from django.db.models import Func, IntegerField, Transform, Value, fields
from django.db.models.functions import Coalesce


class BytesToCharFieldConversionMixin:
    """
    Convert CharField results from bytes to str.

    MySQL returns long data types (bytes) instead of chars when it can't
    determine the length of the result string. For example:
        LPAD(column1, CHAR_LENGTH(column2), ' ')
    returns the LONGTEXT (bytes) instead of VARCHAR.
    """
    def convert_value(self, value, expression, connection):
        if connection.features.db_functions_convert_bytes_to_str:
            if self.output_field.get_internal_type() == 'CharField' and isinstance(value, bytes):
                return value.decode()
        return super().convert_value(value, expression, connection)


class Chr(Transform):
    function = 'CHR'
    lookup_name = 'chr'

    def as_mysql(self, compiler, connection, **extra_context):
        return super().as_sql(
            compiler, connection, function='CHAR',
            template='%(function)s(%(expressions)s USING utf16)',
            **extra_context
        )

    def as_oracle(self, compiler, connection, **extra_context):
        return super().as_sql(
            compiler, connection,
            template='%(function)s(%(expressions)s USING NCHAR_CS)',
            **extra_context
        )

    def as_sqlite(self, compiler, connection, **extra_context):
        return super().as_sql(compiler, connection, function='CHAR', **extra_context)


class ConcatPair(Func):
    """
    Concatenate two arguments together. This is used by `Concat` because not
    all backend databases support more than two arguments.
    """
    function = 'CONCAT'

    def as_sqlite(self, compiler, connection, **extra_context):
        coalesced = self.coalesce()
        return super(ConcatPair, coalesced).as_sql(
            compiler, connection, template='%(expressions)s', arg_joiner=' || ',
            **extra_context
        )

    def as_mysql(self, compiler, connection, **extra_context):
        # Use CONCAT_WS with an empty separator so that NULLs are ignored.
        return super().as_sql(
            compiler, connection, function='CONCAT_WS',
            template="%(function)s('', %(expressions)s)",
            **extra_context
        )

    def coalesce(self):
        # null on either side results in null for expression, wrap with coalesce
        c = self.copy()
        expressions = [
            Coalesce(expression, Value('')) for expression in c.get_source_expressions()
        ]
        c.set_source_expressions(expressions)
        return c


class Concat(Func):
    """
    Concatenate text fields together. Backends that result in an entire
    null expression when any arguments are null will wrap each argument in
    coalesce functions to ensure a non-null result.
    """
    function = None
    template = "%(expressions)s"

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

    def _paired(self, expressions):
        # wrap pairs of expressions in successive concat functions
        # exp = [a, b, c, d]
        # -> ConcatPair(a, ConcatPair(b, ConcatPair(c, d))))
        if len(expressions) == 2:
            return ConcatPair(*expressions)
        return ConcatPair(expressions[0], self._paired(expressions[1:]))


class Left(Func):
    function = 'LEFT'
    arity = 2

    def __init__(self, expression, length, **extra):
        """
        expression: the name of a field, or an expression returning a string
        length: the number of characters to return from the start of the string
        """
        if not hasattr(length, 'resolve_expression'):
            if length < 1:
                raise ValueError("'length' must be greater than 0.")
        super().__init__(expression, length, **extra)

    def get_substr(self):
        return Substr(self.source_expressions[0], Value(1), self.source_expressions[1])

    def use_substr(self, compiler, connection, **extra_context):
        return self.get_substr().as_oracle(compiler, connection, **extra_context)

    as_oracle = use_substr
    as_sqlite = use_substr


class Length(Transform):
    """Return the number of characters in the expression."""
    function = 'LENGTH'
    lookup_name = 'length'
    output_field = fields.IntegerField()

    def as_mysql(self, compiler, connection, **extra_context):
        return super().as_sql(compiler, connection, function='CHAR_LENGTH', **extra_context)


class Lower(Transform):
    function = 'LOWER'
    lookup_name = 'lower'


class LPad(BytesToCharFieldConversionMixin, Func):
    function = 'LPAD'

    def __init__(self, expression, length, fill_text=Value(' '), **extra):
        if not hasattr(length, 'resolve_expression') and length is not None and length < 0:
            raise ValueError("'length' must be greater or equal to 0.")
        super().__init__(expression, length, fill_text, **extra)


class LTrim(Transform):
    function = 'LTRIM'
    lookup_name = 'ltrim'


class Ord(Transform):
    function = 'ASCII'
    lookup_name = 'ord'
    output_field = IntegerField()

    def as_mysql(self, compiler, connection, **extra_context):
        return super().as_sql(compiler, connection, function='ORD', **extra_context)

    def as_sqlite(self, compiler, connection, **extra_context):
        return super().as_sql(compiler, connection, function='UNICODE', **extra_context)


class Repeat(BytesToCharFieldConversionMixin, Func):
    function = 'REPEAT'

    def __init__(self, expression, number, **extra):
        if not hasattr(number, 'resolve_expression') and number is not None and number < 0:
            raise ValueError("'number' must be greater or equal to 0.")
        super().__init__(expression, number, **extra)

    def as_oracle(self, compiler, connection, **extra_context):
        expression, number = self.source_expressions
        length = None if number is None else Length(expression) * number
        rpad = RPad(expression, length, expression)
        return rpad.as_sql(compiler, connection, **extra_context)


class Replace(Func):
    function = 'REPLACE'

    def __init__(self, expression, text, replacement=Value(''), **extra):
        super().__init__(expression, text, replacement, **extra)


class Right(Left):
    function = 'RIGHT'

    def get_substr(self):
        return Substr(self.source_expressions[0], self.source_expressions[1] * Value(-1))


class RPad(LPad):
    function = 'RPAD'


class RTrim(Transform):
    function = 'RTRIM'
    lookup_name = 'rtrim'


class StrIndex(Func):
    """
    Return a positive integer corresponding to the 1-indexed position of the
    first occurrence of a substring inside another string, or 0 if the
    substring is not found.
    """
    function = 'INSTR'
    arity = 2
    output_field = fields.IntegerField()

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


class Substr(Func):
    function = 'SUBSTRING'

    def __init__(self, expression, pos, length=None, **extra):
        """
        expression: the name of a field, or an expression returning a string
        pos: an integer > 0, or an expression returning an integer
        length: an optional number of characters to return
        """
        if not hasattr(pos, 'resolve_expression'):
            if pos < 1:
                raise ValueError("'pos' must be greater than 0")
        expressions = [expression, pos]
        if length is not None:
            expressions.append(length)
        super().__init__(*expressions, **extra)

    def as_sqlite(self, compiler, connection, **extra_context):
        return super().as_sql(compiler, connection, function='SUBSTR', **extra_context)

    def as_oracle(self, compiler, connection, **extra_context):
        return super().as_sql(compiler, connection, function='SUBSTR', **extra_context)


class Trim(Transform):
    function = 'TRIM'
    lookup_name = 'trim'


class Upper(Transform):
    function = 'UPPER'
    lookup_name = 'upper'