Interpret the PDF content stream.

    The stack represents the state of the PDF graphics stack.  We are only
    interested in the current transformation matrix (CTM) so we only track
    this object; a full implementation would need to track many other items.

    The CTM is initialized to the mapping from user space to device space.
    PDF units are 1/72".  In a PDF viewer or printer this matrix is initialized
    to the transformation to device space.  For example if set to
    (1/72, 0, 0, 1/72, 0, 0) then all units would be calculated in inches.

    Images are always considered to be (0, 0) -> (1, 1).  Before drawing an
    image there should be a 'cm' that sets up an image coordinate system
    where drawing from (0, 0) -> (1, 1) will draw on the desired area of the
    page.

    PDF units suit our needs so we initialize ctm to the identity matrix.

    According to the PDF specification, the maximum stack depth is 32. Other
    viewers tolerate some amount beyond this.  We issue a warning if the
    stack depth exceeds the spec limit and set a hard limit beyond this to
    bound our memory requirements.  If the stack underflows behavior is
    undefined in the spec, but we just pretend nothing happened and leave the
    CTM unchanged.

def _interpret_contents(contentstream, initial_shorthand=UNIT_SQUARE):
    """Interpret the PDF content stream.

    The stack represents the state of the PDF graphics stack.  We are only
    interested in the current transformation matrix (CTM) so we only track
    this object; a full implementation would need to track many other items.

    The CTM is initialized to the mapping from user space to device space.
    PDF units are 1/72".  In a PDF viewer or printer this matrix is initialized
    to the transformation to device space.  For example if set to
    (1/72, 0, 0, 1/72, 0, 0) then all units would be calculated in inches.

    Images are always considered to be (0, 0) -> (1, 1).  Before drawing an
    image there should be a 'cm' that sets up an image coordinate system
    where drawing from (0, 0) -> (1, 1) will draw on the desired area of the
    page.

    PDF units suit our needs so we initialize ctm to the identity matrix.

    According to the PDF specification, the maximum stack depth is 32. Other
    viewers tolerate some amount beyond this.  We issue a warning if the
    stack depth exceeds the spec limit and set a hard limit beyond this to
    bound our memory requirements.  If the stack underflows behavior is
    undefined in the spec, but we just pretend nothing happened and leave the
    CTM unchanged.
    """

    stack = []
    ctm = PdfMatrix(initial_shorthand)
    xobject_settings = []
    inline_images = []
    found_vector = False
    vector_ops = set('S s f F f* B B* b b*'.split())
    image_ops = set('BI ID EI q Q Do cm'.split())
    operator_whitelist = ' '.join(vector_ops | image_ops)

    for n, graphobj in enumerate(
        _normalize_stack(
            pikepdf.parse_content_stream(contentstream, operator_whitelist)
        )
    ):
        operands, operator = graphobj
        if operator == 'q':
            stack.append(ctm)
            if len(stack) > 32:  # See docstring
                if len(stack) > 128:
                    raise RuntimeError(
                        "PDF graphics stack overflowed hard limit, operator %i" % n
                    )
                warn("PDF graphics stack overflowed spec limit")
        elif operator == 'Q':
            try:
                ctm = stack.pop()
            except IndexError:
                # Keeping the ctm the same seems to be the only sensible thing
                # to do. Just pretend nothing happened, keep calm and carry on.
                warn("PDF graphics stack underflowed - PDF may be malformed")
        elif operator == 'cm':
            ctm = PdfMatrix(operands) @ ctm
        elif operator == 'Do':
            image_name = operands[0]
            settings = XobjectSettings(
                name=image_name, shorthand=ctm.shorthand, stack_depth=len(stack)
            )
            xobject_settings.append(settings)
        elif operator == 'INLINE IMAGE':  # BI/ID/EI are grouped into this
            iimage = operands[0]
            inline = InlineSettings(
                iimage=iimage, shorthand=ctm.shorthand, stack_depth=len(stack)
            )
            inline_images.append(inline)
        elif operator in vector_ops:
            found_vector = True

    return ContentsInfo(
        xobject_settings=xobject_settings,
        inline_images=inline_images,
        found_vector=found_vector,
    )