"""
Helpers for building SQL expressions.

Some of this code is concerned with SQL factories.  These are functions
with the signature::

	func(field, val, outPars) -> fragment

outPars is a dictionary that is used to transmit literal values into SQL.
The result must be an SQL boolean expression for embedding into a WHERE clause
(use None to signal no constraint).  Field is the field for which the
expression is being generated.

The factories currently are never called when val is a sequence; there's
special hard-coded behaviour for that in getSQLFactory.

To enter values in outPars, use getSQLKey.  Its docstring contains
an example that shows how that would look like.
"""

#c Copyright 2008-2021, the GAVO project <gavo@ari.uni-heidelberg.de>
#c
#c This program is free software, covered by the GNU GPL.  See the
#c COPYING file in the source distribution.


from gavo.stc import mjdToDateTime


plusInfinity = float("Inf")
minusInfinity = float("-Inf")


def joinOperatorExpr(operator, operands):
	"""filters empty operands and joins the rest using operator.

	The function returns an expression string or None for the empty expression.
	"""
	operands = [_f for _f in operands if _f]
	if not operands:
		return None
	elif len(operands)==1:
		return operands[0]
	else:
		return operator.join([" (%s) "%op for op in operands]).strip()


def getSQLKey(key, value, sqlPars):
	"""adds value to sqlPars and returns a key for inclusion in a SQL query.

	This function is used to build parameter dictionaries for SQL queries, 
	avoiding overwriting parameters with accidental name clashes.

	As an extra service, if value is a list, it is turned into a set
	(rather than the default, which would be an array).  We don't believe
	there's a great need to match against arrays.  If you must match against
	arrays, use numpy arrays.

	>>> sqlPars = {}
	>>> getSQLKey("foo", 13, sqlPars)
	'foo0'
	>>> getSQLKey("foo", 14, sqlPars)
	'foo1'
	>>> getSQLKey("foo", 13, sqlPars)
	'foo0'
	>>> sqlPars["foo0"], sqlPars["foo1"]; sqlPars = {}
	(13, 14)
	>>> "WHERE foo<%%(%s)s OR foo>%%(%s)s"%(getSQLKey("foo", 1, sqlPars),
	...   getSQLKey("foo", 15, sqlPars))
	'WHERE foo<%(foo0)s OR foo>%(foo1)s'
	"""
	if isinstance(value, list):
		value = frozenset(value)

	ct = 0
	while True:
		dataKey = "%s%d"%(key, ct)
		if dataKey not in sqlPars or sqlPars[dataKey]==value:
			break
		ct += 1
	sqlPars[dataKey] = value
	return dataKey


_REGISTRED_SQL_FACTORIES = {}
def registerSQLFactory(type, factory):
	"""registers factory as an SQL factory for the type type (a string).
	"""
	_REGISTRED_SQL_FACTORIES[type] = factory


def _getSQLForSequence(field, val, sqlPars):
	if len(val)==0 or (len(val)==1 and val[0] is None):
		return ""
	return "%s IN %%(%s)s"%(field.name, getSQLKey(field.name,
		set(val), sqlPars))


def _convertIfFinite(val, converter):
	if minusInfinity<val<plusInfinity:
		return converter(val)
	return val


def _getSQLForInterval(field, val, sqlPars):
	"""returns SQL for DALI intervals.

	This presumes that val is a 2-array of numbers and will return
	an empty condition otherwise.
	"""
	if len(val)!=2:
		return ""

	if field.hasProperty("database-column-is-date"):
		val = [_convertIfFinite(v, mjdToDateTime) for v in val]

	if val[1]==plusInfinity:
		return "%s > %%(%s)s"%(field.name, getSQLKey(field.name,
		val[0], sqlPars))

	elif val[0]==minusInfinity:
		return "%s < %%(%s)s"%(field.name, getSQLKey(field.name,
		val[1], sqlPars))

	else:
		return "%s BETWEEN %%(%s)s AND %%(%s)s"%(field.name, 
			getSQLKey(field.name, val[0], sqlPars),
			getSQLKey(field.name, val[1], sqlPars))


def _getSQLForSimple(field, val, sqlPars):
	return "%s=%%(%s)s"%(field.name, getSQLKey(field.name,
		val, sqlPars))


def _getSQLFactory(field, value):
	"""returns an SQL factory for matching field's values against value.
	"""
	if field.xtype=="interval":
		return _getSQLForInterval

	elif isinstance(value, (list, tuple)):
		return _getSQLForSequence

	elif field.type in _REGISTRED_SQL_FACTORIES:
		return _REGISTRED_SQL_FACTORIES[field.type]

	else:
		return _getSQLForSimple


def getSQLForField(field, inPars, sqlPars):
	"""returns an SQL fragment for a column-like thing.

	This will be empty if no input in inPars is present.  If it is, (a) new
	key(s) will be left in sqlPars.

	getSQLForField defines the default behaviour; in DBCore condDescs,
	it can be overridden using phrase makers.

	inPars is supposed to be "typed"; we do not catch general parse errors
	here.
	"""
	val = inPars.get(field.name)
	if val is None:
		return None
	if isinstance(val, (list, set, tuple)) and len(val)==1:
		val = val[0]

	factory = _getSQLFactory(field, val)
	return factory(field, val, sqlPars)


def _test():
	import doctest
	doctest.testmod()


if __name__=="__main__":
	_test()
