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• Parser

Unread-tokens

Unread a specified number of tokens.

Signature

(unread-tokens n parstate) → new-parstate

Arguments

n — Guard (natp n).

parstate — Guard (parstatep parstate).

Returns

new-parstate — Type (parstatep new-parstate), given (parstatep parstate).

We move tokens from the sequence of read tokens to the sequence of unread tokens by incrementing the number of unread tokens by n and decrementing the number of read tokens by n.

It is an internal error if n exceeds the number of tokens read so far. In this case, besides raising the error, we increment tokens-unread so that the theorem on the parser state size holds unconditionally.

Definitions and Theorems

Function: unread-tokens

(defun unread-tokens (n parstate)
 (declare (xargs :stobjs (parstate)))
 (declare (xargs :guard (and (natp n) (parstatep parstate))))
 (let ((__function__ 'unread-tokens))
  (declare (ignorable __function__))
  (b*
   ((n (nfix n))
    (tokens-read (parstate->tokens-read parstate))
    (tokens-unread (parstate->tokens-unread parstate))
    (size (parstate->size parstate))
    ((unless (<= n tokens-read))
     (raise
      "Internal error: ~
                attempting to unread ~x0 tokens ~
                from ~x1 read tokens."
      n tokens-read)
     (b*
      ((parstate (update-parstate->tokens-unread (+ tokens-unread n)
                                                 parstate))
       (parstate (update-parstate->size (+ size n)
                                        parstate)))
      parstate))
    (new-tokens-read (- tokens-read n))
    (new-tokens-unread (+ tokens-unread n))
    (new-size (+ size n))
    (parstate
         (update-parstate->tokens-read new-tokens-read parstate))
    (parstate
        (update-parstate->tokens-unread new-tokens-unread parstate))
    (parstate (update-parstate->size new-size parstate)))
   parstate)))

Theorem: parstatep-of-unread-tokens

(defthm parstatep-of-unread-tokens
  (implies (parstatep parstate)
           (b* ((new-parstate (unread-tokens n parstate)))
             (parstatep new-parstate)))
  :rule-classes :rewrite)

Theorem: parsize-of-unread-tokens

(defthm parsize-of-unread-tokens
  (b* ((?new-parstate (unread-tokens n parstate)))
    (equal (parsize new-parstate)
           (+ (parsize parstate) (nfix n)))))