"""Database functions that do comparisons or type conversions."""
from django.db import NotSupportedError
from django.db.models.expressions import Func, Value
from django.db.models.fields.json import JSONField
from django.utils.regex_helper import _lazy_re_compile


class Cast(Func):
    """Coerce an expression to a new field type."""
    function = 'CAST'
    template = '%(function)s(%(expressions)s AS %(db_type)s)'

    def __init__(self, expression, output_field):
        super().__init__(expression, output_field=output_field)

    def as_sql(self, compiler, connection, **extra_context):
        extra_context['db_type'] = self.output_field.cast_db_type(connection)
        return super().as_sql(compiler, connection, **extra_context)

    def as_sqlite(self, compiler, connection, **extra_context):
        db_type = self.output_field.db_type(connection)
        if db_type in {'datetime', 'time'}:
            # Use strftime as datetime/time don't keep fractional seconds.
            template = 'strftime(%%s, %(expressions)s)'
            sql, params = super().as_sql(compiler, connection, template=template, **extra_context)
            format_string = '%H:%M:%f' if db_type == 'time' else '%Y-%m-%d %H:%M:%f'
            params.insert(0, format_string)
            return sql, params
        elif db_type == 'date':
            template = 'date(%(expressions)s)'
            return super().as_sql(compiler, connection, template=template, **extra_context)
        return self.as_sql(compiler, connection, **extra_context)

    def as_mysql(self, compiler, connection, **extra_context):
        template = None
        output_type = self.output_field.get_internal_type()
        # MySQL doesn't support explicit cast to float.
        if output_type == 'FloatField':
            template = '(%(expressions)s + 0.0)'
        # MariaDB doesn't support explicit cast to JSON.
        elif output_type == 'JSONField' and connection.mysql_is_mariadb:
            template = "JSON_EXTRACT(%(expressions)s, '$')"
        return self.as_sql(compiler, connection, template=template, **extra_context)

    def as_postgresql(self, compiler, connection, **extra_context):
        # CAST would be valid too, but the :: shortcut syntax is more readable.
        # 'expressions' is wrapped in parentheses in case it's a complex
        # expression.
        return self.as_sql(compiler, connection, template='(%(expressions)s)::%(db_type)s', **extra_context)

    def as_oracle(self, compiler, connection, **extra_context):
        if self.output_field.get_internal_type() == 'JSONField':
            # Oracle doesn't support explicit cast to JSON.
            template = "JSON_QUERY(%(expressions)s, '$')"
            return super().as_sql(compiler, connection, template=template, **extra_context)
        return self.as_sql(compiler, connection, **extra_context)


class Coalesce(Func):
    """Return, from left to right, the first non-null expression."""
    function = 'COALESCE'

    def __init__(self, *expressions, **extra):
        if len(expressions) < 2:
            raise ValueError('Coalesce must take at least two expressions')
        super().__init__(*expressions, **extra)

    @property
    def empty_result_set_value(self):
        for expression in self.get_source_expressions():
            result = expression.empty_result_set_value
            if result is NotImplemented or result is not None:
                return result
        return None

    def as_oracle(self, compiler, connection, **extra_context):
        # Oracle prohibits mixing TextField (NCLOB) and CharField (NVARCHAR2),
        # so convert all fields to NCLOB when that type is expected.
        if self.output_field.get_internal_type() == 'TextField':
            clone = self.copy()
            clone.set_source_expressions([
                Func(expression, function='TO_NCLOB') for expression in self.get_source_expressions()
            ])
            return super(Coalesce, clone).as_sql(compiler, connection, **extra_context)
        return self.as_sql(compiler, connection, **extra_context)


class Collate(Func):
    function = 'COLLATE'
    template = '%(expressions)s %(function)s %(collation)s'
    # Inspired from https://www.postgresql.org/docs/current/sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS
    collation_re = _lazy_re_compile(r'^[\w\-]+$')

    def __init__(self, expression, collation):
        if not (collation and self.collation_re.match(collation)):
            raise ValueError('Invalid collation name: %r.' % collation)
        self.collation = collation
        super().__init__(expression)

    def as_sql(self, compiler, connection, **extra_context):
        extra_context.setdefault('collation', connection.ops.quote_name(self.collation))
        return super().as_sql(compiler, connection, **extra_context)


class Greatest(Func):
    """
    Return the maximum expression.

    If any expression is null the return value is database-specific:
    On PostgreSQL, the maximum not-null expression is returned.
    On MySQL, Oracle, and SQLite, if any expression is null, null is returned.
    """
    function = 'GREATEST'

    def __init__(self, *expressions, **extra):
        if len(expressions) < 2:
            raise ValueError('Greatest must take at least two expressions')
        super().__init__(*expressions, **extra)

    def as_sqlite(self, compiler, connection, **extra_context):
        """Use the MAX function on SQLite."""
        return super().as_sqlite(compiler, connection, function='MAX', **extra_context)


class JSONObject(Func):
    function = 'JSON_OBJECT'
    output_field = JSONField()

    def __init__(self, **fields):
        expressions = []
        for key, value in fields.items():
            expressions.extend((Value(key), value))
        super().__init__(*expressions)

    def as_sql(self, compiler, connection, **extra_context):
        if not connection.features.has_json_object_function:
            raise NotSupportedError(
                'JSONObject() is not supported on this database backend.'
            )
        return super().as_sql(compiler, connection, **extra_context)

    def as_postgresql(self, compiler, connection, **extra_context):
        return self.as_sql(
            compiler,
            connection,
            function='JSONB_BUILD_OBJECT',
            **extra_context,
        )

    def as_oracle(self, compiler, connection, **extra_context):
        class ArgJoiner:
            def join(self, args):
                args = [' VALUE '.join(arg) for arg in zip(args[::2], args[1::2])]
                return ', '.join(args)

        return self.as_sql(
            compiler,
            connection,
            arg_joiner=ArgJoiner(),
            template='%(function)s(%(expressions)s RETURNING CLOB)',
            **extra_context,
        )


class Least(Func):
    """
    Return the minimum expression.

    If any expression is null the return value is database-specific:
    On PostgreSQL, return the minimum not-null expression.
    On MySQL, Oracle, and SQLite, if any expression is null, return null.
    """
    function = 'LEAST'

    def __init__(self, *expressions, **extra):
        if len(expressions) < 2:
            raise ValueError('Least must take at least two expressions')
        super().__init__(*expressions, **extra)

    def as_sqlite(self, compiler, connection, **extra_context):
        """Use the MIN function on SQLite."""
        return super().as_sqlite(compiler, connection, function='MIN', **extra_context)


class NullIf(Func):
    function = 'NULLIF'
    arity = 2

    def as_oracle(self, compiler, connection, **extra_context):
        expression1 = self.get_source_expressions()[0]
        if isinstance(expression1, Value) and expression1.value is None:
            raise ValueError('Oracle does not allow Value(None) for expression1.')
        return super().as_sql(compiler, connection, **extra_context)
