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

(doc fmt "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]
  (fmt-internal s args))