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

package dataplane

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

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

// parseReportPart decodes one TRAIN_REPORT payload back into rows, checking the header.
func parseReportPart(t *testing.T, b []byte) (id uint32, received int, part, parts int, truncated bool, rows []session.TrainEntry) {
	t.Helper()
	if len(b) < trainReportHeader {
		t.Fatalf("report part shorter than its header: %d", len(b))
	}
	id = binary.BigEndian.Uint32(b[0:4])
	received = int(binary.BigEndian.Uint32(b[4:8]))
	part = int(binary.BigEndian.Uint16(b[8:10]))
	parts = int(binary.BigEndian.Uint16(b[10:12]))
	truncated = b[12]&1 != 0
	n := int(binary.BigEndian.Uint16(b[14:16]))
	if want := trainReportHeader + n*trainReportRow; len(b) != want {
		t.Fatalf("part length %d, want %d for %d rows", len(b), want, n)
	}
	off := trainReportHeader
	rows = make([]session.TrainEntry, n)
	for i := range rows {
		rows[i].Seq = binary.BigEndian.Uint32(b[off+i*4:])
	}
	off += n * 4
	for i := range rows {
		rows[i].TRxNs = int64(binary.BigEndian.Uint64(b[off+i*8:]))
	}
	off += n * 8
	for i := range rows {
		rows[i].Size = binary.BigEndian.Uint16(b[off+i*2:])
	}
	off += n * 2
	for i := range rows {
		rows[i].TTL = b[off+i]
	}
	off += n
	for i := range rows {
		rows[i].DSCP = b[off+i]
	}
	off += n
	for i := range rows {
		rows[i].ECN = b[off+i]
	}
	return
}

func TestBuildTrainReportSplitsAndRoundTrips(t *testing.T) {
	const count = 250 // enough to need several parts
	train := session.Train{ID: 42, Received: count, Truncated: true}
	for i := 0; i < count; i++ {
		train.Entries = append(train.Entries, session.TrainEntry{
			Seq: uint32(i), TRxNs: int64(i) * 1_000_000, Size: uint16(100 + i),
			TTL: 64, DSCP: 46, ECN: 1,
		})
	}

	parts := buildTrainReport(train)
	if len(parts) < 2 {
		t.Fatalf("250 rows should not fit one ≤%d-byte datagram", trainReportMaxDatagram)
	}
	var got []session.TrainEntry
	for i, p := range parts {
		if HeaderSize+len(p) > trainReportMaxDatagram {
			t.Fatalf("part %d would be a %d-byte datagram, cap is %d", i, HeaderSize+len(p), trainReportMaxDatagram)
		}
		id, received, part, total, truncated, rows := parseReportPart(t, p)
		if id != 42 || received != count || part != i || total != len(parts) || !truncated {
			t.Fatalf("part %d header: id=%d received=%d part=%d/%d truncated=%v",
				i, id, received, part, total, truncated)
		}
		got = append(got, rows...)
	}
	if len(got) != count {
		t.Fatalf("round-tripped %d rows, want %d", len(got), count)
	}
	for i, r := range got {
		want := train.Entries[i]
		if r != want {
			t.Fatalf("row %d = %+v, want %+v", i, r, want)
		}
	}
}

func TestBuildTrainReportEmptyTrainStillAnswers(t *testing.T) {
	parts := buildTrainReport(session.Train{ID: 9})
	if len(parts) != 1 {
		t.Fatalf("empty train: %d parts, want 1 — 'nothing arrived' is the answer, not silence", len(parts))
	}
	id, received, _, total, _, rows := parseReportPart(t, parts[0])
	if id != 9 || received != 0 || total != 1 || len(rows) != 0 {
		t.Fatalf("empty report: id=%d received=%d parts=%d rows=%d", id, received, total, len(rows))
	}
}

func TestTrainDataThenReportOverTheWire(t *testing.T) {
	mgr, addr := startServer(t)
	sess, _, err := mgr.New("dev1", "credential-ikm", netip.MustParseAddr("127.0.0.1"))
	if err != nil {
		t.Fatal(err)
	}
	client, err := net.DialUDP("udp", nil, net.UDPAddrFromAddrPort(addr))
	if err != nil {
		t.Fatal(err)
	}
	defer client.Close()
	client.SetDeadline(time.Now().Add(3 * time.Second))

	// A short train: id 5 in payload[0:4], plus padding.
	const trainID, count = 5, 4
	for i := 0; i < count; i++ {
		payload := make([]byte, 60)
		binary.BigEndian.PutUint32(payload[0:4], trainID)
		if _, err := client.Write(craft(t, sess, TypeTrainData, uint32(i+1), payload)); err != nil {
			t.Fatal(err)
		}
	}
	// TRAIN_DATA must be silent (spec §3.2).
	client.SetReadDeadline(time.Now().Add(300 * time.Millisecond))
	if _, err := client.Read(make([]byte, 1500)); err == nil {
		t.Fatal("TRAIN_DATA got a response, want none")
	}

	// Ask for the report.
	reqPayload := make([]byte, 4)
	binary.BigEndian.PutUint32(reqPayload, trainID)
	client.SetDeadline(time.Now().Add(3 * time.Second))
	if _, err := client.Write(craft(t, sess, TypeTrainReportReq, 100, reqPayload)); err != nil {
		t.Fatal(err)
	}
	buf := make([]byte, 2000)
	n, err := client.Read(buf)
	if err != nil {
		t.Fatalf("no TRAIN_REPORT: %v", err)
	}
	if buf[4] != TypeTrainReport {
		t.Fatalf("type = %#x, want TRAIN_REPORT", buf[4])
	}
	id, received, part, parts, truncated, rows := parseReportPart(t, buf[HeaderSize:n])
	if id != trainID || received != count || part != 0 || parts != 1 || truncated {
		t.Fatalf("report header: id=%d received=%d part=%d/%d truncated=%v", id, received, part, parts, truncated)
	}
	if len(rows) != count {
		t.Fatalf("%d rows, want %d", len(rows), count)
	}
	for i, r := range rows {
		if r.Seq != uint32(i+1) {
			t.Fatalf("row %d seq = %d, want %d", i, r.Seq, i+1)
		}
		if r.Size != HeaderSize+60 {
			t.Fatalf("row %d size = %d, want %d", i, r.Size, HeaderSize+60)
		}
	}

	// Unknown train id: one zero-row report, not silence.
	binary.BigEndian.PutUint32(reqPayload, 999)
	if _, err := client.Write(craft(t, sess, TypeTrainReportReq, 101, reqPayload)); err != nil {
		t.Fatal(err)
	}
	n, err = client.Read(buf)
	if err != nil {
		t.Fatalf("no report for unknown train: %v", err)
	}
	id, received, _, _, _, rows = parseReportPart(t, buf[HeaderSize:n])
	if id != 999 || received != 0 || len(rows) != 0 {
		t.Fatalf("unknown-train report: id=%d received=%d rows=%d", id, received, len(rows))
	}
}