SQLite

             * (p->a[ii*2+1] + p->a[1])              /* (yN + y0) */
             * 0.5;
    sqlite3_result_double(context, rArea);
    sqlite3_free(p);
  }            
}

/*
** Determine if point (x0,y0) is beneath line segment (x1,y1)->(x2,y2).
** Returns:
**
**    +2  x0,y0 is on the line segement
**
**    +1  x0,y0 is beneath line segment
**
**    0   x0,y0 is not on or beneath the line segment or the line segment
**        is vertical and x0,y0 is not on the line segment
**
** The left-most coordinate min(x1,x2) is not considered to be part of
** the line segment for the purposes of this analysis.
*/
static int pointBeneathLine(
  double x0, double y0,
  double x1, double y1,
  double x2, double y2
){
  double y;
  if( x0==x1 && y0==y1 ) return 2;
  if( x1<x2 ){
    if( x0<=x1 || x0>x2 ) return 0;
  }else if( x1>x2 ){
    if( x0<=x2 || x0>x1 ) return 0;
  }else{
    /* Vertical line segment */
    if( x0!=x1 ) return 0;
    if( y0<y1 && y0<y2 ) return 0;
    if( y0>y1 && y0>y2 ) return 0;
    return 2;
  }
  y = y1 + (y2-y1)*(x0-x1)/(x2-x1);
  if( y0==y ) return 2;
  if( y0<y ) return 1;
  return 0;
}

/*
** SQL function:    geopoly_within(P,X,Y)
**
** Return +2 if point X,Y is within polygon P.
** Return +1 if point X,Y is on the polygon boundary.
** Return 0 if point X,Y is outside the polygon
*/
static void geopolyWithinFunc(
  sqlite3_context *context,
  int argc,
  sqlite3_value **argv
){
  GeoPoly *p = geopolyFuncParam(context, argv[0]);
  double x0 = sqlite3_value_double(argv[1]);
  double y0 = sqlite3_value_double(argv[2]);
  if( p ){
    int v = 0;
    int cnt = 0;
    int ii;
    for(ii=0; ii<p->nVertex-1; ii++){
      v = pointBeneathLine(x0,y0,p->a[ii*2],p->a[ii*2+1],
                                 p->a[ii*2+2],p->a[ii*2+3]);
      if( v==2 ) break;
      cnt += v;
    }
    if( v!=2 ){
      v = pointBeneathLine(x0,y0,p->a[ii*2],p->a[ii*2+1],
                                 p->a[0],p->a[1]);
    }
    if( v==2 ){
      sqlite3_result_int(context, 1);
    }else if( ((v+cnt)&1)==0 ){
      sqlite3_result_int(context, 0);
    }else{
      sqlite3_result_int(context, 2);
    }
    sqlite3_free(p);
  }            
}


#ifdef _WIN32
__declspec(dllexport)
#endif
int sqlite3_geopoly_init(
  sqlite3 *db, 
  char **pzErrMsg, 
  const sqlite3_api_routines *pApi
){
  int rc = SQLITE_OK;
  static const struct {
    void (*xFunc)(sqlite3_context*,int,sqlite3_value**);
    int nArg;
    const char *zName;
  } aFunc[] = {
     { geopolyAreaFunc,          1,    "geopoly_area"   },
     { geopolyBlobFunc,          1,    "geopoly_blob"   },
     { geopolyJsonFunc,          1,    "geopoly_json"   },
     { geopolyWithinFunc,        3,    "geopoly_within" },
  };
  int i;
  SQLITE_EXTENSION_INIT2(pApi);
  (void)pzErrMsg;  /* Unused parameter */
  for(i=0; i<sizeof(aFunc)/sizeof(aFunc[0]) && rc==SQLITE_OK; i++){
    rc = sqlite3_create_function(db, aFunc[i].zName, aFunc[i].nArg,
                                 SQLITE_UTF8, 0,
                                 aFunc[i].xFunc, 0, 0);
  }
  return rc;
}