defmodule Polymarket.RLP do
  @moduledoc """
  Hand-rolled RLP (Recursive Length Prefix) **encoder** for Ethereum
  legacy (type-0) transaction serialization.

  This module implements only the encoding side — there is no decoder.
  The set of inputs we encode is narrow: bytes, non-negative integers,
  and lists of already-encoded items. That's enough for legacy
  EIP-155 transactions and pure RLP test vectors.

  ## Encoding rules (per the Ethereum spec)

  Bytes / strings:

    * Empty: `0x80`.
    * Single byte `b` where `0x00 <= b <= 0x7f`: encoded as itself
      (single byte, no length prefix).
    * Other byte strings of length `len`:
      * `1 <= len <= 55`: `<<0x80 + len, ...bytes>>`.
      * `len > 55`: `<<0xb7 + size_of_len, ...len_bytes, ...bytes>>`
        where `len_bytes` is the minimal big-endian encoding of `len`.

  Lists with combined-payload size `len`:

    * `0 <= len <= 55`: `<<0xc0 + len, ...payload>>`.
    * `len > 55`: `<<0xf7 + size_of_len, ...len_bytes, ...payload>>`.

  Non-negative integers are encoded as their minimal-byte big-endian
  representation, then RLP-encoded as a byte string. Zero collapses to
  the empty string `0x80` — the integer `0` does NOT encode as
  `<<0x00>>`.

  Output is byte-for-byte verified against `ethers.encodeRlp` in
  `test/fixtures/tx.json` § rlp_primitives.
  """

  @doc """
  Encodes a binary as an RLP string.
  """
  @spec encode_bytes(binary()) :: binary()
  def encode_bytes(<<>>), do: <<0x80>>
  def encode_bytes(<<b>>) when b < 0x80, do: <<b>>

  def encode_bytes(bin) when is_binary(bin) and byte_size(bin) <= 55 do
    <<0x80 + byte_size(bin)>> <> bin
  end

  def encode_bytes(bin) when is_binary(bin) do
    len_bytes = :binary.encode_unsigned(byte_size(bin), :big)
    <<0xB7 + byte_size(len_bytes)>> <> len_bytes <> bin
  end

  @doc """
  Encodes a non-negative integer as an RLP byte string carrying the
  minimal big-endian representation. Zero encodes as `0x80` (empty
  string), matching the canonical Ethereum/RLP convention.
  """
  @spec encode_integer(non_neg_integer()) :: binary()
  def encode_integer(0), do: <<0x80>>

  def encode_integer(n) when is_integer(n) and n > 0 do
    encode_bytes(:binary.encode_unsigned(n, :big))
  end

  @doc """
  Encodes a list of already-RLP-encoded items as an RLP list.

  The caller is responsible for encoding each element first
  (`encode_bytes/1`, `encode_integer/1`, or a nested `encode_list/1`).
  This module deliberately keeps element-encoding decisions in the
  caller — there is no value tagging that would let one function
  decide between "string" and "integer" interpretations of arbitrary
  Elixir terms.
  """
  @spec encode_list([binary()]) :: binary()
  def encode_list(items) when is_list(items) do
    payload = IO.iodata_to_binary(items)
    payload_size = byte_size(payload)

    if payload_size <= 55 do
      <<0xC0 + payload_size>> <> payload
    else
      len_bytes = :binary.encode_unsigned(payload_size, :big)
      <<0xF7 + byte_size(len_bytes)>> <> len_bytes <> payload
    end
  end

  @doc """
  Hex-encodes raw RLP output for human display or fixture comparison.
  Always lowercase, always `0x`-prefixed.
  """
  @spec to_hex(binary()) :: String.t()
  def to_hex(bin) when is_binary(bin) do
    "0x" <> Base.encode16(bin, case: :lower)
  end
end