(def reserved-words
  "List of Ludus reserved words."
  ## see ludus-spec repo for more info
  {"as" :as ## impl
   "do" :do ## impl
   "else" :else ## impl
   "false" :false ## impl -> literal word
   "fn" :fn ## impl
   "if" :if ## impl
   "import" :import ## impl
   "let" :let ## impl
   "loop" :loop ## impl
   "match" :match ## impl
   "nil" :nil ## impl -> literal word
   "ns" :ns ## impl
   "panic!" :panic ## impl (should _not_ be a function)
   "recur" :recur ## impl
   "ref" :ref ## impl
   "then" :then ## impl
   "true" :true ## impl -> literal word
   "use" :use ## wip
   "with" :with ## impl
   "when" :when ## impl, replaces cond
   "repeat" :repeat ## syntax sugar over "loop": still unclear what this syntax could be
   "test" :test
   })

(def literal-words {"true" true
                    "false" false
                    "nil" nil
                    })

(defn- new-scanner
  "Creates a new scanner."
  [source input]
  @{:source source
   :input input
   :length (length source)
   :errors @[]
   :start 0
   :current 0
   :line 1
   :tokens @[]})

(defn- at-end?
  "Tests if a scanner is at end of input."
  [scanner]
  (>= (get scanner :current) (get scanner :length)))

(defn- current-char
  "Gets the current character of the scanner."
  [scanner]
  (let [source (get scanner :source)
        current (get scanner :current)
        length (length source)]
    (if (>= current length)
      nil
      (string/from-bytes (get source current)))))

(defn- advance
  "Advances the scanner by a single character."
  [scanner]
  (update scanner :current inc))

(defn- next-char
  "Gets the next character from the scanner."
  [scanner]
  (let [source (get scanner :source)
    current (get scanner :current)
    next (inc current)
    length (length source)]
    (if (>= next length)
      nil
      (string/from-bytes (get source next)))))

(defn- current-lexeme
  [scanner]
  (slice (get scanner :source) (get scanner :start) (get scanner :current)))

(defn- char-code [char] (get char 0))

(defn- char-in-range? [start end char]
  (and char
    (>= (char-code char) (char-code start))
    (<= (char-code char) (char-code end))))

(defn- digit? [c]
  (char-in-range? "0" "9" c))

(defn- nonzero-digit? [c]
  (char-in-range? "1" "9" c))

## for now, use very basic ASCII charset in words
## TODO: research the implications of using the whole 
## (defn- alpha? [c] (boolean (re-find #"\p{L}" (string c))))
(defn- alpha? [c]
  (or (char-in-range? "a" "z" c) (char-in-range? "A" "Z" c)))

(defn- lower? [c] (char-in-range? "a" "z" c))

(defn- upper? [c] (char-in-range? "A" "Z" c))

## legal characters in words
(def word-chars {"_" true "?" true "!" true "*" true "/" true})

(defn- word-char? [c]
  (or (alpha? c) (digit? c) (get word-chars c)))

(defn- whitespace? [c]
  (or (= c " ") (= c "\t")))

(def terminators {
":" true
";" true
"\n" true
"{" true
"}" true
"(" true
")" true
"[" true
"]" true
"$" true
"#" true
"-" true
"=" true
"&" true
"," true
">" true
"\"" true})

(defn- terminates? [c]
  (or (nil? c) (whitespace? c) (get terminators c)))

(defn- add-token
  [scanner token-type &opt literal]
   (update scanner :tokens array/push
      {:type token-type
       :lexeme (current-lexeme scanner)
       :literal literal
       :line (get scanner :line)
       :start (get scanner :start)
       :source (get scanner :source)
       :input (get scanner :input)}))

## TODO: errors should also be in the vector of tokens
## The goal is to be able to be able to hand this to an LSP?
## Do we need a different structure
(defn- add-error [scanner msg]
  (let [token {:type :error
               :lexeme (current-lexeme scanner)
               :literal nil
               :line (get scanner :line)
               :start (get scanner :start)
               :source (get scanner :source)
               :input (get scanner :input)
               :message msg}]
    (-> scanner
      (update :errors array/push token)
      (update :tokens array/push token))))

(defn- add-keyword
  [scanner]
  (defn recur [scanner key]
    (let [char (current-char scanner)]
      (cond
        (terminates? char) (add-token scanner :keyword (keyword key))
        (word-char? char) (recur (advance scanner) (string key char))
        :else (add-error scanner (string "Unexpected " char "after keyword :" key)))))
	(recur scanner ""))

(defn- read-literal [lit] (-> lit parse-all first))

### TODO: consider whether Janet's number rules are right for Ludus
(defn- add-number [char scanner]
  (defn recur [scanner num float?]
    (let [curr (current-char scanner)]
      (cond
        (= curr "_") (recur (advance scanner) num float?) ## consume underscores unharmed
        (= curr ".") (if float?
                      (add-error scanner (string "Unexpected second decimal point after " num "."))
                      (recur (advance scanner) (buffer/push num curr) true))
        (terminates? curr) (add-token scanner :number (read-literal num))
        (digit? curr) (recur (advance scanner) (buffer/push num curr) float?)
        :else (add-error scanner (string "Unexpected " curr " after number " num ".")))))
  (recur scanner (buffer char) false))

(defn- add-string
  [scanner]
  (defn recur [scanner buff interpolate?]
    (let [char (current-char scanner)]
      (case char
        "{" (recur (advance scanner) (buffer/push buff char) true)
        # allow multiline strings
        "\n" (recur (update (advance scanner) :line inc) (buffer/push buff char) interpolate?)
        "\"" (add-token (advance scanner) (if interpolate? :interpolated :string)(string buff))
        "\\" (let [next (next-char scanner)]
                (if (= next "{") 
                  (do
                    (buffer/push buff char)
                    (buffer/push buff next)
                    (recur (advance (advance scanner)) buff interpolate?))
                  (recur (advance scanner) (buffer/push buff char) interpolate?))) 
        (if (at-end? scanner)
          (add-error scanner "Unterminated string.")
          (recur (advance scanner) (buffer/push buff char) interpolate?)))))
  (recur scanner @"" false))

(defn- add-word
  [char scanner]
  (defn recur [scanner word]
    (let [curr (current-char scanner)]
      (cond
        (terminates? curr) (add-token scanner
                             (get reserved-words (string word) :word) 
                             (get literal-words (string word) :none))
        (word-char? curr) (recur (advance scanner) (buffer/push word curr))
        :else (add-error scanner (string "Unexpected " curr " after word " word ".")))))
  (recur scanner (buffer char)))

(defn- add-pkg
  [char scanner]
  (defn recur [scanner pkg]
    (let [curr (current-char scanner)]
      (cond
        (terminates? curr) (add-token scanner :pkg :none)
        (word-char? curr) (recur (advance scanner) (buffer/push pkg curr))
        :else (add-error scanner (string "unexpected " curr " after pkg name " pkg)))))
  (recur scanner (buffer char)))

(defn- add-ignored
  [scanner]
  (defn recur [scanner ignored]
    (let [char (current-char scanner)]
      (cond
        (terminates? char) (add-token scanner :ignored)
        (word-char? char) (recur (advance scanner) (buffer/push ignored char))
        :else (add-error scanner (string "Unexpected " char " after word " ignored ".")))))
  (recur scanner @"_"))

(defn- add-comment [char scanner]
  (defn recur [scanner comm]
    (let [char (current-char scanner)]
      (if (= "\n" char)
        scanner # for now, we don't do anything with comments; can be added later
        (recur (advance scanner) (buffer/push comm char)))))
  (recur scanner (buffer char)))

(defn- scan-token [scanner]
  (let [char (current-char scanner)
        scanner (advance scanner)
        next (current-char scanner)]
    (case char
      ## one-character tokens
      ## :break is a special zero-char token before closing braces
      ## it makes parsing much simpler
      "(" (add-token scanner :lparen)
      ")" (add-token (add-token scanner :break) :rparen)
      "{" (add-token scanner :lbrace)
      "}" (add-token (add-token scanner :break) :rbrace)
      "[" (add-token scanner :lbracket)
      "]" (add-token (add-token scanner :break) :rbracket)
      ";" (add-token scanner :semicolon)
      "," (add-token scanner :comma)
      "\n" (add-token (update scanner :line inc) :newline)
      "\\" (add-token scanner :backslash)
      "=" (add-token scanner :equals)
      ">" (add-token (add-token scanner :break) :pipeline)

      ## two-character tokens
      ## ->
      "-" (cond
           (= next ">") (add-token (advance scanner) :arrow)
           (digit? next) (add-number char scanner)
           :else (add-error scanner (string "Expected -> or negative number after `-`. Got `" char next "`")))

      ## dict #{
      "#" (if (= next "{")
           (add-token (advance scanner) :startdict)
           (add-error scanner (string "Expected beginning of dict: #{. Got " char next)))

      ## set ${
      "$" (if (= next "{")
           (add-token (advance scanner) :startset)
           (add-error scanner (string "Expected beginning of set: ${. Got " char next)))

      ## placeholders
      ## there's a flat _, and then ignored words
      "_" (cond
           (terminates? next) (add-token scanner :placeholder)
           (alpha? next) (add-ignored scanner)
           :else (add-error scanner (string "Expected placeholder: _. Got " char next)))

      ## comments
      ## & starts an inline comment
      "&" (add-comment char scanner)

      ## keywords
      ":" (cond
           (alpha? next) (add-keyword scanner)
           :else (add-error scanner (string "Expected keyword. Got " char next)))

      ## splats
      "." (let [after_next (current-char (advance scanner))]
           (if (= ".." (string next after_next))
             (add-token (advance scanner) :splat)
             (add-error scanner (string "Expected splat: ... . Got " (string "." next after_next)))))

      ## strings
      "\"" (add-string scanner)

      ## word matches
      (cond
        (whitespace? char) scanner ## for now just skip whitespace characters
        (digit? char) (add-number char scanner)
        (upper? char) (add-pkg char scanner) 
        (lower? char) (add-word char scanner)
        :else (add-error scanner (string "Unexpected character: " char))))))

(defn- next-token [scanner]
  (put scanner :start (get scanner :current)))

(defn scan [source &opt input]
  (default input :input)
  (defn recur [scanner]
    (if (at-end? scanner)
      (let [scanner (add-token (add-token scanner :break) :eof)]
        {:tokens (get scanner :tokens)
         :errors (get scanner :errors)})
      (recur (-> scanner (scan-token) (next-token)))))
  (recur (new-scanner source input)))