unit.h

#ifndef _UNIT_H
#define _UNIT_H 1

#include <postgres.h>
#include <utils/hsearch.h>

/* indices */
#define UNIT_m		0 /* meter */
#define UNIT_kg		1 /* kilogram */
#define UNIT_s		2 /* second */
#define UNIT_A		3 /* ampere */
#define UNIT_K		4 /* kelvin */
#define UNIT_mol	5 /* mole */
#define UNIT_cd		6 /* candela */
#define UNIT_B		7 /* byte */

#define N_UNITS		8

/* defined units */

#define UNIT_NAME_LENGTH 32
/* longest unit names (without prefixes) observed in definitions.units:
 * specificheat_glass_silica (25 chars)
 * hardtranslucentarkansas (23 chars)
 * venezuelanbolivarfuerte */

#define MIN_PLURAL_LENGTH 3
/* minimum length of input unit to consider stripping a trailing plural 's'
 * lbs secs
 */

#define DBL_DIG			15
#define TIME_MINUTE		60
#define TIME_HOUR		(60 * TIME_MINUTE)
#define TIME_DAY		(24 * TIME_HOUR)
#define TIME_YEAR		(365 * TIME_DAY)
#define TIME_YEAR_NAME	"commonyear"

extern const char *base_units[N_UNITS]; /* names of base units */

struct derived_unit_t {
	char		*name;
	signed char	 units[N_UNITS];
};

extern const struct derived_unit_t si_derived_units[];

/* type def */

typedef struct Unit {
	double			value;
	signed char		units[N_UNITS];
} Unit;

typedef struct UnitShift {
	Unit			unit;
	double			shift;
} UnitShift;

/* hash table and regex interface */

extern HTAB		*unit_names;

typedef struct unit_names_t {
	char		 name[UNIT_NAME_LENGTH];
	UnitShift	 unit_shift;
} unit_names_t;

typedef struct unit_dimensions_t {
	char		 units[N_UNITS];
	char		 name[UNIT_NAME_LENGTH];
} unit_dimensions_t;

/* parser interface */

int unit_parse (char *s, UnitShift *unit); /* in unit.y */

char *unit_cstring (Unit *unit);

/* static functions */

/* test if two Units have the same dimension */
static inline void
test_same_dimension (char *op, Unit *a, Unit *b)
{
	if (memcmp(a->units, b->units, N_UNITS))
		ereport(ERROR,
				(errcode(ERRCODE_DATA_EXCEPTION),
				 errmsg("dimension mismatch in \"%s\" operation: \"%s\", \"%s\"",
					 op, unit_cstring(a), unit_cstring(b))));
}

static inline void
unit_add_internal (Unit *a, Unit *b, Unit *result)
{
	test_same_dimension("+", a, b);
	result->value = a->value + b->value;
	memcpy(result->units, a->units, N_UNITS);
}

static inline void
unit_sub_internal (Unit *a, Unit *b, Unit *result)
{
	test_same_dimension("-", a, b);
	result->value = a->value - b->value;
	memcpy(result->units, a->units, N_UNITS);
}

static inline void
unit_mult_internal (Unit *a, Unit *b, Unit *result)
{
	int		 i;

	result->value = a->value * b->value;
	for (i = 0; i < N_UNITS; i++)
		result->units[i] = a->units[i] + b->units[i];
}

static inline void
unit_div_internal (Unit *a, Unit *b, Unit *result)
{
	int		 i;

	if (b->value == 0)
		ereport(ERROR,
				(errcode(ERRCODE_DIVISION_BY_ZERO),
				 errmsg("division by zero-valued unit: \"%s\"",
					 unit_cstring(b))));

	result->value = a->value / b->value;
	for (i = 0; i < N_UNITS; i++)
		result->units[i] = a->units[i] - b->units[i];
}

#endif /* _UNIT_H */