220 lines
6.7 KiB
C
220 lines
6.7 KiB
C
/*
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** 2013-05-28
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**
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** The author disclaims copyright to this source code. In place of
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** a legal notice, here is a blessing:
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**
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** May you do good and not evil.
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** May you find forgiveness for yourself and forgive others.
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** May you share freely, never taking more than you give.
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**
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******************************************************************************
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**
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** This file contains code to implement the percentile(Y,P) SQL function
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** as described below:
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**
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** (1) The percentile(Y,P) function is an aggregate function taking
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** exactly two arguments.
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**
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** (2) If the P argument to percentile(Y,P) is not the same for every
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** row in the aggregate then an error is thrown. The word "same"
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** in the previous sentence means that the value differ by less
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** than 0.001.
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**
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** (3) If the P argument to percentile(Y,P) evaluates to anything other
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** than a number in the range of 0.0 to 100.0 inclusive then an
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** error is thrown.
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**
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** (4) If any Y argument to percentile(Y,P) evaluates to a value that
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** is not NULL and is not numeric then an error is thrown.
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**
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** (5) If any Y argument to percentile(Y,P) evaluates to plus or minus
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** infinity then an error is thrown. (SQLite always interprets NaN
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** values as NULL.)
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**
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** (6) Both Y and P in percentile(Y,P) can be arbitrary expressions,
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** including CASE WHEN expressions.
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**
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** (7) The percentile(Y,P) aggregate is able to handle inputs of at least
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** one million (1,000,000) rows.
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**
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** (8) If there are no non-NULL values for Y, then percentile(Y,P)
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** returns NULL.
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**
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** (9) If there is exactly one non-NULL value for Y, the percentile(Y,P)
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** returns the one Y value.
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**
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** (10) If there N non-NULL values of Y where N is two or more and
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** the Y values are ordered from least to greatest and a graph is
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** drawn from 0 to N-1 such that the height of the graph at J is
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** the J-th Y value and such that straight lines are drawn between
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** adjacent Y values, then the percentile(Y,P) function returns
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** the height of the graph at P*(N-1)/100.
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**
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** (11) The percentile(Y,P) function always returns either a floating
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** point number or NULL.
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**
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** (12) The percentile(Y,P) is implemented as a single C99 source-code
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** file that compiles into a shared-library or DLL that can be loaded
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** into SQLite using the sqlite3_load_extension() interface.
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*/
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#include "sqlite3ext.h"
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SQLITE_EXTENSION_INIT1
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#include <assert.h>
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#include <string.h>
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#include <stdlib.h>
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/* The following object is the session context for a single percentile()
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** function. We have to remember all input Y values until the very end.
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** Those values are accumulated in the Percentile.a[] array.
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*/
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typedef struct Percentile Percentile;
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struct Percentile {
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unsigned nAlloc; /* Number of slots allocated for a[] */
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unsigned nUsed; /* Number of slots actually used in a[] */
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double rPct; /* 1.0 more than the value for P */
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double *a; /* Array of Y values */
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};
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/*
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** Return TRUE if the input floating-point number is an infinity.
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*/
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static int isInfinity(double r){
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sqlite3_uint64 u;
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assert( sizeof(u)==sizeof(r) );
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memcpy(&u, &r, sizeof(u));
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return ((u>>52)&0x7ff)==0x7ff;
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}
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/*
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** Return TRUE if two doubles differ by 0.001 or less
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*/
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static int sameValue(double a, double b){
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a -= b;
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return a>=-0.001 && a<=0.001;
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}
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/*
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** The "step" function for percentile(Y,P) is called once for each
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** input row.
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*/
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static void percentStep(sqlite3_context *pCtx, int argc, sqlite3_value **argv){
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Percentile *p;
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double rPct;
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int eType;
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double y;
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assert( argc==2 );
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/* Requirement 3: P must be a number between 0 and 100 */
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eType = sqlite3_value_numeric_type(argv[1]);
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rPct = sqlite3_value_double(argv[1]);
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if( (eType!=SQLITE_INTEGER && eType!=SQLITE_FLOAT) ||
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((rPct = sqlite3_value_double(argv[1]))<0.0 || rPct>100.0) ){
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sqlite3_result_error(pCtx, "2nd argument to percentile() is not "
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"a number between 0.0 and 100.0", -1);
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return;
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}
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/* Allocate the session context. */
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p = (Percentile*)sqlite3_aggregate_context(pCtx, sizeof(*p));
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if( p==0 ) return;
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/* Remember the P value. Throw an error if the P value is different
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** from any prior row, per Requirement (2). */
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if( p->rPct==0.0 ){
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p->rPct = rPct+1.0;
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}else if( !sameValue(p->rPct,rPct+1.0) ){
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sqlite3_result_error(pCtx, "2nd argument to percentile() is not the "
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"same for all input rows", -1);
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return;
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}
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/* Ignore rows for which Y is NULL */
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eType = sqlite3_value_type(argv[0]);
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if( eType==SQLITE_NULL ) return;
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/* If not NULL, then Y must be numeric. Otherwise throw an error.
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** Requirement 4 */
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if( eType!=SQLITE_INTEGER && eType!=SQLITE_FLOAT ){
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sqlite3_result_error(pCtx, "1st argument to percentile() is not "
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"numeric", -1);
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return;
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}
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/* Throw an error if the Y value is infinity or NaN */
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y = sqlite3_value_double(argv[0]);
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if( isInfinity(y) ){
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sqlite3_result_error(pCtx, "Inf input to percentile()", -1);
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return;
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}
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/* Allocate and store the Y */
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if( p->nUsed>=p->nAlloc ){
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unsigned n = p->nAlloc*2 + 250;
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double *a = sqlite3_realloc64(p->a, sizeof(double)*n);
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if( a==0 ){
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sqlite3_free(p->a);
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memset(p, 0, sizeof(*p));
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sqlite3_result_error_nomem(pCtx);
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return;
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}
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p->nAlloc = n;
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p->a = a;
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}
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p->a[p->nUsed++] = y;
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}
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/*
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** Compare to doubles for sorting using qsort()
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*/
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static int SQLITE_CDECL doubleCmp(const void *pA, const void *pB){
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double a = *(double*)pA;
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double b = *(double*)pB;
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if( a==b ) return 0;
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if( a<b ) return -1;
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return +1;
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}
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/*
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** Called to compute the final output of percentile() and to clean
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** up all allocated memory.
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*/
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static void percentFinal(sqlite3_context *pCtx){
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Percentile *p;
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unsigned i1, i2;
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double v1, v2;
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double ix, vx;
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p = (Percentile*)sqlite3_aggregate_context(pCtx, 0);
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if( p==0 ) return;
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if( p->a==0 ) return;
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if( p->nUsed ){
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qsort(p->a, p->nUsed, sizeof(double), doubleCmp);
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ix = (p->rPct-1.0)*(p->nUsed-1)*0.01;
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i1 = (unsigned)ix;
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i2 = ix==(double)i1 || i1==p->nUsed-1 ? i1 : i1+1;
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v1 = p->a[i1];
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v2 = p->a[i2];
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vx = v1 + (v2-v1)*(ix-i1);
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sqlite3_result_double(pCtx, vx);
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}
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sqlite3_free(p->a);
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memset(p, 0, sizeof(*p));
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}
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#ifdef _WIN32
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__declspec(dllexport)
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#endif
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int sqlite3_percentile_init(
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sqlite3 *db,
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char **pzErrMsg,
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const sqlite3_api_routines *pApi
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){
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int rc = SQLITE_OK;
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SQLITE_EXTENSION_INIT2(pApi);
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(void)pzErrMsg; /* Unused parameter */
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rc = sqlite3_create_function(db, "percentile", 2, SQLITE_UTF8, 0,
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0, percentStep, percentFinal);
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return rc;
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}
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