package aastro

import (
	"io"
	"net/http"
	"sync"
	"time"

	"go.opentelemetry.io/otel"
	"go.opentelemetry.io/otel/attribute"
	"go.opentelemetry.io/otel/codes"
	"go.opentelemetry.io/otel/trace"
	"go.uber.org/zap"

	"github.com/voidrunner3074/aastro/internal/metric"
	"github.com/voidrunner3074/aastro/internal/tracing"
)

const maxBodySize = 5 << 20 // 5 MB

type scatter interface {
	scatter(f *flow, original *http.Request) []upstreamResponse
	call(f *flow, original *http.Request) (upstreamResponse, bool)
}

type defaultScatter struct {
	log     *zap.Logger
	metrics *metric.Metrics
}

// scatter reads the original request body once, then fans out to all upstreams concurrently.
// Returns nil when the body is unreadable or exceeds maxBodySize -
// the caller treats nil as a signal to respond with 413.
func (d *defaultScatter) scatter(f *flow, original *http.Request) []upstreamResponse {
	log := d.log.With(zap.String("request_id", requestIDFromContext(original.Context())))

	tracer := otel.Tracer(tracing.TracerName)
	ctx, span := tracer.Start(original.Context(), "aastro.scatter",
		trace.WithAttributes(
			attribute.Int("aastro.upstream.count", len(f.upstreams)),
			attribute.String("aastro.aggregation.strategy", f.aggregation.strategy.String()),
		),
	)
	defer span.End()

	original = original.WithContext(ctx)

	body, ok := d.readBody(original, log)
	if !ok {
		span.SetStatus(codes.Error, "body too large")
		return nil
	}

	results := make([]upstreamResponse, len(f.upstreams))

	var wg sync.WaitGroup
	wg.Add(len(f.upstreams))

	for i, u := range f.upstreams {
		go func() {
			defer wg.Done()
			results[i] = d.callUpstream(f, u, original, body, log)
		}()
	}

	wg.Wait()

	return results
}

// call reads the request body once, then invokes the flow's single upstream
// directly - no goroutine, no result slice. It backs single-upstream flows,
// which bypass the aggregator entirely (see Router.buildProxyResponse).
// Returns (response, true) on success, or (zero value, false) when the body
// is unreadable or exceeds maxBodySize - the caller treats false as a signal
// to respond with 413.
func (d *defaultScatter) call(f *flow, original *http.Request) (upstreamResponse, bool) {
	log := d.log.With(zap.String("request_id", requestIDFromContext(original.Context())))

	tracer := otel.Tracer(tracing.TracerName)
	ctx, span := tracer.Start(original.Context(), "aastro.scatter",
		trace.WithAttributes(
			attribute.Int("aastro.upstream.count", 1),
		),
	)
	defer span.End()

	original = original.WithContext(ctx)

	body, ok := d.readBody(original, log)
	if !ok {
		span.SetStatus(codes.Error, "body too large")
		return upstreamResponse{}, false
	}

	return d.callUpstream(f, f.upstreams[0], original, body, log), true
}

// readBody consumes and closes original.Body, enforcing maxBodySize.
// Returns (body, true) on success or (nil, false) on read error or oversized body.
func (d *defaultScatter) readBody(req *http.Request, log *zap.Logger) ([]byte, bool) {
	body, err := io.ReadAll(io.LimitReader(req.Body, maxBodySize+1))
	if err != nil {
		log.Error("cannot read body", zap.Error(err))
		return nil, false
	}

	if err = req.Body.Close(); err != nil {
		log.Warn("cannot close original request body", zap.Error(err))
	}

	if len(body) > maxBodySize {
		d.metrics.IncFailedRequestsTotal(metric.FailReasonBodyTooLarge)
		return nil, false
	}

	return body, true
}

// callUpstream acquires the semaphore, calls the upstream, records metrics,
// and returns the response. Policy validation is handled by the upstream itself.
func (d *defaultScatter) callUpstream(
	f *flow,
	u upstream,
	original *http.Request,
	body []byte,
	log *zap.Logger,
) upstreamResponse {
	start := time.Now()

	tracer := otel.Tracer(tracing.TracerName)
	ctx, span := tracer.Start(original.Context(), "aastro.upstream",
		trace.WithSpanKind(trace.SpanKindClient),
		trace.WithAttributes(
			attribute.String("aastro.upstream.name", u.name()),
			attribute.String("aastro.flow.path", f.path),
		),
	)
	defer span.End()

	d.metrics.IncUpstreamRequestsTotal(f.path, u.name())

	resp := u.call(ctx, original, body)
	if resp.err != nil {
		d.metrics.IncUpstreamErrorsTotal(f.path, u.name(), string(resp.err.kind))

		span.RecordError(resp.err.Unwrap())
		span.SetAttributes(attribute.String("aastro.upstream.error_kind", resp.err.Error()))
		span.SetStatus(codes.Error, "upstream call failed")

		log.Error("upstream call failed",
			zap.String("name", u.name()),
			zap.Error(resp.err.Unwrap()),
		)
	}

	d.metrics.UpdateUpstreamLatency(f.path, u.name(), start)

	return *resp
}