Statistics for MySQL  0.9
skewness_pop.c
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00001 /* skewness_pop.c (population moment coefficient of skewness) */
00002 
00003 /***********************************************************************
00004 *  This code is part of Statistics for MySQL.
00005 *
00006 *  Copyright (C) 2011 Heinrich Schuchardt (xypron.glpk@gmx.de)
00007 *
00008 *  Licensed under the Apache License, Version 2.0 (the "License");
00009 *  you may not use this file except in compliance with the License.
00010 *  You may obtain a copy of the License at
00011 *
00012 *      http://www.apache.org/licenses/LICENSE-2.0
00013 *
00014 *  Unless required by applicable law or agreed to in writing, software
00015 *  distributed under the License is distributed on an "AS IS" BASIS,
00016 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
00017 *  See the License for the specific language governing permissions and
00018 *  limitations under the License.
00019 ***********************************************************************/
00020 
00030 #include "sqlstat.h"
00031 
00035 struct skewness_pop_storage {
00036   int     argc;  
00037   double  count;  
00038   double  sumX;   
00039   double  sumXX;  
00040   double  sumXXX; 
00041 };
00042 
00055 my_bool skewness_pop_init(UDF_INIT *initid, UDF_ARGS *args, char *message) {
00056   struct skewness_pop_storage * data;
00057   
00058   if (args->arg_count < 1 || args->arg_count > 2) {
00059     strcpy(message,"skewness_pop() requires one or two arguments");
00060     return 1;
00061   }
00062   args->arg_type[0] = REAL_RESULT;
00063   if (args->arg_count > 1) {
00064     args->arg_type[1] = REAL_RESULT;
00065   }
00066 
00067   data = (struct skewness_pop_storage *) malloc( sizeof(struct skewness_pop_storage));
00068   if (data == NULL) {
00069     strcpy(message,"Couldn't allocate memory");
00070     return 1;
00071   }  
00072   data->argc = args->arg_count;
00073   
00074   initid->maybe_null = 1;
00075   initid->decimals   = NOT_FIXED_DEC;
00076   initid->max_length = 13 + initid->decimals;
00077   initid->ptr        = (char *) data;
00078   initid->const_item = 0;
00079   
00080   return 0;
00081 }
00082 
00094 void skewness_pop_reset(UDF_INIT *initid, UDF_ARGS *args, char *is_null, char *error) {
00095   skewness_pop_clear(initid, is_null, error);
00096   skewness_pop_add(initid, args, is_null, error);
00097 }
00098 
00108 void skewness_pop_clear(UDF_INIT *initid, char *is_null, char *error) {
00109   struct skewness_pop_storage *data;
00110   data = (struct skewness_pop_storage *) initid->ptr;
00111   data->count = 0;
00112   data->sumX  = 0;
00113   data->sumXX = 0;
00114   data->sumXXX = 0;
00115 }
00116 
00127 void skewness_pop_add(UDF_INIT *initid, UDF_ARGS *args, char *is_null, char *error) {
00128   struct skewness_pop_storage * data;
00129   double x;
00130   double w;
00131   
00132   if (!args->args[0]) {
00133     return;
00134   }
00135   data = (struct skewness_pop_storage *) initid->ptr;
00136   if (data->argc > 1) {
00137     if (!args->args[1]) {
00138       return;
00139     }
00140     w = *((double*) args->args[1]);
00141   } else {
00142     w = 1.;
00143   }
00144   x = *((double*) args->args[0]);
00145   
00146   data->count  += w;
00147   data->sumX   += w * x;
00148   data->sumXX  += w * x*x;
00149   data->sumXXX += w * x*x*x;
00150 }
00151 
00161 double skewness_pop(UDF_INIT *initid, UDF_ARGS *args, char *is_null, char *error) {
00162   struct skewness_pop_storage * data;
00163   double m3;
00164   double m2;
00165   double mean;
00166   
00167   double ret;
00168 
00169   data = (struct skewness_pop_storage *) initid->ptr;
00170 
00171   if (data->count <= 0) {
00172     *is_null = 1;
00173     return 0;
00174   }
00175   
00176   mean = data->sumX / data->count;
00177   
00178   m3 = (data->sumXXX - 3 * mean * data->sumXX + 2 * mean * mean * data->sumX) / data->count;
00179   m2 = (data->sumXX - mean * data->sumX) / data->count;
00180   
00181   if (m2 <= 0) {
00182     *is_null = 1;
00183     return 0;
00184   }
00185   
00186   ret = m3 / sqrt(m2 * m2 * m2);
00187   
00188   return ret;
00189 }
00190 
00198 void skewness_pop_deinit(UDF_INIT *initid) {
00199   if (initid->ptr) {
00200     free(initid->ptr);
00201   }
00202 }
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