package balancer

import (
	"io"
	"math"
	"net/http"
	"strings"
	"testing"

	"git.codelab.vc/pkg/httpx/middleware"
)

func mockTransport(fn func(*http.Request) (*http.Response, error)) http.RoundTripper {
	return middleware.RoundTripperFunc(fn)
}

func okResponse() *http.Response {
	return &http.Response{
		StatusCode: http.StatusOK,
		Body:       io.NopCloser(strings.NewReader("")),
		Header:     make(http.Header),
	}
}

func TestTransport_PicksEndpointAndReplacesURL(t *testing.T) {
	endpoints := []Endpoint{
		{URL: "https://backend1.example.com"},
	}

	var captured *http.Request
	base := mockTransport(func(req *http.Request) (*http.Response, error) {
		captured = req
		return okResponse(), nil
	})

	mw, _ := Transport(endpoints)
	rt := mw(base)

	req, err := http.NewRequest(http.MethodGet, "https://original.example.com/api/v1/users", nil)
	if err != nil {
		t.Fatal(err)
	}

	resp, err := rt.RoundTrip(req)
	if err != nil {
		t.Fatalf("unexpected error: %v", err)
	}
	defer resp.Body.Close()

	if captured == nil {
		t.Fatal("base transport was not called")
	}
	if captured.URL.Scheme != "https" {
		t.Errorf("scheme = %q, want %q", captured.URL.Scheme, "https")
	}
	if captured.URL.Host != "backend1.example.com" {
		t.Errorf("host = %q, want %q", captured.URL.Host, "backend1.example.com")
	}
	if captured.URL.Path != "/api/v1/users" {
		t.Errorf("path = %q, want %q", captured.URL.Path, "/api/v1/users")
	}
}

func TestTransport_ErrNoHealthyWhenNoEndpoints(t *testing.T) {
	var endpoints []Endpoint
	base := mockTransport(func(req *http.Request) (*http.Response, error) {
		t.Fatal("base transport should not be called")
		return nil, nil
	})

	mw, _ := Transport(endpoints)
	rt := mw(base)

	req, err := http.NewRequest(http.MethodGet, "https://example.com/test", nil)
	if err != nil {
		t.Fatal(err)
	}

	_, err = rt.RoundTrip(req)
	if err != ErrNoHealthy {
		t.Fatalf("err = %v, want %v", err, ErrNoHealthy)
	}
}

func TestRoundRobin_DistributesEvenly(t *testing.T) {
	endpoints := []Endpoint{
		{URL: "https://a.example.com"},
		{URL: "https://b.example.com"},
		{URL: "https://c.example.com"},
	}

	rr := RoundRobin()
	counts := make(map[string]int)

	const iterations = 300
	for i := 0; i < iterations; i++ {
		ep, err := rr.Next(endpoints)
		if err != nil {
			t.Fatalf("iteration %d: unexpected error: %v", i, err)
		}
		counts[ep.URL]++
	}

	expected := iterations / len(endpoints)
	for _, ep := range endpoints {
		got := counts[ep.URL]
		if got != expected {
			t.Errorf("endpoint %s: got %d calls, want %d", ep.URL, got, expected)
		}
	}
}

func TestRoundRobin_ErrNoHealthy(t *testing.T) {
	rr := RoundRobin()
	_, err := rr.Next(nil)
	if err != ErrNoHealthy {
		t.Fatalf("err = %v, want %v", err, ErrNoHealthy)
	}
}

func TestFailover_AlwaysPicksFirst(t *testing.T) {
	endpoints := []Endpoint{
		{URL: "https://primary.example.com"},
		{URL: "https://secondary.example.com"},
		{URL: "https://tertiary.example.com"},
	}

	fo := Failover()

	for i := 0; i < 10; i++ {
		ep, err := fo.Next(endpoints)
		if err != nil {
			t.Fatalf("iteration %d: unexpected error: %v", i, err)
		}
		if ep.URL != "https://primary.example.com" {
			t.Errorf("iteration %d: got %q, want %q", i, ep.URL, "https://primary.example.com")
		}
	}
}

func TestFailover_ErrNoHealthy(t *testing.T) {
	fo := Failover()
	_, err := fo.Next(nil)
	if err != ErrNoHealthy {
		t.Fatalf("err = %v, want %v", err, ErrNoHealthy)
	}
}

func TestWeightedRandom_RespectsWeights(t *testing.T) {
	endpoints := []Endpoint{
		{URL: "https://heavy.example.com", Weight: 80},
		{URL: "https://light.example.com", Weight: 20},
	}

	wr := WeightedRandom()
	counts := make(map[string]int)

	const iterations = 10000
	for i := 0; i < iterations; i++ {
		ep, err := wr.Next(endpoints)
		if err != nil {
			t.Fatalf("iteration %d: unexpected error: %v", i, err)
		}
		counts[ep.URL]++
	}

	totalWeight := 0
	for _, ep := range endpoints {
		totalWeight += ep.Weight
	}

	for _, ep := range endpoints {
		got := float64(counts[ep.URL]) / float64(iterations)
		want := float64(ep.Weight) / float64(totalWeight)
		if math.Abs(got-want) > 0.05 {
			t.Errorf("endpoint %s: got ratio %.3f, want ~%.3f (tolerance 0.05)", ep.URL, got, want)
		}
	}
}

func TestWeightedRandom_DefaultWeightForZero(t *testing.T) {
	endpoints := []Endpoint{
		{URL: "https://a.example.com", Weight: 0},
		{URL: "https://b.example.com", Weight: 0},
	}

	wr := WeightedRandom()
	counts := make(map[string]int)

	const iterations = 1000
	for i := 0; i < iterations; i++ {
		ep, err := wr.Next(endpoints)
		if err != nil {
			t.Fatalf("iteration %d: unexpected error: %v", i, err)
		}
		counts[ep.URL]++
	}

	// With equal default weights, distribution should be roughly even.
	for _, ep := range endpoints {
		got := float64(counts[ep.URL]) / float64(iterations)
		if math.Abs(got-0.5) > 0.1 {
			t.Errorf("endpoint %s: got ratio %.3f, want ~0.5 (tolerance 0.1)", ep.URL, got)
		}
	}
}

func TestWeightedRandom_ErrNoHealthy(t *testing.T) {
	wr := WeightedRandom()
	_, err := wr.Next(nil)
	if err != ErrNoHealthy {
		t.Fatalf("err = %v, want %v", err, ErrNoHealthy)
	}
}