/*
 * This shows how to store values of type numeric in a PostgreSQL database
 *   using C datatypes
 *
 * Change your database credentials as given to PQconnectdb().
 *   Then compile with:
 * cc -W -Wall -O3 -s -pipe -I/usr/local/include -L/usr/local/lib -o pgnumeric pgnumeric.c -lpq
 *
 * Create table for testing:
 * CREATE TABLE t1 (n numeric NOT NULL);
 */

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <err.h>
/* for htons() */
#include <arpa/inet.h>
/* PostgreSQL */
#include "libpq-fe.h"

int main ()
{
  PGconn *pg = PQconnectdb("host=/tmp dbname=... user=... password=...");
  PGresult *res;
  /*
    Keep in sync with struct NumericVar and numeric_send() from numeric.c:
    https://github.com/postgres/postgres/blob/master/src/backend/utils/adt/numeric.c
   */
  struct
  {
    int16_t ndigits; /* # of digits in digits[] - can be 0! */
    int16_t weight;  /* weight of first digit */
    int16_t sign;    /* NUMERIC_POS, _NEG, _NAN, _PINF, or _NINF */
    int16_t dscale;  /* display scale */
    /* XXX dont use a fixed size buffer */
    int16_t buf[8];  /* start of palloc'd space for digits[] */
  } NumericVar;
  int bin = 1, len, empty;
  uint8_t srcbuf[16];
  uint16_t i, tmp, dstidx, srclen;
  uint32_t divi, modu;
  uint64_t ui64src, ui64tmp;

  /* We want to store these unsigned 64 bit and 72 bit integers as numerics */
  ui64src = ~(uint64_t) 0; /* maximum uint64 */
  srcbuf[0] = 1;           /* maximum uint64 + 1 */
  for (srclen = 1; srclen < 9; srclen++)
    srcbuf[srclen] = 0;

  /* First, store native C uint64 */
  dstidx = 0;
  for (ui64tmp = ui64src; ui64tmp; ui64tmp /= 10000) {
    dstidx++;
  }
  if (dstidx >= sizeof(NumericVar.buf)/sizeof(NumericVar.buf[0]))
    errx(1, "%s", "NumericVar.buf too small for uint64");

  NumericVar.ndigits = htons(dstidx);
  NumericVar.weight = htons(dstidx - 1);
  NumericVar.sign = 0;
  NumericVar.dscale = 0;

  len = 4 * 2 + dstidx * 2; // sizeof(int16_t) = 2
  for (ui64tmp = ui64src; ui64tmp; ui64tmp /= 10000) {
    NumericVar.buf[--dstidx] = htons(ui64tmp % 10000);
  }

  if (PQstatus(pg) != CONNECTION_OK)
    errx(1, "%s", "PGconn");

  res = PQexecParams(pg, "INSERT INTO t1 VALUES ($1)", 1, NULL,
                     (const char*[]){ (char*)&NumericVar },
                     (const int[]){ len },
                     (const int[]){ bin }, 0);
  if (PQresultStatus(res) != PGRES_COMMAND_OK)
    errx(1, "%s", "INSERT");

  /* Now store 72 bit unsigned integer */
  dstidx = 0;
  do {
    empty = 1;
    modu = srcbuf[0];

    for (i = 0; i < srclen - 1; i++) {
      divi = modu / 10000;
      modu = ((modu - divi * 10000) << 8) + srcbuf[i + 1];
      srcbuf[i] = divi;
      empty &= (srcbuf[i] == 0);
    }

    divi = modu / 10000;
    modu = modu - divi * 10000;
    srcbuf[i] = divi;
    empty &= (srcbuf[i] == 0);

    if (dstidx >= sizeof(NumericVar.buf)/sizeof(NumericVar.buf[0]))
      errx(1, "%s", "NumericVar.buf too small for large integer");

    NumericVar.buf[dstidx] = htons(modu);
    dstidx++;
  } while (!empty);

  for (i = 0; i < dstidx / 2; i++) {
    tmp = NumericVar.buf[i];
    NumericVar.buf[i] = NumericVar.buf[dstidx - 1 - i];
    NumericVar.buf[dstidx - 1 - i] = tmp;
  }

  NumericVar.ndigits = htons(dstidx);
  NumericVar.weight = htons(dstidx - 1);
  NumericVar.sign = 0;
  NumericVar.dscale = 0;

  len = 4 * 2 + dstidx * 2; // sizeof(int16_t) = 2

  if (PQstatus(pg) != CONNECTION_OK)
    errx(1, "%s", "PGconn");

  res = PQexecParams(pg, "INSERT INTO t1 VALUES ($1)", 1, NULL,
                     (const char * const []){ (char*)&NumericVar },
                     (const int[]){ len },
                     (const int[]){ bin }, 0);
  if (PQresultStatus(res) != PGRES_COMMAND_OK)
    errx(1, "%s", "INSERT");

  PQfinish(pg);
}