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Std/io/read-byte$

Read one byte from an open :byte stream.

Signature: (read-byte$ channel state-state) returns (mv byte/nil state).

channel is a symbol that must refer to an open :byte input channel; see open-input-channel.
state is the ACL2 state.

This is a logic-mode function, but its logical definition is tricky; see logical-story-of-io. The main logical consequence is the updating of state.

Under the hood, we read a byte (i.e., a number between 0 and 255, inclusive) from the Lisp input stream that is associated with channel. If we reach the end of the file, nil is returned.

Definitions and Theorems

Theorem: state-p1-of-read-byte$

(defthm state-p1-of-read-byte$
  (implies (and (state-p1 state)
                (symbolp channel)
                (open-input-channel-p1 channel
                                       :byte state))
           (state-p1 (mv-nth 1 (read-byte$ channel state)))))

Theorem: open-input-channel-p1-of-read-byte$

(defthm open-input-channel-p1-of-read-byte$
  (implies (and (state-p1 state)
                (open-input-channel-p1 channel
                                       :byte state))
           (open-input-channel-p1
                channel
                :byte (mv-nth 1 (read-byte$ channel state)))))

Theorem: bytep-of-read-byte$

(defthm bytep-of-read-byte$
 (implies
     (and (state-p1 state)
          (open-input-channel-p1 channel
                                 :byte state)
          (mv-nth 0 (read-byte$ channel state)))
     (and (bytep (mv-nth 0 (read-byte$ channel state)))
          (natp (mv-nth 0 (read-byte$ channel state)))
          (integerp (mv-nth 0 (read-byte$ channel state)))
          (unsigned-byte-p 8
                           (mv-nth 0 (read-byte$ channel state))))))

Theorem: bytep-of-read-byte$-type

(defthm bytep-of-read-byte$-type
  (implies (and (state-p1 state)
                (open-input-channel-p1 channel
                                       :byte state))
           (or (null (mv-nth 0 (read-byte$ channel state)))
               (natp (mv-nth 0 (read-byte$ channel state)))))
  :rule-classes :type-prescription)

Theorem: bytep-of-read-byte$-linear

(defthm bytep-of-read-byte$-linear
  (implies (and (state-p1 state)
                (open-input-channel-p1 channel
                                       :byte state))
           (and (< (mv-nth 0 (read-byte$ channel state))
                   256)
                (<= 0
                    (mv-nth 0 (read-byte$ channel state)))))
  :rule-classes :linear)

Theorem: no-bytes-after-read-byte$-null

(defthm no-bytes-after-read-byte$-null
  (let ((byte1 (mv-nth 0 (read-byte$ channel state)))
        (state2 (mv-nth 1 (read-byte$ channel state))))
    (implies (and (not byte1)
                  (state-p1 state)
                  (open-input-channel-p1 channel
                                         :byte state))
             (not (mv-nth 0 (read-byte$ channel state2))))))

Theorem: state-preserved-by-read-byte$-when-eof

(defthm state-preserved-by-read-byte$-when-eof
  (implies (and (not (mv-nth 0 (read-byte$ channel state)))
                (state-p1 state)
                (open-input-channel-p1 channel
                                       :byte state))
           (equal (mv-nth 1 (read-byte$ channel state))
                  state)))