"""Read-only client for the actualbudget HTTP API (per-user).

The API is `jhonderson/actual-http-api`, an HTTP wrapper over the
Actual Budget node API. Endpoints used:

- `GET /v1/budgets/{syncId}/months` → list of "YYYY-MM" with data
- `GET /v1/budgets/{syncId}/months/{month}` → per-category-group
   spending; the `totalSpent` field is in pence (negative = outflow)

The wrapper is cluster-internal — `ACTUALBUDGET_API_URL` points at
`http://budget-http-api-viktor.actualbudget.svc.cluster.local`. Auth
is a static API key set on the http-api Deployment (header
`x-api-key`); fire-planner reads both via Vault → ESO.

This module deliberately stays thin: no caching, no retries, no
background sync. The /spending endpoint pulls live each call (~12
upstream HTTP requests for one year, <2s).
"""
from __future__ import annotations

import os
from dataclasses import dataclass
from datetime import date
from decimal import Decimal

import httpx


@dataclass(frozen=True)
class MonthSpend:
    """One calendar month's spending broken out by category group.

    All amounts are in pence and represent OUTFLOWS as positive numbers
    (the upstream API returns negative for spend; we flip the sign at
    parse time so callers don't have to remember the convention).

    Income groups (`is_income=True` upstream) are excluded — we only
    want consumption-style buckets.
    """

    month: str  # "YYYY-MM"
    by_group: dict[str, int]  # group name → pence outflow

    @property
    def total_pence(self) -> int:
        return sum(self.by_group.values())


def _required_env(name: str) -> str:
    v = os.environ.get(name)
    if not v:
        raise RuntimeError(f"{name} not set; cannot reach actualbudget")
    return v


class ActualBudgetClient:
    """Per-user actualbudget HTTP API client.

    Constructed once per request (cheap; httpx.AsyncClient internally
    reuses connections within the call) so we don't have to thread a
    long-lived client through FastAPI.
    """

    def __init__(self,
                 api_url: str | None = None,
                 api_key: str | None = None,
                 sync_id: str | None = None,
                 timeout: float = 10.0) -> None:
        self.api_url = (api_url or _required_env("ACTUALBUDGET_API_URL")).rstrip("/")
        self.api_key = api_key or _required_env("ACTUALBUDGET_API_KEY")
        self.sync_id = sync_id or _required_env("ACTUALBUDGET_SYNC_ID")
        self.timeout = timeout

    async def list_months(self, client: httpx.AsyncClient) -> list[str]:
        r = await client.get(
            f"{self.api_url}/v1/budgets/{self.sync_id}/months",
            headers={"x-api-key": self.api_key},
            timeout=self.timeout,
        )
        r.raise_for_status()
        return list(r.json()["data"])

    async def fetch_month(self, client: httpx.AsyncClient, month: str) -> MonthSpend:
        r = await client.get(
            f"{self.api_url}/v1/budgets/{self.sync_id}/months/{month}",
            headers={"x-api-key": self.api_key},
            timeout=self.timeout,
        )
        r.raise_for_status()
        groups = r.json()["data"]["categoryGroups"]
        by_group: dict[str, int] = {}
        for g in groups:
            if g.get("is_income"):
                continue
            spent = sum(int(c.get("spent", 0)) for c in g.get("categories", []))
            # API uses negative for outflows; flip so callers see positive £.
            by_group[g["name"]] = -spent
        return MonthSpend(month=month, by_group=by_group)


def _trailing_months(today: date, count: int) -> list[str]:
    """Return the last `count` complete calendar months ending the
    month before `today`. The current month is excluded because it's
    incomplete and would skew the trailing average.
    """
    if count <= 0:
        return []
    months: list[str] = []
    y, m = today.year, today.month
    # Walk back: skip current month, then take `count` complete months.
    m -= 1
    if m == 0:
        y -= 1
        m = 12
    for _ in range(count):
        months.append(f"{y:04d}-{m:02d}")
        m -= 1
        if m == 0:
            y -= 1
            m = 12
    months.reverse()
    return months


def _today() -> date:
    """Indirection so tests can patch the clock without subclassing date."""
    return date.today()


def _months_between(older: str, newer: str) -> int:
    """Calendar months from `older` to `newer` (both YYYY-MM)."""
    yo, mo = (int(p) for p in older.split("-"))
    yn, mn = (int(p) for p in newer.split("-"))
    return (yn - yo) * 12 + (mn - mo)


def _inflation_factor(months_age: int, annual_inflation_pct: float) -> float:
    """Multiplier to convert nominal £ at a point `months_age` months
    in the past into today's real £. Compounds monthly so the curve
    is smooth, not stair-stepped."""
    if months_age <= 0:
        return 1.0
    monthly = (1.0 + annual_inflation_pct) ** (1.0 / 12.0)
    return float(monthly ** months_age)


async def fetch_trailing_spending(
    months: int,
    exclude_groups: frozenset[str],
    annual_inflation_pct: float = 0.03,
    client_factory: ActualBudgetClient | None = None,
    today: date | None = None,
) -> tuple[list[MonthSpend], Decimal, Decimal]:
    """Fetch trailing-N-months spending and return (per-month
    breakdown, nominal-£ total, today's-£ inflation-adjusted total).

    `exclude_groups` is a set of category-group names whose spending
    should NOT count towards the consumption total — typical example
    is "Investments and Savings", which is a wealth transfer rather
    than consumption.

    Months older than the current one are revalued to today's £ using
    `annual_inflation_pct` compounded monthly. The fire-planner
    simulator works entirely in REAL GBP, so the real-£ total is the
    one to feed back as the "Annual spending" default.

    Missing months (e.g. user joined actualbudget mid-year) come back
    with empty `by_group`.
    """
    cli = client_factory or ActualBudgetClient()
    today_d = today or _today()
    target_months = _trailing_months(today_d, months)
    available_set = None  # only fetch if the API surfaces fewer months than asked

    async with httpx.AsyncClient() as http:
        if len(target_months) > 0:
            available = await cli.list_months(http)
            available_set = set(available)
        kept = [m for m in target_months if available_set is None or m in available_set]
        # Sequential — actualbudget single-user instance can't parallelise the
        # SQLite-backed API anyway, and 12 calls @ ~80ms each is well under 2s.
        spends: list[MonthSpend] = []
        for m in kept:
            spends.append(await cli.fetch_month(http, m))

    today_month = f"{today_d.year:04d}-{today_d.month:02d}"
    nominal_pence = 0
    real_pence_float = 0.0
    for ms in spends:
        age = _months_between(ms.month, today_month)
        factor = _inflation_factor(age, annual_inflation_pct)
        for name, amt in ms.by_group.items():
            if name in exclude_groups:
                continue
            nominal_pence += amt
            real_pence_float += amt * factor

    nominal_gbp = (Decimal(nominal_pence) / Decimal(100)).quantize(Decimal("0.01"))
    real_gbp = (Decimal(str(round(real_pence_float / 100, 2)))).quantize(Decimal("0.01"))
    return spends, nominal_gbp, real_gbp