"""SHA256Generator — SHA-256 hash computation.

Ported from net.i2p.crypto.SHA256Generator.
Wraps Python's hashlib.sha256.
"""

import hashlib

from i2p_crypto.hash_data import Hash


class SHA256Generator:
    """SHA-256 hash generator. Thread-safe (hashlib creates new state per call)."""

    _instance: "SHA256Generator | None" = None

    def __init__(self) -> None:
        pass

    @classmethod
    def get_instance(cls) -> "SHA256Generator":
        if cls._instance is None:
            cls._instance = SHA256Generator()
        return cls._instance

    def calculate_hash(self, source: bytes, start: int = 0, length: int = -1) -> Hash:
        """Calculate SHA-256 and return a Hash object."""
        if length < 0:
            length = len(source) - start
        digest = hashlib.sha256(source[start : start + length]).digest()
        return Hash(digest)

    def calculate_hash_into(self, source: bytes, start: int, length: int,
                            out: bytearray, out_offset: int) -> None:
        """Calculate SHA-256 into an existing buffer."""
        digest = hashlib.sha256(source[start : start + length]).digest()
        out[out_offset : out_offset + Hash.HASH_LENGTH] = digest

    @staticmethod
    def digest(data: bytes) -> bytes:
        """Convenience: return raw 32-byte SHA-256 digest."""
        return hashlib.sha256(data).digest()