(hidden fmt-internal)
(defndynamic fmt-internal [s args]
  (let [idx (String.index-of s \%)
        len (String.length s)]
    (if (= idx -1)
      (if (= (length args) 0)
        s ; no more splits found, just return string
        (macro-error
          (str "error in format string: too many arguments to format string (missing directive for '"
               (car args)
               "')")))
      (if (= len 1)
        (macro-error "error in format string: expected expression after last %")
        (if (= \% (String.char-at s (inc idx))) ; this is an escaped %
          (list 'ref
            (list 'String.append
                  "%"
                  (fmt-internal (String.slice s (+ idx 2) len) args)))
          (if (= 0 (length args)) ; we need to insert something, but have nothing
            (macro-error
              (str "error in format string: not enough arguments to format string (missing argument for '%"
                   (String.slice s (inc idx) (inc (inc idx)))
                   "')"))
            ; okay, this is the meat:
            ; get the next % after our escaper
            (let [next (String.index-of (String.slice s (inc idx) len) \%)]
              (if (= -1 next)
                (if (< 1 (length args))
                  (macro-error
                    (str "error in format string: too many arguments to format string (missing directive for '"
                         (cadr args)
                         "')"))
                  (list 'ref (list 'format s (car args))))
                (let [slice (String.slice s 0 (+ (inc idx) next))]
                  (list 'ref
                    (list 'String.append
                      (list 'ref (list 'format slice (car args)))
                      (fmt-internal (String.slice s (+ (inc idx) next) len)
                                        (cdr args)))))))))))))

(doc fmt "formats a string. It supports all of the string interpolations defined in format of the type that should be interpolated (e.g. %d and %x on integers).")
(defmacro fmt [s :rest args]
  (list 'copy (fmt-internal s args)))