package osd

import (
	"errors"
	"os"
	"path/filepath"
	"testing"
	"time"
)

func testSnapshot() Snapshot {
	var snapshot Snapshot
	snapshot.Sequence = 0xbEEF
	snapshot.TimestampMS = 123456
	for i := range snapshot.Channels {
		snapshot.Channels[i] = int16(-1024 + i*137)
	}
	snapshot.Armed = true
	snapshot.Video = true
	snapshot.OpConfig = OperatorConfig{Table: Table{Gen: 1}, MaxPower: 95}
	snapshot.Policy = GSPolicy{Pending: true}
	snapshot.Ack = VTXAck{Pending: true}
	snapshot.Link = GSObservedLink{Known: false}
	snapshot.PublishedMS = time.Now().UnixMilli()
	return snapshot
}

func TestSnapshotSizeMatchesLayout(t *testing.T) {
	// Header (magic, version, sequence, timestamp) + all 16 channels +
	// flags + publish time + the four contract values + CRC must stay a
	// small fixed record.
	if Size != 50+1+12+17+14+8+2 {
		t.Fatalf("Size=%d must match the documented layout", Size)
	}
	if encoded := Encode(testSnapshot()); len(encoded) != Size {
		t.Fatalf("encoded=%d bytes want %d", len(encoded), Size)
	}
}

func TestEncodeDecodeSnapshotRoundtrip(t *testing.T) {
	want := testSnapshot()

	got, err := Decode(Encode(want))
	if err != nil {
		t.Fatal(err)
	}
	if got != want {
		t.Fatalf("decoded=%+v want %+v", got, want)
	}
}

func TestDecodeRejectsDamagedSnapshots(t *testing.T) {
	encoded := Encode(testSnapshot())

	if _, err := Decode(encoded[:len(encoded)-1]); !errors.Is(err, ErrLength) {
		t.Fatalf("truncated decode=%v want ErrLength", err)
	}
	magic := append([]byte(nil), encoded...)
	magic[1] = 'X'
	if _, err := Decode(magic); !errors.Is(err, ErrMagic) {
		t.Fatalf("magic decode=%v want ErrMagic", err)
	}
	version := append([]byte(nil), encoded...)
	version[2] = Version + 1
	if _, err := Decode(version); !errors.Is(err, ErrVersion) {
		t.Fatalf("version decode=%v want ErrVersion", err)
	}
	corrupt := append([]byte(nil), encoded...)
	corrupt[10] ^= 0xff
	if _, err := Decode(corrupt); !errors.Is(err, ErrCRC) {
		t.Fatalf("corrupt payload decode=%v want ErrCRC", err)
	}
	corruptCRC := append([]byte(nil), encoded...)
	corruptCRC[len(corruptCRC)-1] ^= 0xff
	if _, err := Decode(corruptCRC); !errors.Is(err, ErrCRC) {
		t.Fatalf("corrupt crc decode=%v want ErrCRC", err)
	}
}

func TestFileSourceAgeIsNotResetByRewritingIdenticalBytes(t *testing.T) {
	// Publisher heartbeats rewrite the last snapshot's identical bytes; the
	// display's staleness must key on the snapshot's own publish time, so a
	// rewrite can never present stale data as current.
	path := filepath.Join(t.TempDir(), "osd-state.bin")
	now := time.UnixMilli(10_000_000)
	source := NewFileSource(path)
	source.now = func() time.Time { return now }

	want := testSnapshot()
	want.PublishedMS = now.UnixMilli()
	if err := os.WriteFile(path, Encode(want), 0644); err != nil {
		t.Fatal(err)
	}
	if _, state := source.Latest(); state != SnapshotFresh {
		t.Fatalf("state=%v want fresh at publish time", state)
	}

	// Heartbeat rewrite of identical bytes at a later wall-clock time.
	if err := os.WriteFile(path, Encode(want), 0644); err != nil {
		t.Fatal(err)
	}
	source.now = func() time.Time { return now.Add(2 * Freshness) }
	got, state := source.Latest()
	if state != SnapshotStale {
		t.Fatalf("state=%v want stale despite the heartbeat rewrite", state)
	}
	if got != want {
		t.Fatalf("last-known snapshot=%+v want %+v", got, want)
	}
}

func TestFileSourceReportsFreshStaleAndMissingStates(t *testing.T) {
	path := filepath.Join(t.TempDir(), "osd-state.bin")
	source := NewFileSource(path)
	fixed := time.UnixMilli(10_000_000)
	source.now = func() time.Time { return fixed }

	if _, state := source.Latest(); state != SnapshotNone {
		t.Fatalf("a missing state file must report state %v, got %v", SnapshotNone, state)
	}
	if err := os.WriteFile(path, []byte("garbage that is not a snapshot"), 0644); err != nil {
		t.Fatal(err)
	}
	if _, state := source.Latest(); state != SnapshotNone {
		t.Fatalf("a corrupt state file must report state %v, got %v", SnapshotNone, state)
	}

	want := testSnapshot()
	want.PublishedMS = fixed.UnixMilli()
	if err := os.WriteFile(path, Encode(want), 0644); err != nil {
		t.Fatal(err)
	}
	got, state := source.Latest()
	if state != SnapshotFresh {
		t.Fatalf("a fresh snapshot must report state %v, got %v", SnapshotFresh, state)
	}
	if got != want {
		t.Fatalf("got=%+v want %+v", got, want)
	}

	// The last successfully decoded snapshot must survive past the
	// freshness window, marked stale instead of being hidden: the display
	// shows last-known values as explicitly stale.
	stale := want
	stale.PublishedMS = fixed.Add(-Freshness - time.Second).UnixMilli()
	if err := os.WriteFile(path, Encode(stale), 0644); err != nil {
		t.Fatal(err)
	}
	got, state = source.Latest()
	if state != SnapshotStale {
		t.Fatalf("a snapshot beyond the freshness window must report state %v, got %v", SnapshotStale, state)
	}
	if got != stale {
		t.Fatalf("last-known snapshot=%+v want %+v", got, stale)
	}
}