"""Memory-mapped frontmatter processor for markdown files.

Provides efficient in-place processing of YAML frontmatter blocks in markdown
files using memory-mapped I/O. Files that already pass linting are exited
immediately without any write operations. Files that require fixes are rewritten
atomically via a temporary file and ``pathlib.Path.replace``, making the processor safe
to run across large collections of files.

Also provides a fast in-memory frontmatter parser that avoids the python-frontmatter
dependency overhead while maintaining API compatibility.
"""

from __future__ import annotations

import mmap
import pathlib
from dataclasses import dataclass
from io import StringIO
from typing import TypeAlias

from ruamel.yaml import YAML

DELIMITER = b"---\n"

# Recursive type for YAML-serializable frontmatter values.
FrontmatterValue: TypeAlias = dict[str, "FrontmatterValue"] | list["FrontmatterValue"] | str | int | float | bool | None

# Shared ruamel.yaml instance (round-trip mode for formatting preservation)
_yaml = YAML(typ="rt")
_yaml.preserve_quotes = False
_yaml.width = 2147483647


@dataclass
class Post:
    """Simple frontmatter post object compatible with python-frontmatter API.

    Attributes:
        metadata: Parsed YAML frontmatter as a dict.
        content: Body content after the frontmatter block.
    """

    metadata: dict[str, FrontmatterValue]
    content: str


def loads_frontmatter(text: str) -> Post:
    """Parse frontmatter from a string.

    Fast in-memory parser that extracts YAML frontmatter and body content
    without the python-frontmatter dependency overhead.

    Args:
        text: Markdown string potentially containing YAML frontmatter.

    Returns:
        A Post object with metadata and content attributes.
    """
    # Fast path: check for opening delimiter
    if not text.startswith("---"):
        return Post(metadata={}, content=text)

    # Find closing delimiter (must be on its own line)
    end_match = text.find("\n---", 3)
    if end_match == -1:
        # Malformed: no closing delimiter
        return Post(metadata={}, content=text)

    # Extract frontmatter (between delimiters)
    frontmatter_text = text[3:end_match]

    # Find where body starts (after closing delimiter line)
    body_start = end_match + 4  # Skip past "\n---"
    # Skip any newlines immediately after the closing delimiter
    while body_start < len(text) and text[body_start] == "\n":
        body_start += 1
    content = text[body_start:] if body_start < len(text) else ""
    # Strip trailing newline for consistency with python-frontmatter
    content = content.removesuffix("\n")

    # Parse YAML
    metadata: dict[str, FrontmatterValue] = {}
    if frontmatter_text.strip():
        try:
            parsed = _yaml.load(frontmatter_text)
            if isinstance(parsed, dict):
                metadata = parsed
        except (OSError, ValueError, KeyError, TypeError):
            # On parse error, return empty metadata
            pass

    return Post(metadata=metadata, content=content)


def load_frontmatter(path: str | pathlib.Path) -> Post:
    """Load frontmatter from a file path.

    Args:
        path: Path to a markdown file with YAML frontmatter.

    Returns:
        A Post object with metadata and content attributes.
    """
    text = pathlib.Path(path).read_text(encoding="utf-8")
    return loads_frontmatter(text)


def dump_frontmatter(post: Post) -> str:
    """Serialize a Post object to a string.

    Args:
        post: A Post object with metadata and content.

    Returns:
        Full markdown string with YAML frontmatter delimiters and body.
    """
    buf = StringIO()
    _yaml.dump(post.metadata, buf)
    yaml_str = buf.getvalue().strip()
    return f"---\n{yaml_str}\n---\n{post.content}"


def dumps_frontmatter(post: Post, path: str | pathlib.Path) -> None:
    """Write a Post object to a file.

    Args:
        post: A Post object with metadata and content.
        path: Destination file path.
    """
    content = dump_frontmatter(post)
    pathlib.Path(path).write_text(content, encoding="utf-8")


def update_field(path: str | pathlib.Path, key: str, value: FrontmatterValue) -> None:
    """Load a file, update one key, and write back.

    Args:
        path: Path to the markdown file.
        key: Frontmatter field name to set.
        value: New value for the field.
    """
    post = load_frontmatter(path)
    post.metadata[key] = value
    dumps_frontmatter(post, path)


def process_markdown_file(file_path: str) -> None:
    """Process a single markdown file's frontmatter in-place.

    Reads the file using a memory-mapped buffer to locate the YAML frontmatter
    block bounded by ``---`` delimiters. If the frontmatter passes linting
    (``lint_and_fix`` returns ``False``), the function returns immediately with
    no I/O side effects. When fixes are required, the corrected file is written
    to a sibling ``.tmp`` file and atomically renamed over the original.

    Args:
        file_path: Absolute or relative path to the markdown file to process.
    """
    path = pathlib.Path(file_path)

    with path.open("rb") as f:
        try:
            mm = mmap.mmap(f.fileno(), 0, access=mmap.ACCESS_READ)
        except ValueError:
            # mmap raises ValueError on zero-length files — skip silently
            return
        with mm:
            # 1. Find the frontmatter boundaries
            if mm[: len(DELIMITER)] != DELIMITER:
                return  # No frontmatter found

            end_pos = mm.find(DELIMITER, len(DELIMITER))
            if end_pos == -1:
                return  # Malformed: No closing delimiter

            frontmatter_end_index = end_pos + len(DELIMITER)

            # 2. Extract and parse frontmatter
            raw_yaml = mm[len(DELIMITER) : end_pos]

            # (Insert your linting/YAML parsing logic here)
            needs_fix, new_yaml_bytes = lint_and_fix(raw_yaml)

            if not needs_fix:
                return  # Fast exit! Lint passed, do nothing.

            # 3. The "Zero-Copy Body" Write
            temp_path = path.with_suffix(path.suffix + ".tmp")
            try:
                with temp_path.open("wb") as temp_file:
                    # Write the new frontmatter
                    temp_file.write(DELIMITER)
                    temp_file.write(new_yaml_bytes)
                    if not new_yaml_bytes.endswith(b"\n"):
                        temp_file.write(b"\n")
                    temp_file.write(DELIMITER)

                    # Dump the rest of the file directly from the memory map
                    # Python handles this as a highly optimized block transfer
                    temp_file.write(mm[frontmatter_end_index:])
                # 4. Atomic Replace (Safe across thousands of files)
                temp_path.replace(path)
            except BaseException:
                temp_path.unlink(missing_ok=True)
                raise


def lint_and_fix(raw_yaml: bytes) -> tuple[bool, bytes]:
    """Lint raw YAML frontmatter bytes and return a corrected version if needed.

    Args:
        raw_yaml: Raw bytes of the YAML frontmatter block, without delimiters.

    Returns:
        A two-element tuple ``(needs_fix, fixed_bytes)`` where ``needs_fix`` is
        ``True`` when the frontmatter requires correction and ``fixed_bytes``
        contains the replacement bytes, or ``(False, raw_yaml)`` when the
        frontmatter is already valid.

    Raises:
        NotImplementedError: This function is a scaffold pending integration
            with the skilllint validation and fix pipeline.
    """
    raise NotImplementedError(
        "lint_and_fix is not yet implemented. Integrate with skilllint's FrontmatterValidator and fixers."
    )
