// Package scheduler contains the request-scheduling strategies used by the
// router's baseRouter. A Scheduler owns the queue, in-flight tracking, and the
// decision tree for when to start a swap versus queue a request. The baseRouter
// owns the channels, run loop, and process machinery, and exposes the
// side-effects a scheduler needs through the Effects interface.
//
// Splitting these apart lets the scheduling strategy be swapped out
// independently of both the process machinery (baseRouter) and the eviction
// policy (Swapper). FIFO is the first and currently only implementation.
package scheduler

import (
	"context"
	"fmt"
	"net/http"
	"time"

	"github.com/mostlygeek/llama-swap/internal/config"
	"github.com/mostlygeek/llama-swap/internal/logmon"
	"github.com/mostlygeek/llama-swap/internal/process"
	"github.com/mostlygeek/llama-swap/internal/shared"
)

// ErrModelNotFound is granted to callers whose model is not handled by this
// router. It is an alias for shared.ErrNoLocalModelFound.
var ErrModelNotFound = shared.ErrNoLocalModelFound

// Swapper is the eviction policy: it decides which running models must be
// stopped before a target can serve. It is orthogonal to the scheduling
// strategy — any Scheduler works with any Swapper.
type Swapper interface {
	// EvictionFor returns running model IDs that must be stopped before
	// target can serve. running is the complete set the scheduler considers
	// live: every process that is not stopped, unioned with the targets of
	// in-flight swaps the scheduler has already committed to (which are not yet
	// visible in process state). The planner does not inspect process state
	// itself. Pure decision; must not log.
	EvictionFor(target string, running []string) []string

	// OnSwapStart runs once at the start of every swap, with the same running
	// set EvictionFor was given for this decision. Planners may log their
	// decision here at whatever verbosity they choose.
	OnSwapStart(target string, running []string)
}

// Scheduler decides what happens to each event the router's run loop receives.
// All methods run on that single run-loop goroutine, so implementations need no
// internal locking for their own state.
type Scheduler interface {
	// OnRequest handles one incoming ServeHTTP request.
	OnRequest(req HandlerReq)
	// OnCancel handles a request whose client has disconnected before it was
	// granted. The scheduler must remove the request from its queue and from
	// any in-flight swap's waiters so it never triggers a model load or grant
	// for a caller that is no longer there.
	OnCancel(req HandlerReq)
	// OnSwapDone handles a swap goroutine reporting completion.
	OnSwapDone(ev SwapDone)
	// OnServeDone handles a tracked ServeHTTP finishing (in-flight decrement).
	OnServeDone(ev ServeDoneEvent)
	// OnUnload reconciles scheduler state for an unload, stops the targeted
	// processes via Effects, and drains the queue. It must block until the
	// targeted processes have stopped.
	OnUnload(targets []string, timeout time.Duration)
	// OnShutdown grants err to every waiter the scheduler still holds (active
	// swap waiters and queued requests). Process teardown is the baseRouter's
	// responsibility.
	OnShutdown(err error)
}

// Effects is implemented by the baseRouter. The scheduler calls back through it
// for every side-effect: inspecting process state, launching swaps, responding
// to callers, and stopping processes.
type Effects interface {
	// ModelState returns the current state of a model's process. ok is false
	// when the model is not handled by this router.
	ModelState(modelID string) (process.ProcessState, bool)
	// RunningModels returns the state of every process that is not stopped or
	// shut down, keyed by model ID. The scheduler uses it to build the running
	// set it hands the Swapper.
	RunningModels() map[string]process.ProcessState
	// StartSwap launches the swap goroutine for modelID, stopping evict first.
	StartSwap(modelID string, evict []string)
	// GrantError responds to a caller with an error.
	GrantError(req HandlerReq, err error)
	// GrantServe hands a caller the wrapped handler for modelID and reports
	// whether the caller was still there to receive it. The scheduler bumps
	// its in-flight count only when this returns true.
	GrantServe(req HandlerReq, modelID string) bool
	// StopProcesses stops the named processes in parallel and blocks until all
	// have stopped. Unknown IDs are skipped.
	StopProcesses(timeout time.Duration, ids []string)
}

// New returns a Scheduler selected by conf.Routing.Scheduler.Use, configured from
// conf and bound to the given planner and effects. Supported values are "fifo"
// (throughput-oriented, batches same-model requests) and "serial" (strict
// one-model-at-a-time, highest-priority-first).
//
// The deployment default is applied by config loading (LoadConfig sets Use to
// "serial" when unset). The "" fallback here is the library default and remains
// "fifo" so callers that build a Config directly keep the original behavior.
func New(conf config.Config, name string, logger *logmon.Monitor, planner Swapper, eff Effects) (Scheduler, error) {
	use := conf.Routing.Scheduler.Use
	if use == "" {
		use = "fifo"
	}
	switch use {
	case "fifo":
		return NewFIFO(name, logger, planner, conf.Routing.Scheduler.Settings.Fifo, conf.Models, eff), nil
	case "serial":
		// Serial ignores the group planner: it always evicts every other model.
		return NewSerial(name, logger, conf.Routing.Scheduler.Settings.Serial, eff), nil
	default:
		return nil, fmt.Errorf("unsupported scheduler type: %q", use)
	}
}

// HandlerReq is one in-flight ServeHTTP request waiting for a routing decision.
type HandlerReq struct {
	Model string
	// Priority is the caller's scheduling priority from the
	// X-LlamaSwap-Priority header. Higher is more urgent, 0 is normal, and
	// negative is batch work. Schedulers use it to order their queues.
	Priority   int
	Ctx        context.Context
	Respond    chan HandlerResp
	PositionCh chan int
}

// BandStats is the queue state for one priority band.
type BandStats struct {
	// Depth is how many requests are waiting in this band.
	Depth int
	// OldestWait is how long the longest-waiting request in this band has
	// been queued, or 0 when the band is empty.
	OldestWait time.Duration
	// Dispatched counts requests in this band handed off since start.
	Dispatched uint64
}

// QueueStats is a point-in-time snapshot of a scheduler's queue, for metrics.
// The reorder counters answer "did the scoring function actually change
// anything?" — without them, tuning aging and swap affinity is guesswork.
type QueueStats struct {
	// Bands is keyed by shared.PriorityBand and always carries an entry for
	// every band in shared.PriorityBands, so metric series stay stable.
	Bands map[string]BandStats
	// AgingReorders counts dispatches where the aging term changed which
	// request was picked.
	AgingReorders uint64
	// AffinityReorders counts dispatches where the swap-affinity term changed
	// which request was picked.
	AffinityReorders uint64
}

// StatsReporter is implemented by schedulers that can report queue statistics.
// It is optional: callers type-assert and skip schedulers that do not provide
// it. Unlike the Scheduler methods, QueueStats is called from arbitrary
// goroutines (the /metrics handler) and must be safe for concurrent use.
type StatsReporter interface {
	QueueStats() QueueStats
}

// HandlerResp is the routing decision returned to a HandlerReq's caller: either
// a handler to serve with, or an error.
type HandlerResp struct {
	HandleFunc http.HandlerFunc
	Err        error
}

// SwapDone is reported by a swap goroutine when its target is ready (or failed).
type SwapDone struct {
	ModelID string
	Err     error
}

// ServeDoneEvent is reported when a tracked ServeHTTP handler returns.
type ServeDoneEvent struct {
	ModelID string
}