defmodule CljElixir.Reader do
  @moduledoc """
  Reader for CljElixir: tokenizes source text and parses it into CljElixir AST.

  The reader has two phases:
  1. Tokenizer — converts source text into a flat list of tokens
  2. Parser — recursive descent over the token list, producing CljElixir AST nodes

  ## AST representation

  Literals are themselves: integers, floats, strings, booleans, nil, atoms (keywords).

  Compound forms use tagged tuples:
    {:symbol, meta, name}
    {:list, meta, [elements]}
    {:vector, meta, [elements]}
    {:map, meta, [k1, v1, k2, v2, ...]}
    {:set, meta, [elements]}
    {:tuple, meta, [elements]}
    {:regex, meta, pattern}
    {:quote, meta, form}
    {:with_meta, meta, {metadata, target}}
    {:anon_fn, meta, body}
    {:quasiquote, meta, form}
    {:unquote, meta, form}
    {:splice_unquote, meta, form}
    {:deref, meta, form}
  """

  alias CljElixir.Reader.Token

  # ── Public API ──────────────────────────────────────────────────────

  @doc """
  Read a string of CljElixir source into a list of AST forms.

  Returns `{:ok, [form]}` on success, `{:error, message}` on failure.
  """
  @spec read_string(String.t()) :: {:ok, list()} | {:error, String.t()}
  def read_string(source) when is_binary(source) do
    case tokenize(source) do
      {:ok, tokens} ->
        parse_all(tokens, [])

      {:error, _} = err ->
        err
    end
  end

  # ════════════════════════════════════════════════════════════════════
  # TOKENIZER
  # ════════════════════════════════════════════════════════════════════

  @doc false
  def tokenize(source) do
    chars = String.to_charlist(source)
    tokenize_loop(chars, 1, 1, [])
  end

  # ---------- end of input ----------
  defp tokenize_loop([], _line, _col, acc), do: {:ok, Enum.reverse(acc)}

  # ---------- newline ----------
  defp tokenize_loop([?\n | rest], line, _col, acc),
    do: tokenize_loop(rest, line + 1, 1, acc)

  defp tokenize_loop([?\r, ?\n | rest], line, _col, acc),
    do: tokenize_loop(rest, line + 1, 1, acc)

  defp tokenize_loop([?\r | rest], line, _col, acc),
    do: tokenize_loop(rest, line + 1, 1, acc)

  # ---------- whitespace / commas ----------
  defp tokenize_loop([c | rest], line, col, acc) when c in [?\s, ?\t, ?,],
    do: tokenize_loop(rest, line, col + 1, acc)

  # ---------- comments ----------
  defp tokenize_loop([?; | rest], line, _col, acc) do
    rest = skip_comment(rest)
    # skip_comment stops at (but does not consume) the newline or EOF.
    # Let the main loop's newline handler increment line/col.
    tokenize_loop(rest, line, 1, acc)
  end

  # ---------- strings ----------
  defp tokenize_loop([?" | rest], line, col, acc) do
    case read_string_literal(rest, line, col + 1, [])  do
      {:ok, value, rest2, end_line, end_col} ->
        token = %Token{type: :string, value: value, line: line, col: col}
        tokenize_loop(rest2, end_line, end_col, [token | acc])

      {:error, msg} ->
        {:error, msg}
    end
  end

  # ---------- dispatch sequences: #{ #el[ #( #" ----------
  defp tokenize_loop([?#, ?e, ?l, ?[ | rest], line, col, acc) do
    token = %Token{type: :hash_el_lbracket, value: "#el[", line: line, col: col}
    tokenize_loop(rest, line, col + 4, [token | acc])
  end

  defp tokenize_loop([?#, ?{ | rest], line, col, acc) do
    token = %Token{type: :hash_lbrace, value: "\#{", line: line, col: col}
    tokenize_loop(rest, line, col + 2, [token | acc])
  end

  defp tokenize_loop([?#, ?( | rest], line, col, acc) do
    token = %Token{type: :hash_lparen, value: "#(", line: line, col: col}
    tokenize_loop(rest, line, col + 2, [token | acc])
  end

  defp tokenize_loop([?#, ?" | rest], line, col, acc) do
    case read_string_literal(rest, line, col + 2, []) do
      {:ok, value, rest2, end_line, end_col} ->
        token = %Token{type: :hash_string, value: value, line: line, col: col}
        tokenize_loop(rest2, end_line, end_col, [token | acc])

      {:error, msg} ->
        {:error, msg}
    end
  end

  # ---------- splice-unquote ~@ (must come before unquote ~) ----------
  defp tokenize_loop([?~, ?@ | rest], line, col, acc) do
    token = %Token{type: :splice_unquote, value: "~@", line: line, col: col}
    tokenize_loop(rest, line, col + 2, [token | acc])
  end

  # ---------- unquote ~ ----------
  defp tokenize_loop([?~ | rest], line, col, acc) do
    token = %Token{type: :unquote, value: "~", line: line, col: col}
    tokenize_loop(rest, line, col + 1, [token | acc])
  end

  # ---------- delimiters ----------
  defp tokenize_loop([?( | rest], line, col, acc) do
    token = %Token{type: :lparen, value: "(", line: line, col: col}
    tokenize_loop(rest, line, col + 1, [token | acc])
  end

  defp tokenize_loop([?) | rest], line, col, acc) do
    token = %Token{type: :rparen, value: ")", line: line, col: col}
    tokenize_loop(rest, line, col + 1, [token | acc])
  end

  defp tokenize_loop([?[ | rest], line, col, acc) do
    token = %Token{type: :lbracket, value: "[", line: line, col: col}
    tokenize_loop(rest, line, col + 1, [token | acc])
  end

  defp tokenize_loop([?] | rest], line, col, acc) do
    token = %Token{type: :rbracket, value: "]", line: line, col: col}
    tokenize_loop(rest, line, col + 1, [token | acc])
  end

  defp tokenize_loop([?{ | rest], line, col, acc) do
    token = %Token{type: :lbrace, value: "{", line: line, col: col}
    tokenize_loop(rest, line, col + 1, [token | acc])
  end

  defp tokenize_loop([?} | rest], line, col, acc) do
    token = %Token{type: :rbrace, value: "}", line: line, col: col}
    tokenize_loop(rest, line, col + 1, [token | acc])
  end

  # ---------- quote ' ----------
  defp tokenize_loop([?' | rest], line, col, acc) do
    token = %Token{type: :quote, value: "'", line: line, col: col}
    tokenize_loop(rest, line, col + 1, [token | acc])
  end

  # ---------- quasiquote ` ----------
  defp tokenize_loop([?` | rest], line, col, acc) do
    token = %Token{type: :quasiquote, value: "`", line: line, col: col}
    tokenize_loop(rest, line, col + 1, [token | acc])
  end

  # ---------- metadata ^ ----------
  defp tokenize_loop([?^ | rest], line, col, acc) do
    token = %Token{type: :meta, value: "^", line: line, col: col}
    tokenize_loop(rest, line, col + 1, [token | acc])
  end

  # ---------- deref @ ----------
  defp tokenize_loop([?@ | rest], line, col, acc) do
    token = %Token{type: :deref, value: "@", line: line, col: col}
    tokenize_loop(rest, line, col + 1, [token | acc])
  end

  # ---------- keywords ----------
  defp tokenize_loop([?: | rest], line, col, acc) do
    case read_keyword(rest, line, col) do
      {:ok, kw_value, rest2, end_col} ->
        token = %Token{type: :keyword, value: kw_value, line: line, col: col}
        tokenize_loop(rest2, line, end_col, [token | acc])

      {:error, msg} ->
        {:error, msg}
    end
  end

  # ---------- negative numbers: -<digit> ----------
  # Since whitespace is always consumed before reaching tokenize_loop,
  # a standalone `-` followed by a digit is always a negative number literal.
  # The `-` inside symbol names (like `my-func`) is consumed by the symbol reader
  # and never reaches this clause as a standalone character.
  defp tokenize_loop([?- | rest], line, col, acc) do
    if starts_with_digit?(rest) do
      {:ok, token, rest2, end_col} = read_number(rest, line, col + 1, [?-])
      token = %{token | line: line, col: col}
      tokenize_loop(rest2, line, end_col, [token | acc])
    else
      # It's a symbol starting with -
      case read_symbol([?- | rest], line, col) do
        {:ok, token, rest2, end_col} ->
          tokenize_loop(rest2, line, end_col, [token | acc])
      end
    end
  end

  # ---------- numbers ----------
  defp tokenize_loop([c | _] = chars, line, col, acc) when c in ?0..?9 do
    {:ok, token, rest2, end_col} = read_number(chars, line, col, [])
    tokenize_loop(rest2, line, end_col, [token | acc])
  end

  # ---------- symbols (and true/false/nil) ----------
  defp tokenize_loop([c | _] = chars, line, col, acc)
       when c in ?a..?z or c in ?A..?Z or
              c == ?_ or c == ?* or c == ?! or c == ?? or
              c == ?< or c == ?> or c == ?= or c == ?+ or
              c == ?. or c == ?& or c == ?% do
    case read_symbol(chars, line, col) do
      {:ok, token, rest, end_col} ->
        tokenize_loop(rest, line, end_col, [token | acc])
    end
  end

  # ---------- catch-all: unexpected character ----------
  defp tokenize_loop([c | _], line, col, _acc) do
    {:error, "Unexpected character '#{<<c::utf8>>}' at line #{line}, col #{col}"}
  end

  # ── Tokenizer helpers ───────────────────────────────────────────────

  # Characters that can continue a symbol (after the start)
  defp symbol_continue_char?(c) when c in ?a..?z, do: true
  defp symbol_continue_char?(c) when c in ?A..?Z, do: true
  defp symbol_continue_char?(c) when c in ?0..?9, do: true
  defp symbol_continue_char?(c) when c in [?_, ?*, ?!, ??, ?<, ?>, ?=, ?+, ?-, ?/, ?., ?%, ?&, ?#], do: true
  defp symbol_continue_char?(_), do: false

  defp starts_with_digit?([c | _]) when c in ?0..?9, do: true
  defp starts_with_digit?(_), do: false

  defp skip_comment([?\n | _] = rest), do: rest
  defp skip_comment([?\r | _] = rest), do: rest
  defp skip_comment([]), do: []
  defp skip_comment([_ | rest]), do: skip_comment(rest)

  # ── String literal reader ──────────────────────────────────────────

  defp read_string_literal([], line, _col, _acc),
    do: {:error, "Unterminated string starting at line #{line}"}

  defp read_string_literal([?" | rest], line, col, acc),
    do: {:ok, IO.chardata_to_string(Enum.reverse(acc)), rest, line, col + 1}

  defp read_string_literal([?\\, ?" | rest], line, col, acc),
    do: read_string_literal(rest, line, col + 2, [?" | acc])

  defp read_string_literal([?\\, ?\\ | rest], line, col, acc),
    do: read_string_literal(rest, line, col + 2, [?\\ | acc])

  defp read_string_literal([?\\, ?n | rest], line, col, acc),
    do: read_string_literal(rest, line, col + 2, [?\n | acc])

  defp read_string_literal([?\\, ?t | rest], line, col, acc),
    do: read_string_literal(rest, line, col + 2, [?\t | acc])

  defp read_string_literal([?\\, ?r | rest], line, col, acc),
    do: read_string_literal(rest, line, col + 2, [?\r | acc])

  defp read_string_literal([?\n | rest], line, _col, acc),
    do: read_string_literal(rest, line + 1, 1, [?\n | acc])

  defp read_string_literal([c | rest], line, col, acc),
    do: read_string_literal(rest, line, col + 1, [c | acc])

  # ── Keyword reader ─────────────────────────────────────────────────

  # Quoted keyword: :"some-name"
  defp read_keyword([?" | rest], line, col) do
    case read_string_literal(rest, line, col + 2, []) do
      {:ok, value, rest2, _end_line, end_col} ->
        {:ok, String.to_atom(value), rest2, end_col}

      {:error, msg} ->
        {:error, msg}
    end
  end

  # Regular keyword: :name, :my-key, :ok
  defp read_keyword(chars, _line, col) do
    {name_chars, rest} = take_keyword_chars(chars, [])

    case name_chars do
      [] ->
        {:error, "Expected keyword name after ':'"}

      _ ->
        name = IO.chardata_to_string(Enum.reverse(name_chars))
        atom_val = String.to_atom(name)
        {:ok, atom_val, rest, col + 1 + length(name_chars)}
    end
  end

  defp take_keyword_chars([c | rest], acc) when c in ?a..?z or c in ?A..?Z or c in ?0..?9 or c in [?_, ?-, ?!, ??, ?., ?/, ?*, ?+, ?>, ?<, ?=, ?&, ?#],
    do: take_keyword_chars(rest, [c | acc])

  defp take_keyword_chars(rest, acc), do: {acc, rest}

  # ── Number reader ──────────────────────────────────────────────────

  defp read_number(chars, line, col, prefix) do
    {digit_chars, rest} = take_digits(chars, prefix)

    case rest do
      [?. | after_dot] ->
        case after_dot do
          [d | _] when d in ?0..?9 ->
            {frac_chars, rest2} = take_digits(after_dot, [?. | digit_chars])
            str = IO.chardata_to_string(Enum.reverse(frac_chars))
            {float_val, ""} = Float.parse(str)
            end_col = col + String.length(str) - length(prefix)
            token = %Token{type: :float, value: float_val, line: line, col: col}
            {:ok, token, rest2, end_col}

          _ ->
            # dot not followed by digit — just an integer, leave dot for next token
            str = IO.chardata_to_string(Enum.reverse(digit_chars))
            {int_val, ""} = Integer.parse(str)
            end_col = col + String.length(str) - length(prefix)
            token = %Token{type: :integer, value: int_val, line: line, col: col}
            {:ok, token, rest, end_col}
        end

      _ ->
        str = IO.chardata_to_string(Enum.reverse(digit_chars))
        {int_val, ""} = Integer.parse(str)
        end_col = col + String.length(str) - length(prefix)
        token = %Token{type: :integer, value: int_val, line: line, col: col}
        {:ok, token, rest, end_col}
    end
  end

  defp take_digits([c | rest], acc) when c in ?0..?9,
    do: take_digits(rest, [c | acc])

  defp take_digits(rest, acc), do: {acc, rest}

  # ── Symbol reader ──────────────────────────────────────────────────

  defp read_symbol(chars, line, col) do
    {sym_chars, rest} = take_symbol_chars(chars, [])
    name = IO.chardata_to_string(Enum.reverse(sym_chars))
    end_col = col + String.length(name)

    token =
      case name do
        "true" -> %Token{type: :boolean, value: true, line: line, col: col}
        "false" -> %Token{type: :boolean, value: false, line: line, col: col}
        "nil" -> %Token{type: :nil, value: nil, line: line, col: col}
        _ -> %Token{type: :symbol, value: name, line: line, col: col}
      end

    {:ok, token, rest, end_col}
  end

  defp take_symbol_chars([c | rest], acc) do
    if (acc == [] && symbol_start_char?(c)) || (acc != [] && symbol_continue_char?(c)) do
      take_symbol_chars(rest, [c | acc])
    else
      {acc, [c | rest]}
    end
  end

  defp take_symbol_chars([], acc), do: {acc, []}

  defp symbol_start_char?(c) when c in ?a..?z, do: true
  defp symbol_start_char?(c) when c in ?A..?Z, do: true
  defp symbol_start_char?(c) when c in [?_, ?*, ?!, ??, ?<, ?>, ?=, ?+, ?-, ?., ?&, ?%], do: true
  defp symbol_start_char?(_), do: false

  # ════════════════════════════════════════════════════════════════════
  # PARSER — Recursive Descent
  # ════════════════════════════════════════════════════════════════════

  # Parse all top-level forms until tokens are exhausted
  defp parse_all([], acc), do: {:ok, Enum.reverse(acc)}

  defp parse_all(tokens, acc) do
    case parse_form(tokens) do
      {:ok, form, rest} ->
        parse_all(rest, [form | acc])

      {:error, _} = err ->
        err
    end
  end

  # ── Parse a single form ────────────────────────────────────────────

  # Literals
  defp parse_form([%Token{type: :integer, value: v} | rest]),
    do: {:ok, v, rest}

  defp parse_form([%Token{type: :float, value: v} | rest]),
    do: {:ok, v, rest}

  defp parse_form([%Token{type: :string, value: v} | rest]),
    do: {:ok, v, rest}

  defp parse_form([%Token{type: :keyword, value: v} | rest]),
    do: {:ok, v, rest}

  defp parse_form([%Token{type: :boolean, value: v} | rest]),
    do: {:ok, v, rest}

  defp parse_form([%Token{type: :nil} | rest]),
    do: {:ok, nil, rest}

  # Symbol
  defp parse_form([%Token{type: :symbol, value: name, line: l, col: c} | rest]),
    do: {:ok, {:symbol, %{line: l, col: c}, name}, rest}

  # List ( ... )
  defp parse_form([%Token{type: :lparen, line: l, col: c} | rest]) do
    case parse_until(rest, :rparen) do
      {:ok, elements, rest2} ->
        {:ok, {:list, %{line: l, col: c}, elements}, rest2}

      {:error, _} = err ->
        err
    end
  end

  # Vector [ ... ]
  defp parse_form([%Token{type: :lbracket, line: l, col: c} | rest]) do
    case parse_until(rest, :rbracket) do
      {:ok, elements, rest2} ->
        {:ok, {:vector, %{line: l, col: c}, elements}, rest2}

      {:error, _} = err ->
        err
    end
  end

  # Map { ... }
  defp parse_form([%Token{type: :lbrace, line: l, col: c} | rest]) do
    case parse_until(rest, :rbrace) do
      {:ok, elements, rest2} ->
        {:ok, {:map, %{line: l, col: c}, elements}, rest2}

      {:error, _} = err ->
        err
    end
  end

  # Set #{ ... }
  defp parse_form([%Token{type: :hash_lbrace, line: l, col: c} | rest]) do
    case parse_until(rest, :rbrace) do
      {:ok, elements, rest2} ->
        {:ok, {:set, %{line: l, col: c}, elements}, rest2}

      {:error, _} = err ->
        err
    end
  end

  # BEAM tuple #el[ ... ]
  defp parse_form([%Token{type: :hash_el_lbracket, line: l, col: c} | rest]) do
    case parse_until(rest, :rbracket) do
      {:ok, elements, rest2} ->
        {:ok, {:tuple, %{line: l, col: c}, elements}, rest2}

      {:error, _} = err ->
        err
    end
  end

  # Anonymous function #( ... )
  defp parse_form([%Token{type: :hash_lparen, line: l, col: c} | rest]) do
    case parse_until(rest, :rparen) do
      {:ok, elements, rest2} ->
        body = {:list, %{line: l, col: c}, elements}
        {:ok, {:anon_fn, %{line: l, col: c}, body}, rest2}

      {:error, _} = err ->
        err
    end
  end

  # Regex #"..."
  defp parse_form([%Token{type: :hash_string, value: pattern, line: l, col: c} | rest]),
    do: {:ok, {:regex, %{line: l, col: c}, pattern}, rest}

  # Quote '
  defp parse_form([%Token{type: :quote, line: l, col: c} | rest]) do
    case parse_form(rest) do
      {:ok, form, rest2} ->
        {:ok, {:quote, %{line: l, col: c}, form}, rest2}

      {:error, _} = err ->
        err
    end
  end

  # Quasiquote `
  defp parse_form([%Token{type: :quasiquote, line: l, col: c} | rest]) do
    case parse_form(rest) do
      {:ok, form, rest2} ->
        {:ok, {:quasiquote, %{line: l, col: c}, form}, rest2}

      {:error, _} = err ->
        err
    end
  end

  # Unquote ~
  defp parse_form([%Token{type: :unquote, line: l, col: c} | rest]) do
    case parse_form(rest) do
      {:ok, form, rest2} ->
        {:ok, {:unquote, %{line: l, col: c}, form}, rest2}

      {:error, _} = err ->
        err
    end
  end

  # Splice-unquote ~@
  defp parse_form([%Token{type: :splice_unquote, line: l, col: c} | rest]) do
    case parse_form(rest) do
      {:ok, form, rest2} ->
        {:ok, {:splice_unquote, %{line: l, col: c}, form}, rest2}

      {:error, _} = err ->
        err
    end
  end

  # Deref @
  defp parse_form([%Token{type: :deref, line: l, col: c} | rest]) do
    case parse_form(rest) do
      {:ok, form, rest2} ->
        {:ok, {:deref, %{line: l, col: c}, form}, rest2}

      {:error, _} = err ->
        err
    end
  end

  # Metadata ^
  defp parse_form([%Token{type: :meta, line: l, col: c} | rest]) do
    case parse_meta_value(rest, l, c) do
      {:ok, meta_form, rest2} ->
        case parse_form(rest2) do
          {:ok, target, rest3} ->
            {:ok, {:with_meta, %{line: l, col: c}, {meta_form, target}}, rest3}

          {:error, _} = err ->
            err
        end

      {:error, _} = err ->
        err
    end
  end

  # Unexpected token
  defp parse_form([%Token{type: type, line: l, col: c} | _]),
    do: {:error, "Unexpected token #{type} at line #{l}, col #{c}"}

  defp parse_form([]),
    do: {:error, "Unexpected end of input"}

  # ── Parse helpers ──────────────────────────────────────────────────

  # Parse elements until a closing delimiter token type is found
  defp parse_until(tokens, closer) do
    parse_until_loop(tokens, closer, [])
  end

  defp parse_until_loop([], closer, _acc) do
    name = delimiter_name(closer)
    {:error, "Unexpected end of input, expected '#{name}'"}
  end

  defp parse_until_loop([%Token{type: type} | rest], closer, acc) when type == closer do
    {:ok, Enum.reverse(acc), rest}
  end

  defp parse_until_loop(tokens, closer, acc) do
    case parse_form(tokens) do
      {:ok, form, rest} ->
        parse_until_loop(rest, closer, [form | acc])

      {:error, _} = err ->
        err
    end
  end

  # Parse the value after ^ (metadata)
  # ^{...} — map metadata
  defp parse_meta_value([%Token{type: :lbrace, line: l, col: c} | rest], _ml, _mc) do
    case parse_until(rest, :rbrace) do
      {:ok, elements, rest2} ->
        {:ok, {:map, %{line: l, col: c}, elements}, rest2}

      {:error, _} = err ->
        err
    end
  end

  # ^:keyword — sugar for ^{:keyword true}
  defp parse_meta_value([%Token{type: :keyword, value: kw, line: l, col: c} | rest], _ml, _mc) do
    meta_map = {:map, %{line: l, col: c}, [kw, true]}
    {:ok, meta_map, rest}
  end

  # ^symbol — sugar for ^{:tag symbol}
  defp parse_meta_value([%Token{type: :symbol} | _] = tokens, _ml, _mc) do
    case parse_form(tokens) do
      {:ok, form, rest} -> {:ok, form, rest}
      {:error, _} = err -> err
    end
  end

  defp parse_meta_value(_tokens, ml, mc) do
    {:error, "Expected metadata value (map, keyword, or symbol) at line #{ml}, col #{mc}"}
  end

  defp delimiter_name(:rparen), do: ")"
  defp delimiter_name(:rbracket), do: "]"
  defp delimiter_name(:rbrace), do: "}"
end