/**
 * Solana tx wire-format helpers (pure byte ops, no @solana/web3.js).
 * Wire format: [shortvec sigCount] [sigCount × 64-byte sigs] [message bytes].
 * Used to confirm a wallet only ADDED a signature and didn't change the message.
 */

/** Offset where the message begins, or -1 if the tx bytes are malformed. */
export function solanaMessageOffset(serializedTx: Uint8Array): number {
  let offset = 0
  let sigCount = 0
  let shift = 0
  let terminated = false
  // shortvec: little-endian base-128, continuation bit 0x80.
  while (offset < serializedTx.length) {
    const byte = serializedTx[offset]!
    sigCount |= (byte & 0x7f) << shift
    offset++
    if ((byte & 0x80) === 0) {
      terminated = true
      break
    }
    shift += 7
    if (shift >= 32) return -1
  }
  if (!terminated) return -1
  const messageStart = offset + sigCount * 64
  return messageStart >= serializedTx.length ? -1 : messageStart
}

/** V0 messages set the high bit (0x80) on the first message byte; legacy don't. */
export function isVersionedTransactionBytes(serializedTx: Uint8Array): boolean {
  const messageStart = solanaMessageOffset(serializedTx)
  if (messageStart < 0) return false
  return (serializedTx[messageStart]! & 0x80) !== 0
}

/**
 * Throw unless `returned` keeps the byte-identical message of `sent` — signing
 * may only add a signature. Compares the message region only (sigs differ).
 */
export function assertSameMessage(
  sent: Uint8Array,
  returned: Uint8Array
): void {
  const sentStart = solanaMessageOffset(sent)
  const returnedStart = solanaMessageOffset(returned)
  if (sentStart < 0 || returnedStart < 0) {
    throw new Error('Malformed Solana transaction: cannot locate message bytes')
  }
  // Signing fills a pre-allocated slot, so the sig count (hence the offset)
  // can't change. A different offset = added/dropped signer → reject.
  if (sentStart !== returnedStart) {
    throw new Error(
      'Signed transaction does not match the request — signature layout changed'
    )
  }
  const sentMsg = sent.subarray(sentStart)
  const returnedMsg = returned.subarray(returnedStart)
  const tampered =
    sentMsg.length !== returnedMsg.length ||
    sentMsg.some((byte, i) => byte !== returnedMsg[i])
  if (tampered) {
    throw new Error(
      'Signed transaction does not match the request — wallet returned a different transaction'
    )
  }
}