Added a "stddev()" aggregate function for testing the new user aggregate

function interface. (CVS 393)

FossilOrigin-Name: 2198109712ccf988f93db5740a4f248e80fb9f5d

src/func.c

@@ -16,9 +16,11 @@
 ** sqliteRegisterBuildinFunctions() found at the bottom of the file.
 ** All other code has file scope.
 **
-** $Id: func.c,v 1.1 2002/02/24 01:55:17 drh Exp $
+** $Id: func.c,v 1.2 2002/02/24 17:12:54 drh Exp $
 */
 #include <ctype.h>
+#include <math.h>
+#include <stdlib.h>
 #include "sqlite.h"
 
 /*
@@ -45,6 +47,48 @@ static void lowerFunc(void *context, int argc, const char **argv){
   }
 }
 
+/*
+** An instance of the following structure holds the context of a
+** standard deviation computation.
+*/
+typedef struct StdDevCtx StdDevCtx;
+struct StdDevCtx {
+  double sum;     /* Sum of terms */
+  double sum2;    /* Sum of the squares of terms */
+  int n;          /* Number of terms seen so far */
+};
+
+/*
+** Routines used to compute the standard deviation as an aggregate.
+*/
+static void *stdDevStep(void *stddev, int argc, char **argv){
+  StdDevCtx *p;
+  double x;
+  if( argc<1 ) return 0;
+  if( stddev==0 ){
+    p = malloc( sizeof(*p) );
+    p->n = 0;
+    p->sum = 0.0;
+    p->sum2 = 0.0;
+  }else{
+    p = (StdDevCtx*)stddev;
+  }
+  x = atof(argv[0]);
+  p->sum += x;
+  p->sum2 += x*x;
+  p->n++;
+  return p;
+}
+static void stdDevFinalize(void *stddev, void *context){
+  StdDevCtx *p = (StdDevCtx*)stddev;
+  if( context && p && p->n>1 ){
+    double rN = p->n;
+    sqlite_set_result_double(context, 
+       sqrt((p->sum2 - p->sum*p->sum/rN)/(rN-1.0)));
+  }
+  if( stddev ) free(stddev);
+}
+
 /*
 ** This file registered all of the above C functions as SQL
 ** functions.
@@ -52,4 +96,5 @@ static void lowerFunc(void *context, int argc, const char **argv){
 void sqliteRegisterBuildinFunctions(sqlite *db){
   sqlite_create_function(db, "upper", 1, upperFunc);
   sqlite_create_function(db, "lower", 1, lowerFunc);
+  sqlite_create_aggregate(db, "stddev", 1, stdDevStep, stdDevFinalize);
 }