"""Type codecs: IDS wire bytes ↔ Python values.

Phase 2 implements decoders for the MVP type set (SMALLINT, INT, BIGINT,
SMFLOAT, FLOAT, CHAR, VARCHAR, BOOL, DATE). Encoders are stubbed —
they land in Phase 4 with parameter binding.

Decoder dispatch: ``decode(type_code, raw_bytes) → python value`` looks
up the codec in ``DECODERS`` keyed by ``IfxType`` (after stripping
high-bit flags via ``_types.base_type``). NULL values are signaled by
the row decoder, not by sentinel bytes here.

For DATE we use the Informix epoch (1899-12-31). The raw bytes are a
4-byte big-endian signed int representing day count.
"""

from __future__ import annotations

import datetime
import decimal
import struct
from collections.abc import Callable

from ._types import IfxType, base_type

# Informix DATE epoch — day 0 is December 31, 1899 (per Informix convention).
_INFORMIX_DATE_EPOCH = datetime.date(1899, 12, 31)

DecoderFn = Callable[[bytes], object]


# Informix uses sentinel values for NULL per type — see DECISION_LOG.md
# entry on null sentinel discovery (2026-05-04).
_INT_NULL = 0x80000000  # INT_MIN
_SMALLINT_NULL = 0x8000  # SHORT_MIN
_BIGINT_NULL = 0x8000000000000000  # LONG_MIN
_REAL_NULL = b"\xff\xff\xff\xff"
_DOUBLE_NULL = b"\xff\xff\xff\xff\xff\xff\xff\xff"
_DATE_NULL = 0x80000000


def _decode_smallint(raw: bytes) -> int | None:
    val = struct.unpack("!h", raw)[0]
    return None if val == -0x8000 else val


def _decode_int(raw: bytes) -> int | None:
    val = struct.unpack("!i", raw)[0]
    return None if val == -0x80000000 else val


def _decode_bigint(raw: bytes) -> int | None:
    val = struct.unpack("!q", raw)[0]
    return None if val == -0x8000000000000000 else val


def _decode_smfloat(raw: bytes) -> float | None:
    if raw == _REAL_NULL:
        return None
    return struct.unpack("!f", raw)[0]


def _decode_float(raw: bytes) -> float | None:
    if raw == _DOUBLE_NULL:
        return None
    return struct.unpack("!d", raw)[0]


def _decode_char(raw: bytes) -> str:
    """Strip trailing spaces (CHAR is space-padded to declared length)."""
    return raw.rstrip(b" \x00").decode("iso-8859-1")


def _decode_varchar(raw: bytes) -> str | None:
    """VARCHAR — variable-length string. NULL is the special sentinel ``\\x00``
    (single nul byte). The row decoder peels off the length prefix and passes
    the content here. Note: VARCHAR cannot contain embedded nuls anyway, so
    a single-nul value is unambiguously the NULL marker.
    """
    if raw == b"\x00":
        return None
    return raw.rstrip(b"\x00").decode("iso-8859-1")


def _decode_bool(raw: bytes) -> bool:
    """Informix BOOLEAN is one byte: 't'/'T' (true), 'f'/'F' (false)."""
    if not raw:
        raise ValueError("empty BOOL payload")
    return raw[0] in (ord("t"), ord("T"), 1)


def _decode_date(raw: bytes) -> datetime.date | None:
    """4-byte big-endian signed int = day count from 1899-12-31. NULL = 0x80000000."""
    days = struct.unpack("!i", raw)[0]
    if days == -0x80000000:
        return None
    return _INFORMIX_DATE_EPOCH + datetime.timedelta(days=days)


def _decode_decimal(raw: bytes) -> decimal.Decimal | None:
    """Decode IDS DECIMAL/MONEY: base-100 packed BCD with sign/exponent header.

    Wire format (per ``com.informix.lang.Decimal.init``, line 374):
      byte[0]: ``(sign << 7) | (biased_exponent & 0x7F)``
        - sign bit (bit 7): 1 = positive, 0 = negative
        - biased_exponent (bits 0-6): actual exponent = biased - 64,
          measured in BASE-100 digits before the decimal point
      byte[1..]: digit-pair bytes; each byte holds two decimal digits
        as a single base-100 number (0..99). If the value is NEGATIVE,
        each digit-pair is stored as 99-d (i.e., 9's complement in base 100).

    NULL marker: byte[0] == 0 AND byte[1] == 0.
    """
    if len(raw) < 2 or (raw[0] == 0 and raw[1] == 0):
        return None

    expbyte = raw[0]
    is_positive = (expbyte & 0x80) != 0
    # For negative: exponent byte is XOR'd with 0x7F to recover real
    # exponent (per IfxToJavaDecimal.init line 386).
    biased_exp = (expbyte & 0x7F) if is_positive else ((expbyte ^ 0x7F) & 0x7F)
    exponent_base100 = biased_exp - 64  # in base-100 digits

    digits = list(raw[1:])
    if not is_positive:
        # Asymmetric base-100 complement (per Decimal.decComplement, line 447):
        # walk from RIGHT to LEFT; trailing zeros stay zero; the first
        # non-zero is subtracted from 100; subsequent from 99.
        # Without this, trailing 99s appear in the decoded value (a
        # 1234.559999 / 0.4999... rounding-style artifact).
        sub_from = 100
        for i in range(len(digits) - 1, -1, -1):
            if digits[i] == 0 and sub_from == 100:
                continue
            digits[i] = sub_from - digits[i]
            sub_from = 99

    # Build the decimal-string representation.
    # exponent_base100 is the count of BASE-100 digits before the decimal
    # point; multiplying by 2 gives BASE-10 digits before the decimal.
    base10_exp = exponent_base100 * 2

    # Concatenate all digit-pairs as a string, dropping trailing zeros
    # for normalization.
    digit_str = "".join(f"{d:02d}" for d in digits)
    if not digit_str:
        return decimal.Decimal(0)

    sign_str = "" if is_positive else "-"
    # Build "<sign><digits>E<exp>" — Decimal will normalize.
    # Each digit-pair represents 2 base-10 digits; the value is
    # digit_str interpreted as an integer * 10^(base10_exp - len(digit_str))
    if base10_exp >= 0:
        # The decimal point is to the RIGHT of digit_str's start by
        # base10_exp positions.
        if base10_exp >= len(digit_str):
            # All digits are integer; pad with zeros to reach the exp.
            int_part = digit_str + "0" * (base10_exp - len(digit_str))
            return decimal.Decimal(f"{sign_str}{int_part}")
        else:
            int_part = digit_str[:base10_exp] or "0"
            frac_part = digit_str[base10_exp:].rstrip("0")
            if frac_part:
                return decimal.Decimal(f"{sign_str}{int_part}.{frac_part}")
            return decimal.Decimal(f"{sign_str}{int_part}")
    else:
        # base10_exp < 0: leading zeros in the fraction
        frac_zeros = "0" * (-base10_exp)
        frac_part = (frac_zeros + digit_str).rstrip("0")
        if frac_part:
            return decimal.Decimal(f"{sign_str}0.{frac_part}")
        return decimal.Decimal(0)


# Wire byte length per Phase-2-MVP type. Used by the row decoder to
# slice column values out of an SQ_TUPLE payload for fixed-width types.
# Variable-width types (CHAR, VARCHAR, DECIMAL, etc.) are length-prefixed
# on the wire and don't appear in this table.
FIXED_WIDTHS: dict[int, int] = {
    IfxType.SMALLINT: 2,
    IfxType.INT: 4,
    IfxType.SERIAL: 4,
    IfxType.SMFLOAT: 4,
    IfxType.FLOAT: 8,
    IfxType.BIGINT: 8,
    IfxType.BIGSERIAL: 8,
    IfxType.DATE: 4,
    IfxType.BOOL: 1,
}


# Phase 2 MVP decoders. Phase 6+ adds DATETIME, INTERVAL, DECIMAL,
# MONEY, LVARCHAR, BYTE/TEXT, BLOB/CLOB, ROW, COLLECTION.
DECODERS: dict[int, DecoderFn] = {
    IfxType.SMALLINT: _decode_smallint,
    IfxType.INT: _decode_int,
    IfxType.SERIAL: _decode_int,
    IfxType.BIGINT: _decode_bigint,
    IfxType.BIGSERIAL: _decode_bigint,
    IfxType.SMFLOAT: _decode_smfloat,
    IfxType.FLOAT: _decode_float,
    IfxType.CHAR: _decode_char,
    IfxType.VARCHAR: _decode_varchar,
    IfxType.NCHAR: _decode_char,
    IfxType.NVCHAR: _decode_varchar,
    IfxType.LVARCHAR: _decode_varchar,
    IfxType.BOOL: _decode_bool,
    IfxType.DATE: _decode_date,
    IfxType.DECIMAL: _decode_decimal,
    IfxType.MONEY: _decode_decimal,  # MONEY is DECIMAL with implied scale
}


def decode(type_code: int, raw: bytes) -> object:
    """Decode ``raw`` bytes for the given IDS type code into a Python value.

    The high-bit flags (NOTNULLABLE etc.) are stripped before lookup.
    Raises ``KeyError`` for unsupported types — Phase 6+ adds the rest.
    """
    base = base_type(type_code)
    decoder = DECODERS.get(base)
    if decoder is None:
        raise NotImplementedError(
            f"decoder for IDS type code {base} not yet implemented "
            f"(Phase 2 MVP supports: SMALLINT, INT, BIGINT, REAL, FLOAT, "
            f"CHAR, VARCHAR, BOOL, DATE)"
        )
    return decoder(raw)


# ---------------------------------------------------------------------------
# Encoders for parameter binding (Phase 4)
# ---------------------------------------------------------------------------
# Returns ``(type_code, prec_short, raw_bytes)`` per parameter.
# Per-param SQ_BIND format: ``[short type][short ind=0][short prec][data]``
# where data is ``writePadded(raw_bytes)`` (emit + pad-to-even).
#
# JDBC's IfxSqli.sendBind (line 844+) does precision encoding per type:
#   INT/SERIAL: prec = 0x0a00 (packed width=10, scale=0)
#   VARCHAR sent as CHAR (type=0): prec = 0
#   FLOAT (DOUBLE PRECISION): prec = 0
#
# Strings get type=0 (CHAR) on the wire — Informix's server casts them
# to the declared column type via the CIDESCRIBE/IDESCRIBE handshake.

EncodedParam = tuple[int, int, bytes]


def _encode_int(value: int) -> EncodedParam:
    """Encode a Python int as Informix INTEGER (type=2, 4 bytes BE)."""
    return (2, 0x0A00, value.to_bytes(4, "big", signed=True))


def _encode_bigint(value: int) -> EncodedParam:
    """Encode a Python int as Informix BIGINT (type=52, 8 bytes BE)."""
    return (52, 0x1300, value.to_bytes(8, "big", signed=True))


def _encode_str(value: str) -> EncodedParam:
    """Encode a Python str as Informix CHAR (type=0, length-prefixed).

    JDBC sends Java strings as CHAR (type=0) on the wire — the server
    handles conversion to the actual column type (CHAR/VARCHAR/NVARCHAR).
    Format: ``[short length][bytes]`` (writePadded adds even-byte pad).
    """
    encoded = value.encode("iso-8859-1")
    raw = len(encoded).to_bytes(2, "big") + encoded
    return (0, 0, raw)


def _encode_float(value: float) -> EncodedParam:
    """Encode a Python float as Informix FLOAT (type=3, 8-byte IEEE 754)."""
    return (3, 0, struct.pack("!d", value))


def _encode_bool(value: bool) -> EncodedParam:
    """Encode a Python bool as Informix BOOLEAN (type=45, 1 byte)."""
    return (45, 0, b"\x01" if value else b"\x00")


def _encode_decimal(value: decimal.Decimal) -> EncodedParam:
    """Encode a Python ``decimal.Decimal`` as IDS DECIMAL (type=5).

    Inverse of ``_decode_decimal``: produce a base-100 BCD encoding with
    the ``[sign+exponent][digit-pairs]`` header byte. Mirrors
    ``Decimal.javaToIfx`` (line 457).
    """
    sign, digits, exp = value.as_tuple()
    # Total decimal digits in mantissa
    n_digits = len(digits)
    # Compute base-10 exponent of the most significant digit
    # (the "exp" returned by as_tuple is the position of the LSD;
    # we want the position of the MSD relative to the decimal point.)
    base10_exp = n_digits + exp  # number of digits BEFORE the decimal

    # Pad digits to even length on both sides so we can pack into base-100.
    # Compute how many leading-zero-pairs to add (to align base100_exp on
    # a base-100 boundary).
    if base10_exp % 2 != 0:
        # If odd, add a leading 0 to align — base10_exp becomes even.
        digits = (0, *digits)
        base10_exp += 1
        n_digits += 1
    if n_digits % 2 != 0:
        # Pad trailing zero to make digit count even (so we can pair).
        digits = (*digits, 0)
        n_digits += 1

    base100_exp = base10_exp // 2  # exponent in base-100 digits

    # Pack pairs of decimal digits into bytes.
    digit_pairs = bytes(
        digits[i] * 10 + digits[i + 1] for i in range(0, n_digits, 2)
    )

    is_positive = sign == 0
    biased_exp = base100_exp + 64
    if is_positive:
        head_byte = (biased_exp & 0x7F) | 0x80
        out_digits = digit_pairs
    else:
        # Apply asymmetric base-100 complement (mirror of decode).
        complemented = bytearray(digit_pairs)
        sub_from = 100
        for i in range(len(complemented) - 1, -1, -1):
            if complemented[i] == 0 and sub_from == 100:
                continue
            complemented[i] = sub_from - complemented[i]
            sub_from = 99
        # Negative: head byte is biased_exp ^ 0x7F (high bit stays 0)
        head_byte = (biased_exp & 0x7F) ^ 0x7F
        out_digits = bytes(complemented)

    raw = bytes([head_byte]) + out_digits
    # Precision short for DECIMAL: packed (precision << 8) | scale
    # Precision = total significant digits, scale = digits after point.
    precision = max(n_digits, 1)
    scale = max(0, -exp)
    prec_short = (precision << 8) | (scale & 0xFF)
    return (5, prec_short, raw)


def encode_param(value: object) -> EncodedParam:
    """Pick an encoder based on the Python value's type.

    Returns ``(ifx_type, precision_short, raw_bytes)`` for the parameter.
    Returns ``(0, 0, b"")`` and the caller must use indicator=-1 for None.
    """
    if value is None:
        return (0, 0, b"")
    if isinstance(value, bool):  # NB: must come before int (bool is int subclass)
        return _encode_bool(value)
    if isinstance(value, int):
        # Pick INT vs BIGINT based on range.
        if -0x80000000 <= value <= 0x7FFFFFFF:
            return _encode_int(value)
        return _encode_bigint(value)
    if isinstance(value, float):
        return _encode_float(value)
    if isinstance(value, str):
        return _encode_str(value)
    if isinstance(value, decimal.Decimal):
        # _encode_decimal is implemented but the server rejects the
        # bytes (precision packing wrong somewhere) — kept as a
        # Phase 6.x starting point but disabled for now. Workaround:
        # cast Decimal to float at the call site if you need to bind.
        raise NotImplementedError(
            "Decimal parameter binding is Phase 6.x; convert to float "
            "or pass DECIMAL via SQL literal for now"
        )
    raise NotImplementedError(
        f"parameter binding for {type(value).__name__} not yet supported "
        f"(Phase 4 MVP: int, float, str, bool, None)"
    )


# Phase 6+ adds: bytes/Bytes, datetime.date, datetime.datetime, Decimal,
# datetime.timedelta (INTERVAL), bytearray (BYTE), large strings (LVARCHAR).
ENCODERS: dict[int, Callable[[object], bytes]] = {}