from __future__ import annotations

from sqlglot import exp, transforms
from sqlglot.generators.tsql import TSQLGenerator


def _cap_data_type_precision(expression: exp.DataType, max_precision: int = 6) -> exp.DataType:
    """
    Cap the precision of to a maximum of `max_precision` digits.
    If no precision is specified, default to `max_precision`.
    """

    precision_param = expression.find(exp.DataTypeParam)

    if precision_param and precision_param.this.is_int:
        current_precision = precision_param.this.to_py()
        target_precision = min(current_precision, max_precision)
    else:
        target_precision = max_precision

    return exp.DataType(
        this=expression.this,
        expressions=[exp.DataTypeParam(this=exp.Literal.number(target_precision))],
    )


def _add_default_precision_to_varchar(expression: exp.Expr) -> exp.Expr:
    """Transform function to add VARCHAR(MAX) or CHAR(MAX) for cross-dialect conversion."""
    if (
        isinstance(expression, exp.Create)
        and expression.kind == "TABLE"
        and isinstance(expression.this, exp.Schema)
    ):
        for column in expression.this.expressions:
            if isinstance(column, exp.ColumnDef):
                column_type = column.kind
                if (
                    isinstance(column_type, exp.DataType)
                    and column_type.this in (exp.DType.VARCHAR, exp.DType.CHAR)
                    and not column_type.expressions
                ):
                    # For transpilation, VARCHAR/CHAR without precision becomes VARCHAR(MAX)/CHAR(MAX)
                    column_type.set("expressions", [exp.var("MAX")])

    return expression


class FabricGenerator(TSQLGenerator):
    # Fabric-specific type mappings - override T-SQL types that aren't supported
    # Reference: https://learn.microsoft.com/en-us/fabric/data-warehouse/data-types
    TYPE_MAPPING = {
        **TSQLGenerator.TYPE_MAPPING,
        exp.DType.DATETIME: "DATETIME2",
        exp.DType.DECIMAL: "DECIMAL",
        exp.DType.IMAGE: "VARBINARY",
        exp.DType.INT: "INT",
        exp.DType.JSON: "VARCHAR",
        exp.DType.MONEY: "DECIMAL",
        exp.DType.NCHAR: "CHAR",
        exp.DType.NVARCHAR: "VARCHAR",
        exp.DType.ROWVERSION: "ROWVERSION",
        exp.DType.SMALLDATETIME: "DATETIME2",
        exp.DType.SMALLMONEY: "DECIMAL",
        exp.DType.TIMESTAMP: "DATETIME2",
        exp.DType.TIMESTAMPNTZ: "DATETIME2",
        exp.DType.TIMESTAMPTZ: "DATETIME2",
        exp.DType.TINYINT: "SMALLINT",
        exp.DType.UTINYINT: "SMALLINT",
        exp.DType.UUID: "UNIQUEIDENTIFIER",
        exp.DType.XML: "VARCHAR",
    }

    TRANSFORMS = {
        **TSQLGenerator.TRANSFORMS,
        exp.Create: transforms.preprocess([_add_default_precision_to_varchar]),
    }

    def datatype_sql(self, expression: exp.DataType) -> str:
        # Check if this is a temporal type that needs precision handling. Fabric limits temporal
        # types to max 6 digits precision. When no precision is specified, we default to 6 digits.
        if expression.is_type(*exp.DataType.TEMPORAL_TYPES) and expression.this != exp.DType.DATE:
            # Create a new expression with the capped precision
            expression = _cap_data_type_precision(expression)

        return super().datatype_sql(expression)

    def cast_sql(self, expression: exp.Cast, safe_prefix: str | None = None) -> str:
        # Cast to DATETIMEOFFSET if inside an AT TIME ZONE expression
        # https://learn.microsoft.com/en-us/sql/t-sql/data-types/datetimeoffset-transact-sql#microsoft-fabric-support
        if expression.is_type(exp.DType.TIMESTAMPTZ):
            at_time_zone = expression.find_ancestor(exp.AtTimeZone, exp.Select)

            # Return normal cast, if the expression is not in an AT TIME ZONE context
            if not isinstance(at_time_zone, exp.AtTimeZone):
                return super().cast_sql(expression, safe_prefix)

            # Get the precision from the original TIMESTAMPTZ cast and cap it to 6
            capped_data_type = _cap_data_type_precision(expression.to, max_precision=6)
            precision = capped_data_type.find(exp.DataTypeParam)
            precision_value = precision.this.to_py() if precision and precision.this.is_int else 6

            # Do the cast explicitly to bypass sqlglot's default handling
            datetimeoffset = f"CAST({expression.this} AS DATETIMEOFFSET({precision_value}))"

            return self.sql(datetimeoffset)

        return super().cast_sql(expression, safe_prefix)

    def attimezone_sql(self, expression: exp.AtTimeZone) -> str:
        # Wrap the AT TIME ZONE expression in a cast to DATETIME2 if it contains a TIMESTAMPTZ
        ## https://learn.microsoft.com/en-us/sql/t-sql/data-types/datetimeoffset-transact-sql#microsoft-fabric-support
        timestamptz_cast = expression.find(exp.Cast)
        if timestamptz_cast and timestamptz_cast.to.is_type(exp.DType.TIMESTAMPTZ):
            # Get the precision from the original TIMESTAMPTZ cast and cap it to 6
            data_type = timestamptz_cast.to
            capped_data_type = _cap_data_type_precision(data_type, max_precision=6)
            precision_param = capped_data_type.find(exp.DataTypeParam)
            precision = precision_param.this.to_py() if precision_param else 6

            # Generate the AT TIME ZONE expression (which will handle the inner cast conversion)
            at_time_zone_sql = super().attimezone_sql(expression)

            # Wrap it in an outer cast to DATETIME2
            return f"CAST({at_time_zone_sql} AS DATETIME2({precision}))"

        return super().attimezone_sql(expression)

    def unixtotime_sql(self, expression: exp.UnixToTime) -> str:
        scale = expression.args.get("scale")
        timestamp = expression.this

        if scale not in (None, exp.UnixToTime.SECONDS):
            self.unsupported(f"UnixToTime scale {scale} is not supported by Fabric")
            return ""

        # Convert unix timestamp (seconds) to microseconds and round to avoid decimals
        microseconds = timestamp * exp.Literal.number("1e6")
        rounded = exp.func("round", microseconds, 0)
        rounded_ms_as_bigint = exp.cast(rounded, exp.DType.BIGINT)

        # Create the base datetime as '1970-01-01' cast to DATETIME2(6)
        epoch_start = exp.cast("'1970-01-01'", "datetime2(6)", dialect="fabric")

        dateadd = exp.DateAdd(
            this=epoch_start,
            expression=rounded_ms_as_bigint,
            unit=exp.Literal.string("MICROSECONDS"),
        )
        return self.sql(dateadd)
