"""Which machine a node runs on, and the waiting when none of them has room.

The engine used to answer this twice, in two ways that could not see each
other. A node with a ``device`` went to a worker carrying that label, chosen by
which of them had least in flight — a machine's *name*, never its size. A node
with ``resources`` was accounted against the engine's own cores, and only the
engine's: declaring both meant the second answer silently won and nothing was
counted at all.

One question, then, asked once here: of every machine attached — this one and
each worker — which could grant what this node asked for, and which of those
has it free right now. A worker reports its inventory when it attaches, so the
answer covers the whole instance rather than the host the engine happens to
be on.

Two properties are worth stating because they are what make the waiting safe:

*Waiting is central.* One condition variable, woken by a release on any machine
and by a worker attaching. A node waiting inside one machine's books could not
be woken by a second machine appearing, which is exactly the case a cluster is
for.

*Locks go one way.* This condition, then a machine's books, never the reverse —
:meth:`ResourceAccountant.try_take` does not block and
:meth:`ResourceAccountant.give_back` calls back out after dropping its own
lock. The other invariant the engine already had still holds too: the claim is
taken before a worker slot, so a node holding a slot can never be waiting on
resources that a node holding those resources cannot get a slot to release.
"""

from __future__ import annotations

import logging
import threading
import time
from collections.abc import Iterator
from contextlib import contextmanager
from dataclasses import dataclass
from typing import TYPE_CHECKING, Any

from fluksio.flow.events import EventBus
from fluksio.flow.resources import Allocation, ResourceAccountant
from fluksio.flow.schemas import Resources

if TYPE_CHECKING:  # pragma: no cover - imported for types only
    from fluksio.flow.provision import Provisioner
    from fluksio.flow.remote import RemoteWorker, RemoteWorkerHub

logger = logging.getLogger(__name__)

#: How long a waiting node sleeps before looking again without being woken.
#: Everything that frees a machine wakes it, so this only catches a wakeup lost
#: to a race and gives a provisioned machine's timeout somewhere to be noticed.
WAKE_S = 5.0


@dataclass(frozen=True)
class Target:
    """One machine a node could run on: this engine, or an attached worker."""

    name: str
    accountant: ResourceAccountant
    #: None is the engine itself, which runs nodes in its own worker pool.
    worker: RemoteWorker | None = None

    def carries(self, device: str) -> bool:
        """Whether a node asking for this device may run here.

        The engine carries no label: a label names a machine somebody attached
        for the purpose, and the engine is where the node would have run anyway.
        """
        if self.worker is None:
            return False
        return device == self.name or device in self.worker.labels


@dataclass
class _Waiting:
    node: str
    since: float
    reason: str


class Placer:
    """Every machine the engine can reach, and who is queued for one."""

    def __init__(
        self,
        local: ResourceAccountant,
        hub: RemoteWorkerHub | None = None,
        events: EventBus | None = None,
    ) -> None:
        self.local = local
        self.hub = hub
        self.events = events
        self.provisioners: list[Provisioner] = []
        self._condition = threading.Condition()
        self._waiting: dict[int, _Waiting] = {}
        self._ticket = 0
        local.on_release = self.wake

    # -- the machines ----------------------------------------------------------

    def wake(self) -> None:
        """Something changed: a machine freed up, or one attached or left."""
        for provisioner in self.provisioners:
            provisioner.reconcile(self._attached_names())
        with self._condition:
            self._condition.notify_all()

    def _attached_names(self) -> set[str]:
        if self.hub is None:
            return set()
        return {worker.name for worker in self.hub.workers() if not worker.gone}

    def targets(self) -> list[Target]:
        """This engine, and every worker attached right now."""
        found = [Target(name="engine", accountant=self.local)]
        if self.hub is not None:
            for worker in self.hub.workers():
                if worker.gone:
                    continue
                # Assigned here rather than at attach so a worker that
                # reconnects — a new object, with fresh books — is wired up
                # without the hub having to know about placement at all.
                worker.accountant.on_release = self.wake
                found.append(
                    Target(
                        name=worker.name,
                        accountant=worker.accountant,
                        worker=worker,
                    )
                )
        return found

    def _candidates(
        self,
        cpus: int,
        gpus: int,
        ram_mb: int,
        device: str | None,
        policy: str,
    ) -> list[Target]:
        """The machines that could run this node, best first."""
        targets = self.targets()
        remote = [target for target in targets if target.worker is not None]
        engine = [target for target in targets if target.worker is None]

        if device:
            labelled = [target for target in remote if target.carries(device)]
            fitting = self._fitting(labelled, cpus, gpus, ram_mb)
            if fitting or policy != "prefer":
                # `require` waits for the labelled machine even when the engine
                # could take the node: the label is a statement about where
                # this node is correct, not a preference.
                return fitting
            # `prefer` is what makes a flow work before the GPU box exists. It
            # falls back only when nothing labelled could ever take the node —
            # one that is merely busy is worth queueing for.
            return self._fitting(engine, cpus, gpus, ram_mb)

        # No label: the engine first when it fits, because a node that runs
        # here costs no network and reuses a warm worker.
        ordered = engine + sorted(
            remote, key=lambda target: target.worker.in_flight if target.worker else 0
        )
        return self._fitting(ordered, cpus, gpus, ram_mb)

    @staticmethod
    def _fitting(
        targets: list[Target], cpus: int, gpus: int, ram_mb: int
    ) -> list[Target]:
        return [t for t in targets if t.accountant.fits(cpus, gpus, ram_mb)]

    # -- what can actually be granted ------------------------------------------

    def _shapes(self, device: str | None, policy: str) -> list[tuple[int, int, int]]:
        """Every machine's size, as something to measure a request against.

        A provisioner's job shapes count: a machine it can start on demand is
        one this instance has, even when nothing is attached yet.
        """
        shapes = []
        for target in self.targets():
            if device and not target.carries(device):
                # A `prefer` node may end up here, so the engine still counts.
                if policy == "require" or target.worker is not None:
                    continue
            books = target.accountant
            shapes.append((books.cpus, books.gpus, books.ram_mb or 0))
        for provisioner in self.provisioners:
            shapes.extend(provisioner.shapes(device))
        return shapes or [(self.local.cpus, self.local.gpus, self.local.ram_mb or 0)]

    def _clamp(
        self, wanted: Resources, node: str, device: str | None, policy: str
    ) -> tuple[int, int, int]:
        """What can be granted somewhere, which may be less than was asked.

        A flow written against a sixty-four core box should still run on a
        laptop — waiting forever for cores that do not exist is not a smaller
        machine, it is a hung run. What it must not become is an ask no single
        machine can grant: taking the largest of each dimension separately can
        describe a machine that is not there, and waiting on *that* is the hung
        run this exists to avoid. So the GPUs are settled first, and the rest is
        measured only against machines that carry that many.
        """
        ram = wanted.ram or 0
        shapes = self._shapes(device, policy)
        gpus = min(wanted.gpus, max(shape[1] for shape in shapes))
        capable = [shape for shape in shapes if shape[1] >= gpus]
        cpus = min(wanted.cpus, max(shape[0] for shape in capable))
        # A machine that reported no memory is not a memory limit; only those
        # that did have anything to say about it.
        said = [shape[2] for shape in capable if shape[2]]
        ram = min(ram, max(said)) if said and ram else ram
        if (cpus, gpus, ram) != (wanted.cpus, wanted.gpus, wanted.ram or 0):
            # Only NVIDIA's device nodes are counted, so a card behind another
            # vendor's driver reports as none until it is told — which reads as
            # "no GPU here" to a node that then runs unserialised beside every
            # other one.
            hint = (
                "; no machine here has a GPU it knows of — `fluksio serve "
                "--gpus N` (or FLOW_GPUS) says how many this one has"
                if wanted.gpus and not gpus
                else ""
            )
            logger.warning(
                "%s asked for %d cpu(s), %d gpu(s) and %s MB; "
                "the largest machine here can give %d, %d and %s%s",
                node or "a node",
                wanted.cpus,
                wanted.gpus,
                wanted.ram or "no stated",
                cpus,
                gpus,
                ram or "no stated",
                hint,
            )
        return cpus, gpus, ram

    # -- holding a machine for the length of a call ----------------------------

    @contextmanager
    def claim(
        self,
        wanted: Resources,
        *,
        device: str | None = None,
        policy: str = "require",
        node: str = "",
        run: str = "",
    ) -> Iterator[tuple[Target, Allocation]]:
        """Hold a machine's share of itself for as long as this node runs."""
        waited_from = time.monotonic()
        ticket = 0
        chosen: Target | None = None
        allocation: Allocation | None = None

        with self._condition:
            while chosen is None:
                cpus, gpus, ram = self._clamp(wanted, node, device, policy)
                for target in self._candidates(cpus, gpus, ram, device, policy):
                    got = target.accountant.try_take(cpus, gpus, ram)
                    if got is not None:
                        chosen, allocation = target, got
                        break
                if chosen is not None:
                    break
                if not ticket:
                    ticket = self._announce(
                        node, run, waited_from, self._reason(cpus, gpus, device)
                    )
                self._provision(cpus, gpus, ram, device)
                self._condition.wait(timeout=WAKE_S)
            if ticket:
                self._waiting.pop(ticket, None)

        if ticket:
            logger.info(
                "%s waited %.1fs and runs on '%s'",
                node or "a node",
                time.monotonic() - waited_from,
                chosen.name,
            )
        assert allocation is not None
        try:
            yield chosen, allocation
        finally:
            # Every way a call can end comes through here: an answer, a
            # timeout, a cancellation, or the worker going away mid-call.
            chosen.accountant.give_back(allocation)

    def _reason(self, cpus: int, gpus: int, device: str | None) -> str:
        parts = []
        if gpus:
            parts.append(f"{gpus} gpu(s)")
        parts.append(f"{cpus} cpu(s)")
        wants = " and ".join(parts)
        if device:
            return f"waiting for a worker labelled '{device}' with {wants} free"
        return f"waiting for {wants}"

    def _provision(self, cpus: int, gpus: int, ram_mb: int, device: str | None) -> None:
        """Ask for a machine, if something here can start one."""
        for provisioner in self.provisioners:
            if provisioner.covers(cpus, gpus, ram_mb, device):
                provisioner.provision(cpus, gpus, ram_mb, device)
                return

    def provision_for(self, needs: dict[str, Any] | None) -> None:
        """Ask for a machine a queued run is waiting on.

        A run held before it starts never reaches the wait above, so this is
        where the asking happens for it — otherwise it would sit waiting for a
        machine that nothing ever requested.
        """
        if not needs:
            return
        self._provision(
            int(needs.get("cpus") or 1),
            int(needs.get("gpus") or 0),
            int(needs.get("ram_mb") or 0),
            needs.get("device") or None,
        )

    def _announce(self, node: str, run: str, since: float, reason: str) -> int:
        """Say a node is queued, not stuck.

        The failure this exists for looked identical to a hang: a run sitting
        at `running` for twenty minutes with no error and no output. A node
        waiting its turn has to say so somewhere a person will look.
        """
        self._ticket += 1
        ticket = self._ticket
        self._waiting[ticket] = _Waiting(node=node, since=since, reason=reason)
        logger.info("%s is %s", node or "a node", reason)
        if self.events is not None:
            self.events.publish(
                {
                    "type": "node_queued",
                    "flow": node.split(".", 1)[0] if node else "",
                    "node": node,
                    "run": run,
                    "detail": reason,
                    "ts": time.time(),
                }
            )
        return ticket

    # -- what it looks like from outside ---------------------------------------

    def satisfiable(self, needs: dict[str, Any] | None) -> str | None:
        """What a run would have to wait for, or None if it would not wait.

        Only a genuine wait is worth holding a run for. A request nothing can
        grant is cut down to what is here and runs anyway, so answering "no
        machine has 64 cores" would hold a run that was about to work.
        """
        if not needs:
            return None
        cpus = int(needs.get("cpus") or 1)
        gpus = int(needs.get("gpus") or 0)
        ram = int(needs.get("ram_mb") or 0)
        device = needs.get("device") or None
        for target in self.targets():
            if device and not target.carries(device):
                continue
            if target.accountant.fits(cpus, gpus, ram):
                # Something here could take it. Busy is queueing, not waiting.
                return None
        if not any(
            provisioner.covers(cpus, gpus, ram, device)
            for provisioner in self.provisioners
        ):
            # Nothing attached fits and nothing can be started, so the node
            # will be clamped onto what is here and run. Holding the run would
            # be holding it for something that is not going to happen — and
            # where the wait is a missing *label*, `required_labels` says so
            # already.
            return None
        return f"{gpus} gpu(s) and {cpus} cpu(s)" if gpus else f"{cpus} cpu(s)"

    def snapshot(self) -> dict[str, Any]:
        """Every machine, what is free of it, and who is queued.

        A node waiting its turn looks exactly like a node that has hung — the
        run sits at `running` and says nothing — so what is waiting, and for
        what, has to be readable somewhere.
        """
        targets = []
        for target in self.targets():
            entry: dict[str, Any] = {"target": target.name}
            entry.update(target.accountant.snapshot())
            if target.worker is not None:
                entry["labels"] = sorted(target.worker.labels)
                entry["in_flight"] = target.worker.in_flight
            targets.append(entry)
        with self._condition:
            waiting = list(self._waiting.values())
        local = self.local.snapshot()
        return {
            # The engine's own figures stay where they were, so a reader that
            # only ever knew about one machine still finds them.
            "cpus": local["cpus"],
            "gpus": local["gpus"],
            "waiting": [
                {
                    "node": entry.node,
                    "reason": entry.reason,
                    "seconds": round(time.monotonic() - entry.since, 1),
                }
                for entry in waiting
            ],
            "targets": targets,
            "provisioners": [p.status() for p in self.provisioners],
        }