module Quox.Lexer

import public Quox.Token

import Data.String
import Data.String.Extra
import public Text.Lexer
import public Text.Lexer.Tokenizer
import Control.Monad.Either
import Generics.Derive

%default total
%language ElabReflection


public export
record Error where
  constructor Err
  reason    : StopReason
  line, col : Int
  char      : Char



nameStart = pred $ \c => isAlpha c || c == '_'
nameCont  = pred $ \c => isAlphaNum c || c == '_' || c == '\''

name = nameStart <+> many nameCont <+> reject nameCont

wild = exact "_" <+> reject nameCont

%hide Text.Lexer.symbol
symbol = exact "'" <+> name

number : Lexer -> Lexer
number char = char <+> many (opt (is '_') <+> char) <+> reject nameCont

octal       = approx "0o" <+> number octDigit
decimal     = number digit
hexadecimal = approx "0x" <+> number hexDigit


natToNumber : Nat -> Number
natToNumber 0 = Zero
natToNumber 1 = One
natToNumber k = Other k


toHexit : Char -> Nat
toHexit c = cast $
  if '0' <= c && c <= '9' then
    ord c - ord '0'
  else if 'a' <= c && c <= 'f' then
    ord c - ord 'a' + 10
  else if 'A' <= c && c <= 'F' then
    ord c - ord 'A' + 10
  else 0

parameters (base : Nat) (single : Char -> Nat)
  makeNat : Nat -> List Char -> Nat
  makeNat acc []           = acc
  makeNat acc ('_' :: lst) = makeNat acc lst
  makeNat acc (d   :: lst) = makeNat (acc * base + single d) lst

makeOct = makeNat  8 toHexit 0 . unpack
makeDec = makeNat 10 toHexit 0 . unpack
makeHex = makeNat 16 toHexit 0 . unpack


skip : Lexer -> Tokenizer (Maybe a)
skip lex = match lex $ const Nothing

simple : List String -> a -> Tokenizer (Maybe a)
simple strs = match (choice $ map exact strs) . const . Just

keyword : String -> Keyword -> Tokenizer (Maybe Token)
keyword str = match (exact str <+> reject nameCont) . const . Just . K

choice : (xs : List (Tokenizer a)) -> {auto 0 _ : NonEmpty xs} -> Tokenizer a
choice (t :: ts) = foldl (\a, b => a <|> b) t ts

match : Lexer -> (String -> a) -> Tokenizer (Maybe a)
match lex f = Tokenizer.match lex (Just . f)
%hide Tokenizer.match


tokens : Tokenizer (Maybe Token)
tokens = choice [
  skip $ lineComment $ exact "--",
  skip $ blockComment (exact "{-") (exact "-}"),
  skip spaces,

  simple ["("] $ P LParen,  simple [")"] $ P RParen,
  simple ["["] $ P LSquare, simple ["]"] $ P RSquare,
  simple ["{"] $ P LBrace,  simple ["}"] $ P RBrace,
  simple [","] $ P Comma,
  simple ["::", "∷"] $ P DblColon,
  simple [":"] $ P Colon, -- needs to be after "::"
  simple ["."] $ P Dot,

  simple ["->", "→"] $ P Arrow,
  simple ["=>", "⇒"] $ P DblArrow,
  simple ["**", "×"] $ P Times,
  simple ["<<", "⊲"] $ P Triangle,
  match wild $ const $ P Wild,

  keyword "fun"  Fun,   keyword "λ"  Fun,
  keyword "let"  Let,   keyword "in" In,
  keyword "case" Case,  keyword "of" Of,
  keyword "ω"    Omega, simple ["#"] $ K Omega,

  match name   $ Name,
  match symbol $ Symbol . assert_total strTail,

  match decimal     $ N . natToNumber . makeDec,
  match hexadecimal $ N . natToNumber . makeHex . drop 2,
  match octal       $ N . natToNumber . makeOct . drop 2
]


export
lex : String -> Either Error (List BToken)
lex str =
  let (res, (reason, line, col, str)) = lex tokens str in
  case reason of
       EndInput => Right $ mapMaybe sequence res
       _        => let char = assert_total strIndex str 0 in
                   Left $ Err {reason, line, col, char}