package router

import (
	"context"
	"fmt"
	"io"
	"net/http"
	"net/http/httptest"
	"strings"
	"sync"
	"sync/atomic"
	"testing"
	"time"

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

// fakeProcess is an in-memory implementation of process.Process used to drive
// the routers through their state machine without spawning real upstreams.
type fakeProcess struct {
	id string

	mu          sync.Mutex
	state       process.ProcessState
	readyCh     chan struct{}
	stopCh      chan struct{}
	runStarted  chan struct{} // closed on the first Run call
	stopStarted chan struct{} // closed on the first Stop call

	autoReady bool

	// serveBlock, when non-nil, makes ServeHTTP receive from it before
	// writing its response. Tests use this to hold a request in-flight.
	// Closing the channel releases every blocked ServeHTTP caller.
	serveBlock chan struct{}
	// serveStarted is closed on the first ServeHTTP entry, letting tests
	// wait deterministically for the handler to begin executing.
	serveStarted chan struct{}
	// stopBlock, when non-nil, makes Stop receive from it (after signalling
	// stopStarted) before completing. Tests use this to prove that several
	// Stop calls can be in flight simultaneously.
	stopBlock chan struct{}

	runCalls   atomic.Int32
	stopCalls  atomic.Int32
	serveCalls atomic.Int32

	// inFlightServe counts ServeHTTP calls currently inside the handler.
	// stoppedWhileServing flips true if Stop is ever called while that
	// counter is non-zero — a direct, race-free observation of the
	// "swap mid-request" anti-property.
	inFlightServe       atomic.Int32
	stoppedWhileServing atomic.Bool
}

func newFakeProcess(id string) *fakeProcess {
	return &fakeProcess{
		id:           id,
		state:        process.StateStopped,
		readyCh:      make(chan struct{}),
		stopCh:       make(chan struct{}),
		runStarted:   make(chan struct{}),
		stopStarted:  make(chan struct{}),
		serveStarted: make(chan struct{}),
	}
}

func (f *fakeProcess) setState(s process.ProcessState) {
	f.mu.Lock()
	defer f.mu.Unlock()
	f.state = s
	if s == process.StateReady {
		select {
		case <-f.readyCh:
		default:
			close(f.readyCh)
		}
	}
}

func (f *fakeProcess) State() process.ProcessState {
	f.mu.Lock()
	defer f.mu.Unlock()
	return f.state
}

func (f *fakeProcess) markReady() { f.setState(process.StateReady) }

func (f *fakeProcess) Run(_ time.Duration) error {
	f.runCalls.Add(1)
	f.mu.Lock()
	if f.state != process.StateStopped {
		s := f.state
		f.mu.Unlock()
		return fmt.Errorf("fakeProcess %s: Run called while %s", f.id, s)
	}
	f.state = process.StateStarting
	sc := f.stopCh
	select {
	case <-f.runStarted:
	default:
		close(f.runStarted)
	}
	f.mu.Unlock()

	if f.autoReady {
		f.setState(process.StateReady)
	}
	<-sc
	return nil
}

func (f *fakeProcess) Stop(_ time.Duration) error {
	f.stopCalls.Add(1)
	if f.inFlightServe.Load() > 0 {
		f.stoppedWhileServing.Store(true)
	}
	f.mu.Lock()
	select {
	case <-f.stopStarted:
	default:
		close(f.stopStarted)
	}
	f.mu.Unlock()

	// Test hook: hold Stop here so the test can prove multiple Stops are
	// in flight at the same time before any of them complete.
	if f.stopBlock != nil {
		<-f.stopBlock
	}

	f.mu.Lock()
	defer f.mu.Unlock()
	if f.state == process.StateStopped {
		return nil
	}
	f.state = process.StateStopped
	select {
	case <-f.stopCh:
	default:
		close(f.stopCh)
	}
	return nil
}

func (f *fakeProcess) WaitReady(ctx context.Context) error {
	f.mu.Lock()
	if f.state == process.StateReady {
		f.mu.Unlock()
		return nil
	}
	rc := f.readyCh
	f.mu.Unlock()
	select {
	case <-rc:
		return nil
	case <-ctx.Done():
		return ctx.Err()
	}
}

func (f *fakeProcess) Logger() *logmon.Monitor { return logmon.NewWriter(io.Discard) }

func (f *fakeProcess) ServeHTTP(w http.ResponseWriter, _ *http.Request) {
	f.serveCalls.Add(1)
	f.inFlightServe.Add(1)
	defer f.inFlightServe.Add(-1)
	f.mu.Lock()
	select {
	case <-f.serveStarted:
	default:
		close(f.serveStarted)
	}
	f.mu.Unlock()
	if f.serveBlock != nil {
		<-f.serveBlock
	}
	w.WriteHeader(http.StatusOK)
	fmt.Fprintf(w, "ok:%s", f.id)
}

// waitProcessed drains n events from ch, fataling on timeout. One event fires
// per handlerReq or swapDone fully absorbed by run().
func waitProcessed(t *testing.T, ch chan struct{}, n int) {
	t.Helper()
	for i := 0; i < n; i++ {
		select {
		case <-ch:
		case <-time.After(2 * time.Second):
			t.Fatalf("waitProcessed: only %d/%d events received", i, n)
		}
	}
}

func newRequest(model string) *http.Request {
	body := fmt.Sprintf(`{"model":%q}`, model)
	r := httptest.NewRequest(http.MethodPost, "/v1/chat/completions", strings.NewReader(body))
	r.Header.Set("Content-Type", "application/json")
	return r
}

func newRequestCtx(ctx context.Context, model string) *http.Request {
	return newRequest(model).WithContext(ctx)
}