr;

        const int nIdx =

            OGRFeatureFetcherFixFieldIndex(poTargetDefn, op->field_index);

        if (nIdx >= poTargetDefn->GetFieldCount() &&

            nIdx < poTargetDefn->GetFieldCount() + SPECIAL_FIELD_COUNT)

        {

            pszFieldName =

                SpecialFieldNames[nIdx - poTargetDefn->GetFieldCount()];

        }

        else if (nIdx >= 0 && nIdx < poTargetDefn->GetFieldCount())

        {

            auto poFieldDefn = poTargetDefn->GetFieldDefn(nIdx);

            if (!poFieldDefn)

            {

                CPLAssert(false);

                CSLDestroy(papszList);

                return nullptr;

            }

            pszFieldName = poFieldDefn->GetNameRef();

        }

        else

        {

            CSLDestroy(papszList);

            return nullptr;

        }

        if (CSLFindString(papszList, pszFieldName) == -1)

            papszList = CSLAddString(papszList, pszFieldName);

    }

    // Add in fields from subexpressions.

    if (op->eNodeType == SNT_OPERATION)

    {

        for (int iSubExpr = 0; iSubExpr < op->nSubExprCount; iSubExpr++)

        {

            papszList = FieldCollector(op->papoSubExpr[iSubExpr], papszList);

        }

    }

    return papszList;

}

/************************************************************************/

/*                           GetUsedFields()                            */

/************************************************************************/

/**

 * Returns lists of fields in expression.

 *

 * All attribute fields are used in the expression of this feature

 * query are returned as a StringList of field names.  This function would

 * primarily be used within drivers to recognise special case conditions

 * depending only on attribute fields that can be very efficiently

 * fetched.

 *

 * NOTE: If any fields in the expression are from tables other than the

 * primary table then NULL is returned indicating an error.  In successful

 * use, no non-empty expression should return an empty list.

 *

 * @return list of field names.  Free list with CSLDestroy() when no longer

 * required.

 */

char **OGRFeatureQuery::GetUsedFields()

{

    if (pSWQExpr == nullptr)

        return nullptr;

    return FieldCollector(pSWQExpr, nullptr);

}

//! @endcond