package commander

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"sync"
	"time"

	"go.opentelemetry.io/otel"
	"go.opentelemetry.io/otel/attribute"
	"go.opentelemetry.io/otel/codes"
	"go.opentelemetry.io/otel/metric"
	"go.opentelemetry.io/otel/trace"
)

const tracerName = "git.derfenix.pro/fenix/commander"

var ErrNoErrorChannel = errors.New("no error channel provided")

type Command interface {
	Execute(ctx context.Context) error
	Rollback(ctx context.Context) error
}

type CorrelatedCommand interface {
	Command
	CorrelationID() string
}

type Eventer interface {
	EmitEvent(ctx context.Context) error
}

type CommandCache interface {
	CommandCacheGet(context.Context, string, ...Command) bool
	CommandCacheStore(context.Context, string, ...Command)
}

func mustNewMetrics() metrics {
	m, err := newMetrics()
	if err != nil {
		panic(err)
	}

	return m
}

func newMetrics() (metrics, error) {
	meter := otel.GetMeterProvider().Meter("executor")

	commandsCount, err := meter.Int64Histogram("commands", metric.WithDescription("Count of executed commands (can be rolled back)"))
	if err != nil {
		return metrics{}, fmt.Errorf("commands count histogram: %w", err)
	}

	commandsRollbackCount, err := meter.Int64Histogram("commands.rollback", metric.WithDescription("Count of commands rolled back"))
	if err != nil {
		return metrics{}, fmt.Errorf("commands count histogram: %w", err)
	}

	commandsFailedCount, err := meter.Int64Histogram("commands.failed", metric.WithDescription("Count of failed commands"))
	if err != nil {
		return metrics{}, fmt.Errorf("commands failed count histogram: %w", err)
	}

	commandsRollbackFailedCount, err := meter.Int64Histogram("commands.rollback.failed", metric.WithDescription("Count of commands fail to roll back"))
	if err != nil {
		return metrics{}, fmt.Errorf("commands count histogram: %w", err)
	}

	monitorCommands, err := meter.Int64UpDownCounter("commands.running", metric.WithDescription("Command set in progress"))
	if err != nil {
		return metrics{}, fmt.Errorf("monitor commands counter: %w", err)
	}

	cacheHit, err := meter.Int64Counter("cache.hit", metric.WithDescription("Cache hit"))
	if err != nil {
		return metrics{}, fmt.Errorf("cache hit: %w", err)
	}

	cacheMiss, err := meter.Int64Counter("cache.miss", metric.WithDescription("Cache miss"))
	if err != nil {
		return metrics{}, fmt.Errorf("cache miss: %w", err)
	}

	return metrics{
		commandsCount:               commandsCount,
		commandsRollbackCount:       commandsRollbackCount,
		commandsFailedCount:         commandsFailedCount,
		commandsRollbackFailedCount: commandsRollbackFailedCount,
		monitorCommands:             monitorCommands,
		cacheHit:                    cacheHit,
		cacheMiss:                   cacheMiss,
	}, nil
}

type metrics struct {
	commandsCount               metric.Int64Histogram
	commandsRollbackCount       metric.Int64Histogram
	commandsFailedCount         metric.Int64Histogram
	commandsRollbackFailedCount metric.Int64Histogram

	monitorCommands metric.Int64UpDownCounter

	cacheHit  metric.Int64Counter
	cacheMiss metric.Int64Counter
}

func New(commandsLimit int) *Commander {
	tracer := otel.Tracer(tracerName)

	return &Commander{
		metrics: mustNewMetrics(),
		tracer:  tracer,

		correlationIDBuffer: sync.Pool{New: func() any { return bytes.NewBuffer(nil) }},

		commandsSemaphore: NewSemaphore(commandsLimit),
	}
}

type Commander struct {
	metrics
	tracer trace.Tracer
	cache  CommandCache

	correlationIDBuffer sync.Pool

	commandsSemaphore Semaphore
}

func (c *Commander) WithCache(cache Cache) *Commander {
	c.cache = newCommandCache(cache, cacheTTL)

	return c
}

func (c *Commander) WithCacheTTL(cache Cache, ttl time.Duration) *Commander {
	c.cache = newCommandCache(cache, ttl)

	return c
}

func (c *Commander) ExecuteAsync(ctx context.Context, errCh chan<- error, correlationID string, commands ...Command) {
	ctx, span := c.tracer.Start(ctx, "executor.execute_async")
	defer span.End()

	if errCh == nil {
		span.RecordError(ErrNoErrorChannel)
	}

	defer func() {
		if errCh != nil {
			close(errCh)
		}
	}()

	if err := c.Execute(ctx, correlationID, commands...); err != nil {
		if errCh != nil {
			errCh <- err
		}
	}
}

func (c *Commander) Execute(ctx context.Context, correlationID string, commands ...Command) error {
	ctx, span := c.tracer.Start(
		ctx,
		"executor.execute",
		trace.WithAttributes(
			attribute.Int("commands.count", len(commands)),
		),
	)
	defer span.End()

	if c.cache != nil && correlationID == "" {
		correlationID = c.tryGetCorrelationID(commands)
	}

	shouldCache := c.cache != nil && correlationID != ""

	c.commandsSemaphore.Acquire()
	c.monitorCommands.Add(ctx, 1)
	defer func() {
		c.commandsSemaphore.Release()
		c.monitorCommands.Add(ctx, -1)
	}()

	if shouldCache {
		if c.cache.CommandCacheGet(ctx, correlationID, commands...) {
			c.cacheHit.Add(ctx, 1)

			return nil
		} else {
			c.cacheMiss.Add(ctx, 1)
		}
	}

	var actionsCount int64
	defer func() {
		if actionsCount > 0 {
			c.commandsCount.Record(ctx, actionsCount)
		}
	}()

	eventEmitters := make([]Eventer, 0, len(commands))

	for idx, command := range commands {
		if err := command.Execute(ctx); err != nil {
			c.commandsFailedCount.Record(ctx, 1)

			span.RecordError(
				err,
				trace.WithAttributes(attribute.String("step", "execute")),
				trace.WithAttributes(attribute.String("command", fmt.Sprintf("%T", command))),
			)
			span.SetStatus(codes.Error, "failed to execute command")

			if idx > 0 {
				c.Rollback(ctx, commands[:idx]...)
			}

			return fmt.Errorf("execute command %v: %w", command, err)
		}

		if eventer, ok := command.(Eventer); ok {
			eventEmitters = append(eventEmitters, eventer)
		}

		actionsCount++
	}

	for _, emitter := range eventEmitters {
		if err := emitter.EmitEvent(ctx); err != nil {
			span.RecordError(
				err,
				trace.WithAttributes(attribute.String("step", "send events")),
				trace.WithAttributes(attribute.String("command", fmt.Sprintf("%T", emitter))),
			)
		}
	}

	if shouldCache {
		c.cache.CommandCacheStore(ctx, correlationID, commands...)
	}

	return nil
}

func (c *Commander) tryGetCorrelationID(commands []Command) string {
	newCorrelationID := c.correlationIDBuffer.Get().(*bytes.Buffer)
	defer func() {
		newCorrelationID.Reset()
		c.correlationIDBuffer.Put(newCorrelationID)
	}()

	for _, command := range commands {
		correlationIDer, ok := command.(CorrelatedCommand)
		if !ok {
			return ""
		}

		newCorrelationID.WriteString(correlationIDer.CorrelationID())
	}

	return newCorrelationID.String()
}

func (c *Commander) Rollback(ctx context.Context, commands ...Command) {
	ctx, span := c.tracer.Start(ctx, "executor.rollback")
	defer span.End()

	for _, command := range commands {
		if err := command.Rollback(ctx); err != nil {
			c.commandsRollbackFailedCount.Record(ctx, 1)

			span.RecordError(
				err,
				trace.WithAttributes(attribute.String("step", "rollback")),
				trace.WithAttributes(attribute.String("command", fmt.Sprintf("%T", command))),
			)

			continue
		}

		c.commandsRollbackCount.Record(ctx, 1)
	}
}