from __future__ import annotations

import json
from copy import deepcopy
from pathlib import Path
from typing import Any


ALLOWED_COMPONENTS = {
    "task-list",
    "overview-summary",
    "task-progress-grid",
    "task-header",
    "today-record",
    "metric-progress",
    "activity-heatmap",
    "task-settings",
}

FALLBACKS = {
    "home": {
        "schemaVersion": 1,
        "page": "home",
        "regions": {
            "sidebar": [{"component": "task-list"}],
            "main": [{"component": "overview-summary"}, {"component": "task-progress-grid"}],
        },
    },
    "task-detail": {
        "schemaVersion": 1,
        "page": "task-detail",
        "regions": {
            "sidebar": [{"component": "task-list"}],
            "main": [
                {"component": "task-header"},
                {"component": "today-record"},
                {"component": "metric-progress"},
                {"component": "activity-heatmap"},
                {"component": "task-settings"},
            ],
        },
    },
}


def validate_layout(layout: Any, expected_page: str) -> bool:
    if not isinstance(layout, dict):
        return False
    if layout.get("schemaVersion") != 1 or layout.get("page") != expected_page:
        return False
    regions = layout.get("regions")
    if not isinstance(regions, dict) or set(regions) - {"sidebar", "main"}:
        return False
    for items in regions.values():
        if not isinstance(items, list):
            return False
        for item in items:
            if not isinstance(item, dict) or set(item) - {"component", "options"}:
                return False
            if item.get("component") not in ALLOWED_COMPONENTS:
                return False
            if "options" in item and not isinstance(item["options"], dict):
                return False
    return True


def load_layout(layout_dir: Path, page: str) -> dict[str, Any]:
    if page not in FALLBACKS:
        raise KeyError(page)
    try:
        with (layout_dir / f"{page}.json").open("r", encoding="utf-8") as handle:
            layout = json.load(handle)
        if validate_layout(layout, page):
            return layout
    except (OSError, json.JSONDecodeError):
        pass
    return deepcopy(FALLBACKS[page])