// Copyright The OpenTelemetry Authors
// SPDX-License-Identifier: Apache-2.0

package otelhttp // import "go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp"

import (
	"net/http"
	"time"

	"github.com/felixge/httpsnoop"

	"go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp/internal/semconv"
	"go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp/internal/semconvutil"
	"go.opentelemetry.io/otel"
	"go.opentelemetry.io/otel/attribute"
	"go.opentelemetry.io/otel/metric"
	"go.opentelemetry.io/otel/propagation"
	"go.opentelemetry.io/otel/trace"
)

// middleware is an http middleware which wraps the next handler in a span.
type middleware struct {
	operation string
	server    string

	tracer            trace.Tracer
	meter             metric.Meter
	propagators       propagation.TextMapPropagator
	spanStartOptions  []trace.SpanStartOption
	readEvent         bool
	writeEvent        bool
	filters           []Filter
	spanNameFormatter func(string, *http.Request) string
	publicEndpoint    bool
	publicEndpointFn  func(*http.Request) bool

	traceSemconv         semconv.HTTPServer
	requestBytesCounter  metric.Int64Counter
	responseBytesCounter metric.Int64Counter
	serverLatencyMeasure metric.Float64Histogram
}

func defaultHandlerFormatter(operation string, _ *http.Request) string {
	return operation
}

// NewHandler wraps the passed handler in a span named after the operation and
// enriches it with metrics.
func NewHandler(handler http.Handler, operation string, opts ...Option) http.Handler {
	return NewMiddleware(operation, opts...)(handler)
}

// NewMiddleware returns a tracing and metrics instrumentation middleware.
// The handler returned by the middleware wraps a handler
// in a span named after the operation and enriches it with metrics.
func NewMiddleware(operation string, opts ...Option) func(http.Handler) http.Handler {
	h := middleware{
		operation: operation,

		traceSemconv: semconv.NewHTTPServer(),
	}

	defaultOpts := []Option{
		WithSpanOptions(trace.WithSpanKind(trace.SpanKindServer)),
		WithSpanNameFormatter(defaultHandlerFormatter),
	}

	c := newConfig(append(defaultOpts, opts...)...)
	h.configure(c)
	h.createMeasures()

	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			h.serveHTTP(w, r, next)
		})
	}
}

func (h *middleware) configure(c *config) {
	h.tracer = c.Tracer
	h.meter = c.Meter
	h.propagators = c.Propagators
	h.spanStartOptions = c.SpanStartOptions
	h.readEvent = c.ReadEvent
	h.writeEvent = c.WriteEvent
	h.filters = c.Filters
	h.spanNameFormatter = c.SpanNameFormatter
	h.publicEndpoint = c.PublicEndpoint
	h.publicEndpointFn = c.PublicEndpointFn
	h.server = c.ServerName
}

func handleErr(err error) {
	if err != nil {
		otel.Handle(err)
	}
}

func (h *middleware) createMeasures() {
	var err error
	h.requestBytesCounter, err = h.meter.Int64Counter(
		serverRequestSize,
		metric.WithUnit("By"),
		metric.WithDescription("Measures the size of HTTP request messages."),
	)
	handleErr(err)

	h.responseBytesCounter, err = h.meter.Int64Counter(
		serverResponseSize,
		metric.WithUnit("By"),
		metric.WithDescription("Measures the size of HTTP response messages."),
	)
	handleErr(err)

	h.serverLatencyMeasure, err = h.meter.Float64Histogram(
		serverDuration,
		metric.WithUnit("ms"),
		metric.WithDescription("Measures the duration of inbound HTTP requests."),
	)
	handleErr(err)
}

// serveHTTP sets up tracing and calls the given next http.Handler with the span
// context injected into the request context.
func (h *middleware) serveHTTP(w http.ResponseWriter, r *http.Request, next http.Handler) {
	requestStartTime := time.Now()
	for _, f := range h.filters {
		if !f(r) {
			// Simply pass through to the handler if a filter rejects the request
			next.ServeHTTP(w, r)
			return
		}
	}

	ctx := h.propagators.Extract(r.Context(), propagation.HeaderCarrier(r.Header))
	opts := []trace.SpanStartOption{
		trace.WithAttributes(h.traceSemconv.RequestTraceAttrs(h.server, r)...),
	}

	opts = append(opts, h.spanStartOptions...)
	if h.publicEndpoint || (h.publicEndpointFn != nil && h.publicEndpointFn(r.WithContext(ctx))) {
		opts = append(opts, trace.WithNewRoot())
		// Linking incoming span context if any for public endpoint.
		if s := trace.SpanContextFromContext(ctx); s.IsValid() && s.IsRemote() {
			opts = append(opts, trace.WithLinks(trace.Link{SpanContext: s}))
		}
	}

	tracer := h.tracer

	if tracer == nil {
		if span := trace.SpanFromContext(r.Context()); span.SpanContext().IsValid() {
			tracer = newTracer(span.TracerProvider())
		} else {
			tracer = newTracer(otel.GetTracerProvider())
		}
	}

	ctx, span := tracer.Start(ctx, h.spanNameFormatter(h.operation, r), opts...)
	defer span.End()

	readRecordFunc := func(int64) {}
	if h.readEvent {
		readRecordFunc = func(n int64) {
			span.AddEvent("read", trace.WithAttributes(ReadBytesKey.Int64(n)))
		}
	}

	var bw bodyWrapper
	// if request body is nil or NoBody, we don't want to mutate the body as it
	// will affect the identity of it in an unforeseeable way because we assert
	// ReadCloser fulfills a certain interface and it is indeed nil or NoBody.
	if r.Body != nil && r.Body != http.NoBody {
		bw.ReadCloser = r.Body
		bw.record = readRecordFunc
		r.Body = &bw
	}

	writeRecordFunc := func(int64) {}
	if h.writeEvent {
		writeRecordFunc = func(n int64) {
			span.AddEvent("write", trace.WithAttributes(WroteBytesKey.Int64(n)))
		}
	}

	rww := &respWriterWrapper{
		ResponseWriter: w,
		record:         writeRecordFunc,
		ctx:            ctx,
		props:          h.propagators,
		statusCode:     http.StatusOK, // default status code in case the Handler doesn't write anything
	}

	// Wrap w to use our ResponseWriter methods while also exposing
	// other interfaces that w may implement (http.CloseNotifier,
	// http.Flusher, http.Hijacker, http.Pusher, io.ReaderFrom).

	w = httpsnoop.Wrap(w, httpsnoop.Hooks{
		Header: func(httpsnoop.HeaderFunc) httpsnoop.HeaderFunc {
			return rww.Header
		},
		Write: func(httpsnoop.WriteFunc) httpsnoop.WriteFunc {
			return rww.Write
		},
		WriteHeader: func(httpsnoop.WriteHeaderFunc) httpsnoop.WriteHeaderFunc {
			return rww.WriteHeader
		},
		Flush: func(httpsnoop.FlushFunc) httpsnoop.FlushFunc {
			return rww.Flush
		},
	})

	labeler, found := LabelerFromContext(ctx)
	if !found {
		ctx = ContextWithLabeler(ctx, labeler)
	}

	next.ServeHTTP(w, r.WithContext(ctx))

	span.SetStatus(semconv.ServerStatus(rww.statusCode))
	span.SetAttributes(h.traceSemconv.ResponseTraceAttrs(semconv.ResponseTelemetry{
		StatusCode: rww.statusCode,
		ReadBytes:  bw.read.Load(),
		ReadError:  bw.err,
		WriteBytes: rww.written,
		WriteError: rww.err,
	})...)

	// Add metrics
	attributes := append(labeler.Get(), semconvutil.HTTPServerRequestMetrics(h.server, r)...)
	if rww.statusCode > 0 {
		attributes = append(attributes, semconv.HTTPStatusCode(rww.statusCode))
	}
	o := metric.WithAttributeSet(attribute.NewSet(attributes...))
	addOpts := []metric.AddOption{o} // Allocate vararg slice once.
	h.requestBytesCounter.Add(ctx, bw.read.Load(), addOpts...)
	h.responseBytesCounter.Add(ctx, rww.written, addOpts...)

	// Use floating point division here for higher precision (instead of Millisecond method).
	elapsedTime := float64(time.Since(requestStartTime)) / float64(time.Millisecond)

	h.serverLatencyMeasure.Record(ctx, elapsedTime, o)
}

// WithRouteTag annotates spans and metrics with the provided route name
// with HTTP route attribute.
func WithRouteTag(route string, h http.Handler) http.Handler {
	attr := semconv.NewHTTPServer().Route(route)
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		span := trace.SpanFromContext(r.Context())
		span.SetAttributes(attr)

		labeler, _ := LabelerFromContext(r.Context())
		labeler.Add(attr)

		h.ServeHTTP(w, r)
	})
}