Re: dividing money by money

On Apr 1, 2010, at 7:57 AM, Kevin Grittner wrote:
> I'm inclined to think it's better to have an explicit cast from
> money to numeric, as long as it is exact, and leave the division of
> money by money as float8. It does sort of beg the question of
> whether we should support a cast back in the other direction,
> though. I think that would wrap this all up in a tidy package.

OK. Here is the whole thing in C:

#include <postgres.h>
#include <fmgr.h>
#include <utils/cash.h>
#include <utils/numeric.h>
#include <utils/pg_locale.h>

PG_MODULE_MAGIC;

extern Datum int8_numeric(PG_FUNCTION_ARGS);
extern Datum numeric_div(PG_FUNCTION_ARGS);
extern Datum numeric_mul(PG_FUNCTION_ARGS);
extern Datum numeric_int8(PG_FUNCTION_ARGS);

PG_FUNCTION_INFO_V1(cash_div_cash);

/* cash_div_cash()
* Divide cash by cash, returning float8.
*/
Datum
cash_div_cash(PG_FUNCTION_ARGS)
{
Cash dividend = PG_GETARG_CASH(0);
Cash divisor = PG_GETARG_CASH(1);
float8 quotient;

if (divisor == 0)
ereport(ERROR,
(errcode(ERRCODE_DIVISION_BY_ZERO),
errmsg("division by zero")));

quotient = (float8)dividend / (float8)divisor;
PG_RETURN_FLOAT8(quotient);
}

PG_FUNCTION_INFO_V1(cash_numeric);

/* cash_numeric()
* Convert cash to numeric.
*/
Datum
cash_numeric(PG_FUNCTION_ARGS)
{
Cash money = PG_GETARG_CASH(0);
int fpoint;
int64 scale;
int i;
Numeric result;
Datum amount;
Datum numeric_scale;
Datum one;

struct lconv *lconvert = PGLC_localeconv();

/*
* Find the number of digits after the decimal point.
* (These lines were copied from cash_in().)
*/
fpoint = lconvert->frac_digits;
if (fpoint < 0 || fpoint > 10)
fpoint = 2;
scale = 1;
for (i = 0; i < fpoint; i++)
scale *= 10;

amount = DirectFunctionCall1(&int8_numeric, Int64GetDatum(money));
one = DirectFunctionCall1(&int8_numeric, Int64GetDatum(1));
numeric_scale = DirectFunctionCall1(&int8_numeric, Int64GetDatum(scale));
numeric_scale = DirectFunctionCall2(&numeric_div, one, numeric_scale);
result = DatumGetNumeric(DirectFunctionCall2(&numeric_mul, amount, numeric_scale));

result->n_sign_dscale = NUMERIC_SIGN(result) | fpoint; /* Display the right number of decimal digits. */

PG_RETURN_NUMERIC(result);
}

PG_FUNCTION_INFO_V1(numeric_cash);

/* numeric_cash()
* Convert numeric to cash.
*/
Datum
numeric_cash(PG_FUNCTION_ARGS)
{
Datum amount = PG_GETARG_DATUM(0);
Cash result;
int fpoint;
int64 scale;
int i;
Datum numeric_scale;

struct lconv *lconvert = PGLC_localeconv();

/*
* Find the number of digits after the decimal point.
*/
fpoint = lconvert->frac_digits;
if (fpoint < 0 || fpoint > 10)
fpoint = 2;
scale = 1;
for (i = 0; i < fpoint; i++)
scale *= 10;

numeric_scale = DirectFunctionCall1(&int8_numeric, Int64GetDatum(scale));
amount = DirectFunctionCall2(&numeric_mul, amount, numeric_scale);
amount = DirectFunctionCall1(&numeric_int8, amount);

result = DatumGetInt64(amount);
PG_RETURN_CASH(result);
}

--------------------------------------------------------------------------------------

And the SQL to load it:

CREATE FUNCTION cash_div_cash(money, money) RETURNS double precision
LANGUAGE c IMMUTABLE STRICT
AS '$libdir/divide_money', 'cash_div_cash';

CREATE FUNCTION cash_numeric(money) RETURNS numeric
LANGUAGE c IMMUTABLE STRICT
AS '$libdir/divide_money', 'cash_numeric';

CREATE FUNCTION numeric_cash(numeric) RETURNS money
LANGUAGE c IMMUTABLE STRICT
AS '$libdir/divide_money', 'numeric_cash';

CREATE CAST (money AS numeric) WITH FUNCTION public.cash_numeric(money) AS ASSIGNMENT;

CREATE CAST (numeric AS money) WITH FUNCTION public.numeric_cash(numeric) AS ASSIGNMENT;

CREATE OPERATOR / (
PROCEDURE = cash_div_cash,
LEFTARG = money,
RIGHTARG = money
);