package checkpoint_test

import (
	"bytes"
	"crypto/ed25519"
	"crypto/rand"
	"testing"

	"git.n1ko.dev/Niko/niko_trust/internal/checkpoint"
)

func testCkpt() *checkpoint.Checkpoint {
	var root, prev [32]byte
	rand.Read(root[:])
	rand.Read(prev[:])
	return &checkpoint.Checkpoint{
		Epoch:     3,
		Size:      42,
		Root:      root,
		Prev:      prev,
		CreatedAt: 1_700_000_000,
	}
}

func TestEncodeDecodeRoundTrip(t *testing.T) {
	c := testCkpt()
	b, err := c.Encode()
	if err != nil {
		t.Fatal(err)
	}
	got, err := checkpoint.Decode(b)
	if err != nil {
		t.Fatal(err)
	}
	if got.Epoch != c.Epoch || got.Size != c.Size || got.CreatedAt != c.CreatedAt {
		t.Fatal("field mismatch")
	}
	if got.Root != c.Root || got.Prev != c.Prev {
		t.Fatal("hash mismatch")
	}
	again, _ := c.Encode()
	if !bytes.Equal(b, again) {
		t.Fatal("encoding nondeterministic")
	}
}

func TestGenesisPrevIsZeros(t *testing.T) {
	c := &checkpoint.Checkpoint{Epoch: 1, Size: 0, CreatedAt: 1_700_000_000}
	if c.Prev != ([32]byte{}) {
		t.Fatal("genesis prev must be zeros")
	}
}

func TestTamperedBytesRejected(t *testing.T) {
	c := testCkpt()
	pub, priv, _ := ed25519.GenerateKey(rand.Reader)
	b, err := c.Encode()
	if err != nil {
		t.Fatal(err)
	}
	sig, err := checkpoint.Sign(priv, b)
	if err != nil {
		t.Fatal(err)
	}
	if err := checkpoint.Verify(pub, b, sig); err != nil {
		t.Fatal("honest signature rejected")
	}
	for i := len(checkpoint.Magic); i < len(b); i++ {
		bad := append([]byte(nil), b...)
		bad[i] ^= 0x01
		if checkpoint.Verify(pub, bad, sig) == nil {
			t.Fatalf("tampered byte %d verified", i)
		}
		// A mutated body is also a different checkpoint entirely.
		if _, err := checkpoint.Decode(bad); err == nil && i >= len(b)-1 {
			// Mutating created_at's last byte may still decode if it stays
			// in range; signature binding covers that case above.
			continue
		}
	}
}

func TestDecodeRejectsGarbage(t *testing.T) {
	cases := [][]byte{
		nil,
		{},
		bytes.Repeat([]byte{0x00}, 21),
		append(append([]byte(nil), checkpoint.Magic...), 0x02), // unknown version
		checkpoint.Magic[:10],
	}
	for i, b := range cases {
		if _, err := checkpoint.Decode(b); err == nil {
			t.Errorf("case %d decoded garbage", i)
		}
	}
	// Trailing byte after the last field.
	c := testCkpt()
	b, _ := c.Encode()
	if _, err := checkpoint.Decode(append(b, 0x00)); err == nil {
		t.Error("trailing byte accepted")
	}
}

func TestChainLinkage(t *testing.T) {
	c1 := &checkpoint.Checkpoint{Epoch: 1, Size: 5, CreatedAt: 1_700_000_000}
	b1, _ := c1.Encode()
	id1 := checkpoint.ID(b1)

	c2 := &checkpoint.Checkpoint{Epoch: 2, Size: 6, Root: id1, Prev: id1, CreatedAt: 1_700_000_060}
	if c2.Prev != id1 {
		t.Fatal("prev must equal previous checkpoint's ID")
	}
}

func TestSignKeySizeEnforced(t *testing.T) {
	b, _ := testCkpt().Encode()
	if _, err := checkpoint.Sign(ed25519.PrivateKey(make([]byte, 10)), b); err == nil {
		t.Fatal("short private key accepted")
	}
	if err := checkpoint.Verify(ed25519.PublicKey(make([]byte, 10)), b, make([]byte, 64)); err == nil {
		t.Fatal("short public key accepted")
	}
	if err := checkpoint.Verify(ed25519.PublicKey(make([]byte, 32)), b, make([]byte, 63)); err == nil {
		t.Fatal("short signature accepted")
	}
}