package wal

import (
	"encoding/binary"
	"hash/crc32"
	"testing"
)

// Regression guard for C1: verify all WAL CRC uses Castagnoli polynomial
// (crc32c), not IEEE. Per design §3.2 line 359.
//
// The standard CRC-32C test vector from RFC 3720 Appendix B is the 9-byte
// ASCII string "123456789":
//   - crc32c:     0xE3069283
//   - crc32 IEEE: 0xCBF43926
//
// If anyone changes crc32cTable back to IEEE, this test fails immediately.
func TestCRC32CStandardVector(t *testing.T) {
	got := crc32.Checksum([]byte("123456789"), crc32cTable)
	const want = uint32(0xE3069283)
	if got != want {
		t.Errorf("crc32c('123456789') = 0x%X, want 0x%X (Castagnoli)", got, want)
	}
}

// Negative regression: confirm IEEE would produce a DIFFERENT value. This
// catches the case where someone "fixes" crc32cTable to use IEEE by mistake.
func TestCRC32CEdistinctFromIEEE(t *testing.T) {
	data := []byte("123456789")
	ieee := crc32.ChecksumIEEE(data)
	castagnoli := crc32.Checksum(data, crc32cTable)
	if ieee == castagnoli {
		t.Errorf("IEEE and Castagnoli produced the same CRC (impossible unless table is wrong); both = 0x%X", ieee)
	}
}

// Regression guard for C1: header CRC must be computed with crc32c, not IEEE.
// TestHeaderRoundtrip is self-consistent (encode + decode use same polynomial),
// so a paired reversion to IEEE would pass it. This test directly asserts
// the stored CRC matches Castagnoli.
func TestHeaderCRCUsesCastagnoli(t *testing.T) {
	hdr := &WalFileHeader{
		BlockSize:     32 * 1024,
		SegmentID:     42,
		StartSequence: 100,
	}
	encoded := EncodeWalHeader(hdr)
	want := crc32.Checksum(encoded[0:28], crc32cTable)
	got := binary.LittleEndian.Uint32(encoded[28:32])
	if got != want {
		t.Errorf("stored headerCRC = 0x%X, want crc32c value 0x%X", got, want)
	}
	// Sanity: confirm IEEE would produce a different value (catches paired reversion).
	ieeeValue := crc32.ChecksumIEEE(encoded[0:28])
	if got == ieeeValue {
		t.Errorf("stored headerCRC = 0x%X matches IEEE value (C1 regression)", got)
	}
}

// Regression guard for C1: physical record CRC must be Castagnoli.
// Same rationale as TestHeaderCRCUsesCastagnoli — direct value assertion
// catches paired encode/decode reversion.
func TestPhysicalRecordCRCUsesCastagnoli(t *testing.T) {
	payload := []byte("test-payload")
	encoded := EncodePhysicalRecord(RecFull, payload)
	// Record format: [crc u32][length u16][type u8][payload]
	want := crc32.Checksum(encoded[4:7+len(payload)], crc32cTable)
	got := binary.LittleEndian.Uint32(encoded[0:4])
	if got != want {
		t.Errorf("stored record CRC = 0x%X, want crc32c value 0x%X", got, want)
	}
	ieeeValue := crc32.ChecksumIEEE(encoded[4 : 7+len(payload)])
	if got == ieeeValue {
		t.Errorf("stored record CRC = 0x%X matches IEEE value (C1 regression)", got)
	}
}