"""Small, dependency-free RFC 6902 JSON Patch implementation for CDSL revisions."""

from __future__ import annotations

from copy import deepcopy
from typing import Any


class CdslPatchError(ValueError):
    """A JSON Patch operation cannot be applied to the requested CDSL revision."""


def _tokens(path: Any) -> list[str]:
    if not isinstance(path, str) or not path.startswith("/"):
        raise CdslPatchError("JSON Patch paths must be JSON Pointers beginning with '/'")
    return [token.replace("~1", "/").replace("~0", "~") for token in path[1:].split("/")]


def _index(token: str, length: int, *, allow_append: bool = False) -> int:
    if allow_append and token == "-":
        return length
    if not token.isdigit() or (len(token) > 1 and token.startswith("0")):
        raise CdslPatchError(f"Invalid JSON Patch array index: {token}")
    value = int(token)
    if value < 0 or value >= length:
        raise CdslPatchError(f"JSON Patch array index is out of range: {token}")
    return value


def _parent(document: Any, path: str) -> tuple[Any, str]:
    if path == "":
        raise CdslPatchError("Replacing the complete CDSL document is not allowed; call generate_cdsl_model instead")
    tokens = _tokens(path)
    current = document
    for token in tokens[:-1]:
        if isinstance(current, dict):
            if token not in current:
                raise CdslPatchError(f"JSON Patch path does not exist: {path}")
            current = current[token]
        elif isinstance(current, list):
            current = current[_index(token, len(current))]
        else:
            raise CdslPatchError(f"JSON Patch path does not resolve to a container: {path}")
    return current, tokens[-1]


def _get(document: Any, path: str) -> Any:
    current = document
    for token in _tokens(path):
        if isinstance(current, dict):
            if token not in current:
                raise CdslPatchError(f"JSON Patch path does not exist: {path}")
            current = current[token]
        elif isinstance(current, list):
            current = current[_index(token, len(current))]
        else:
            raise CdslPatchError(f"JSON Patch path does not resolve: {path}")
    return current


def _add(document: Any, path: str, value: Any) -> None:
    parent, token = _parent(document, path)
    if isinstance(parent, dict):
        parent[token] = deepcopy(value)
    elif isinstance(parent, list):
        index = _index(token, len(parent), allow_append=True)
        parent.insert(index, deepcopy(value))
    else:
        raise CdslPatchError(f"JSON Patch add target is not a container: {path}")


def _remove(document: Any, path: str) -> Any:
    parent, token = _parent(document, path)
    if isinstance(parent, dict):
        if token not in parent:
            raise CdslPatchError(f"JSON Patch path does not exist: {path}")
        return parent.pop(token)
    if isinstance(parent, list):
        return parent.pop(_index(token, len(parent)))
    raise CdslPatchError(f"JSON Patch remove target is not a container: {path}")


def apply_cdsl_patch(cdsl: dict[str, Any], patches: Any) -> dict[str, Any]:
    if not isinstance(cdsl, dict):
        raise CdslPatchError("The base CDSL must be an object")
    if not isinstance(patches, list) or not patches:
        raise CdslPatchError("patches must be a non-empty JSON Patch array")
    if len(patches) > 32:
        raise CdslPatchError("At most 32 JSON Patch operations are allowed")
    result: Any = deepcopy(cdsl)
    for index, operation in enumerate(patches):
        if not isinstance(operation, dict):
            raise CdslPatchError(f"Patch operation {index} must be an object")
        kind = str(operation.get("op") or "")
        path = operation.get("path")
        if kind not in {"add", "remove", "replace", "move", "copy", "test"}:
            raise CdslPatchError(f"Unsupported JSON Patch operation: {kind or '<missing>'}")
        if kind in {"add", "replace", "test"} and "value" not in operation:
            raise CdslPatchError(f"JSON Patch {kind} requires value")
        if kind in {"move", "copy"} and "from" not in operation:
            raise CdslPatchError(f"JSON Patch {kind} requires from")
        if path == "":
            raise CdslPatchError("Replacing the complete CDSL document is not allowed; call generate_cdsl_model instead")
        if kind == "add":
            _add(result, path, operation["value"])
        elif kind == "remove":
            _remove(result, path)
        elif kind == "replace":
            _get(result, path)
            _remove(result, path)
            _add(result, path, operation["value"])
        elif kind == "move":
            source = str(operation["from"])
            if path == source or str(path).startswith(source.rstrip("/") + "/"):
                raise CdslPatchError("JSON Patch move cannot move a value into itself")
            moved = _remove(result, source)
            _add(result, path, moved)
        elif kind == "copy":
            _add(result, path, _get(result, str(operation["from"])))
        elif kind == "test" and _get(result, path) != operation["value"]:
            raise CdslPatchError(f"JSON Patch test failed at {path}")
    return result