// SPDX-FileCopyrightText: 2026 Echolot contributors
// SPDX-License-Identifier: GPL-3.0-or-later

package control

import (
	"net/netip"
	"testing"
	"time"

	"echo-lot.app/server/internal/session"
)

// The DF ceiling is the difference between "the client's path cannot carry this" and "we could
// never have sent it in the first place". Getting the header arithmetic wrong would silently
// attribute a server limit to the client's network, so it is pinned here.
func TestMaxDFPayload(t *testing.T) {
	mgr := session.NewManager(time.Minute)

	newSess := func(src string) *session.Session {
		s, _, err := mgr.New("dev", "cred", netip.MustParseAddr("203.0.113.9"))
		if err != nil {
			t.Fatalf("new session: %v", err)
		}
		if src != "" {
			s.NoteDataSource(netip.MustParseAddrPort(src))
		}
		return s
	}

	cases := []struct {
		name string
		mtu  func() int
		src  string
		want int
	}{
		{"no egress mtu hook means no clamp", nil, "198.51.100.4:5000", 0},
		{"unknown egress mtu means no clamp", func() int { return 0 }, "198.51.100.4:5000", 0},
		{"ipv4 subtracts ip+udp", func() int { return 1500 }, "198.51.100.4:5000", 1472},
		{"ipv6 subtracts the larger header", func() int { return 1500 }, "[2001:db8::4]:5000", 1452},
		{"pppoe-style 1492 egress", func() int { return 1492 }, "198.51.100.4:5000", 1464},
		{"no data source yet falls back to ipv4 overhead", func() int { return 1500 }, "", 1472},
	}
	for _, tc := range cases {
		t.Run(tc.name, func(t *testing.T) {
			s := &Server{EgressMTU: tc.mtu}
			if got := s.maxDFPayload(newSess(tc.src)); got != tc.want {
				t.Fatalf("maxDFPayload = %d, want %d", got, tc.want)
			}
		})
	}
}