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+Always
+======
+
+* Evaluate issues tagged with `next` and `bug`. Remember to check
+  discussions and pull requests in addition to regular issues.
+* When close to release, make sure external-libs is building and
+  follow instructions in ../external-libs/README
+
+Next
+====
+
+* Fix #874 -- make args in --encrypt to match the json and make
+  positional fill in the gaps
+* Maybe fix #553 -- use file times for attachments
+* std::string_view transition -- work being done by m-holger
+* Break ground on "Document-level work" -- TODO-pages.md lives on a
+  separate branch.
+* Standard for CLI and Job JSON support for JSON-based command-line
+  arguments. Come up with a standard way of supporting command-line
+  arguments that take JSON specifications of things so that
+  * there is a predictable way to indicate whether an argument is a
+    file or a JSON blob
+  * with QPDFJob JSON, make sure it is possible to directly include
+    the JSON rather than having to stringify a JSON blob
+  * One option might be to prepend file:// to a filename or otherwise
+    to take a JSON blob. We could have that as a particular type of
+    argument that would behave properly for both job JSON and CLI.
+
+
+Possible future JSON enhancements
+=================================
+
+* Consider not including unreferenced objects and trimming the trailer
+  in the same way that QPDFWriter does (except don't remove `/ID`).
+  This means excluding the linearization dictionary and hint stream,
+  the encryption dictionary, all keys from trailer that are removed by
+  QPDFWriter::getTrimmedTrailer except `/ID`, any object streams, and
+  the xref stream as long as all those objects are unreferenced. (They
+  always should be, but there could be some bizarre case of someone
+  creating a PDF file that has an indirect reference to one of those,
+  in which case we need to preserve it.) If this is done, make
+  `--preserve-unreferenced` preserve unreference objects and also
+  those extra keys. Search for "linear" and "trailer" in json.rst to
+  update the various places in the documentation that discuss this.
+  Also update the help for --json and --preserve-unreferenced.
+
+* Add to JSON output the information available from a few additional
+  informational options:
+
+  * --check: add but maybe not by default?
+
+  * --show-linearization: add but maybe not by default? Also figure
+    out whether warnings reported for some of the PDF specs (1.7) are
+    qpdf problems. This may not be worth adding in the first
+    increment.
+
+  * --show-xref: add
+
+* Consider having --check, --show-encryption, etc., just select the
+  right keys when in json mode. I don't think I want check on by
+  default, so that might be different.
+
+* Consider having warnings be included in the json in a "warnings" key
+  in json mode.
+
+QPDFJob
+=======
+
+Here are some ideas for QPDFJob that didn't make it into 10.6. Not all
+of these are necessarily good -- just things to consider.
+
+* How do we chain jobs? The idea would be that the input and/or output
+  of a QPDFJob could be a QPDF object rather than a file. For input,
+  it's pretty easy. For output, none of the output-specific options
+  (encrypt, compress-streams, objects-streams, etc.) would have any
+  affect, so we would have to treat this like inspect for error
+  checking. The QPDF object in the state where it's ready to be sent
+  off to QPDFWriter would be used as the input to the next QPDFJob.
+  For the job json, I think we can have the output be an identifier
+  that can be used as the input for another QPDFJob. For a json file,
+  we could the top level detect if it's an array with the convention
+  that exactly one has an output, or we could have a subkey with other
+  job definitions or something. Ideally, any input
+  (copy-attachments-from, pages, etc.) could use a QPDF object. It
+  wouldn't surprise me if this exposes bugs in qpdf around foreign
+  streams as this has been a relatively fragile area before.
+
+Documentation
+=============
+
+* Do a full pass through the documentation.
+
+  * Make sure `qpdf` is consistent. Use QPDF when just referring to
+    the package.
+  * Make sure markup is consistent
+  * Autogenerate where possible
+  * Consider which parts might be good candidates for moving to the
+    wiki.
+
+* Commit 'Manual - enable line wrapping in table cells' from
+  Mon Jan 17 12:22:35 2022 +0000 enables table cell wrapping. See if
+  this can be incorporated directly into sphinx_rtd_theme and the
+  workaround can be removed.
+
+* When possible, update the debian package to include docs again. See
+  https://bugs.debian.org/1004159 for details.
+
+Document-level work
+===================
+
+* Ideas here may by superseded by #593.
+
+* QPDFPageCopier -- object for moving pages around within files or
+  between files and performing various transformations. Reread/rewrite
+  _page-selection in the manual if needed.
+
+  * Handle all the stuff of pages and split-pages
+  * Do n-up, booklet, collation
+  * Look through cli and see what else...flatten-*?
+  * See comments in QPDFPageDocumentHelper.hh for addPage -- search
+    for "a future version".
+  * Make it efficient for bulk operations
+  * Make certain doc-level features selectable
+  * qpdf.cc should do all its page operations, including
+    overlay/underlay, splitting, and merging, using this
+  * There should also be example code
+
+* After doc-level checks are in, call --check on the output files in
+  the "Copy Annotations" tests.
+
+* Document-level checks. For example, for forms, make sure all form
+  fields point to an annotation on exactly one page as well as that
+  all widget annotations are associated with a form field. Hook this
+  into QPDFPageCopier as well as the doc helpers. Make sure it is
+  called from --check.
+
+* See also issues tagged with "pages". Include closed issues.
+
+* Add flags to CLI to select which document-level options to
+  preserve or not preserve. We will probably need a pair of mutually
+  exclusive, repeatable options with a way to specify all, none, only
+  {x,y}, or all but {x,y}.
+
+* If a page contains a reference a file attachment annotation, when
+  that page is copied, if the file attachment appears in the top-level
+  EmbeddedFiles tree, that entry should be preserved in the
+  destination file. Otherwise, we probably will require the use of
+  --copy-attachments-from to preserve these. What will the strategy be
+  for deduplicating in the automatic case?
+
+Text Appearance Streams
+=======================
+
+This is a list of known issues with text appearance streams and things
+we might do about it.
+
+* For variable text, the spec says to pull any resources from /DR that
+  are referenced in /DA but if the resource dictionary already has
+  that resource, just use the one that's there. The current code looks
+  only for /Tf and adds it if needed. We might want to instead merge
+  /DR with resources and then remove anything that's unreferenced. We
+  have all the code required for that in ResourceFinder except
+  TfFinder also gets the font size, which ResourceFinder doesn't do.
+
+* There are things we are missing because we don't look at font
+  metrics. The code from TextBuilder (work) has almost everything in
+  it that is required. Once we have knowledge of character widths, we
+  can support quadding and multiline text fields (/Ff 4096), and we
+  can potentially squeeze text to fit into a field. For multiline,
+  first squeeze vertically down to the font height, then squeeze
+  horizontally with Tz. For single line, squeeze horizontally with Tz.
+  If we use Tz, issue a warning.
+
+* When mapping characters to widths, we will need to care about
+  character encoding. For built-in fonts, we can create a map from
+  Unicode code point to width and then go from the font's encoding to
+  unicode to the width. See misc/character-encoding/ (not on github)
+  and font metric information for the 14 standard fonts in my local
+  pdf-spec directory.
+
+* Once we know about character widths, we can correctly support
+  auto-sized variable text fields (0 Tf). If this is fixed, search for
+  "auto-sized" in cli.rst.
+
+Fuzz Errors
+===========
+
+* https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=<N>
+
+* Ignoring these:
+  * Out of memory in dct: 35001, 32516
+
+External Libraries
+==================
+
+Current state (10.0.2):
+
+* qpdf/external-libs repository builds external-libs on a schedule.
+  It detects and downloads the latest versions of zlib, jpeg, and
+  openssl and creates source and binary distribution zip files in an
+  artifact called "distribution".
+
+* Releases in qpdf/external-libs are made manually. They contain
+  qpdf-external-libs-{bin,src}.zip.
+
+* The qpdf build finds the latest non-prerelease release and downloads
+  the qpdf-external-libs-*.zip files from the releases in the setup
+  stage.
+
+* To upgrade to a new version of external-libs, create a new release
+  of qpdf/external-libs (see README-maintainer in external-libs) from
+  the distribution artifact of the most recent successful build after
+  ensuring that it works.
+
+Desired state:
+
+* The qpdf/external-libs repository should create release candidates.
+  Ideally, every scheduled run would make its zip files available. A
+  personal access token with actions:read scope for the
+  qpdf/external-libs repository is required to download the artifact
+  from an action run, and qpdf/qpdf's secrets.GITHUB_TOKEN doesn't
+  have this access. We could create a service account for this
+  purpose. As an alternative, we could have a draft release in
+  qpdf/external-libs that the qpdf/external-libs build could update
+  with each candidate. It may also be possible to solve this by
+  developing a simple GitHub app.
+
+* Scheduled runs of the qpdf build in the qpdf/qpdf repository (not a
+  fork or pull request) could download external-libs from the release
+  candidate area instead of the latest stable release. Pushes to the
+  build branch should still use the latest release so it always
+  matches the main branch.
+
+* Periodically, we would create a release of external-libs from the
+  release candidate zip files. This could be done safely because we
+  know the latest qpdf works with it. This could be done at least
+  before every release of qpdf, but potentially it could be done at
+  other times, such as when a new dependency version is available or
+  after some period of time.
+
+Other notes:
+
+* The external-libs branch in qpdf/qpdf was never documented. We might
+  be able to get away with deleting it.
+
+* See README-maintainer in qpdf/external-libs for information on
+  creating a release. This could be at least partially scripted in a
+  way that works for the qpdf/qpdf repository as well since they are
+  very similar.
+
+ABI Changes
+===========
+
+This is a list of changes to make next time there is an ABI change.
+Comments appear in the code prefixed by "ABI".
+
+Always:
+* Search for ABI in source and header files
+* Search for "[[deprecated" to find deprecated APIs that can be removed
+* Search for issues, pull requests, and discussions with the "abi" label
+* Check discussion "qpdf X planning" where X is the next major
+  version. This should be tagged `abi`
+
+For qpdf 12, see https://github.com/qpdf/qpdf/discussions/785
+
+C++ Version Changes
+===================
+
+Use
+// C++NN: ...
+to mark places in the code that should be updated when we require at
+least that version of C++.
+
+Page splitting/merging
+======================
+
+ * Update page splitting and merging to handle document-level
+   constructs with page impact such as interactive forms and article
+   threading. Check keys in the document catalog for others, such as
+   outlines, page labels, thumbnails, and zones. For threads,
+   Subramanyam provided a test file; see ../misc/article-threads.pdf.
+   Email Q-Count: 431864 from 2009-11-03.
+
+ * bookmarks (outlines) 12.3.3
+   * support bookmarks when merging
+   * prune bookmarks that don't point to a surviving page when merging
+     or splitting
+   * make sure conflicting named destinations work possibly test by
+     including the same file by two paths in a merge
+   * see also comments in issue 343
+
+   Note: original implementation of bookmark preservation for split
+   pages caused a very high performance hit. The problem was
+   introduced in 313ba081265f69ac9a0324f9fe87087c72918191 and reverted
+   in the commit that adds this paragraph. The revert includes marking
+   a few tests cases as $td->EXPECT_FAILURE. When properly coded, the
+   test cases will need to be adjusted to only include the parts of
+   the outlines that are actually copied. The tests in question are
+   "split page with outlines". When implementing properly, ensure that
+   the performance is not adversely affected by timing split-pages on
+   a large file with complex outlines such as the PDF specification.
+
+   When pruning outlines, keep all outlines in the hierarchy that are
+   above an outline for a page we care about. If one of the ancestor
+   outlines points to a non-existent page, clear its dest. If an
+   outline does not have any children that point to pages in the
+   document, just omit it.
+
+   Possible strategy:
+   * resolve all named destinations to explicit destinations
+   * concatenate top-level outlines
+   * prune outlines whose dests don't point to a valid page
+   * recompute all /Count fields
+
+   Test files
+   * page-labels-and-outlines.pdf: old file with both page labels and
+     outlines. All destinations are explicit destinations. Each page
+     has Potato and a number. All titles are feline names.
+   * outlines-with-actions.pdf: mixture of explicit destinations,
+     named destinations, goto actions with explicit destinations, and
+     goto actions with named destinations; uses /Dests key in names
+     dictionary. Each page has Salad and a number. All titles are
+     silly words. One destination is an indirect object.
+   * outlines-with-old-root-dests.pdf: like outlines-with-actions
+     except it uses the PDF-1.1 /Dests dictionary for named
+     destinations, and each page has Soup and a number. Also pages are
+     numbered with upper-case Roman numerals starting with 0. All
+     titles are silly words preceded by a bullet.
+
+   If outline handling is significantly improved, see
+   ../misc/bad-outlines/bad-outlines.pdf and email:
+   https://mail.google.com/mail/u/0/#search/rfc822msgid%3A02aa01d3d013%249f766990%24de633cb0%24%40mono.hr)
+
+ * Form fields: should be similar to outlines.
+
+Analytics
+=========
+
+Consider features that make it easier to detect certain patterns in
+PDF files. The information below could be computed using an external
+program that reads the existing json, but if it's useful enough, we
+could add it directly to the json output.
+
+ * Add to "pages" in the json:
+   * "inheritsresources": bool; whether there are any inherited
+     attributes from ancestor page tree nodes
+   * "sharedresources": a list of indirect objects that are
+     "/Resources" dictionaries or "XObject" resource dictionary subkeys
+     of either the page itself or of any form XObject referenced by the
+     page.
+
+ * Add to "objectinfo" in json: "directpagerefcount": the number of
+   pages that directly reference this object (i.e., you can find an
+   indirect reference to the object in the page dictionary without
+   traversing over any indirect objects)
+
+General
+=======
+
+NOTE: Some items in this list refer to files in my personal home
+directory or that are otherwise not publicly accessible. This includes
+things sent to me by email that are specifically not public. Even so,
+I find it useful to make reference to them in this list.
+
+* Consider enabling code scanning on GitHub.
+
+* Add an option --ignore-encryption to ignore encryption information
+  and treat encrypted files as if they weren't encrypted. This should
+  make it possible to solve #598 (--show-encryption without a
+  password). We'll need to make sure we don't try to filter any
+  streams in this mode. Ideally we should be able to combine this with
+  --json so we can look at the raw encrypted strings and streams if we
+  want to, though be sure to document that the resulting JSON won't be
+  convertible back to a valid PDF. Since providing the password may
+  reveal additional details, --show-encryption could potentially retry
+  with this option if the first time doesn't work. Then, with the file
+  open, we can read the encryption dictionary normally. If this is
+  done, search for "raw, encrypted" in json.rst.
+
+* In libtests, separate executables that need the object library
+  from those that strictly use public API. Move as many of the test
+  drivers from the qpdf directory into the latter category as long
+  as doing so isn't too troublesome from a coverage standpoint.
+
+* Consider generating a non-flat pages tree before creating output to
+  better handle files with lots of pages. If there are more than 256
+  pages, add a second layer with the second layer nodes having no more
+  than 256 nodes and being as evenly sizes as possible. Don't worry
+  about the case of more than 65,536 pages. If the top node has more
+  than 256 children, we'll live with it. This is only safe if all
+  intermediate page nodes have only /Kids, /Parent, /Type, and /Count.
+
+* Look at https://bestpractices.coreinfrastructure.org/en
+
+* Consider adding fuzzer code for JSON
+
+* Rework tests so that nothing is written into the source directory.
+  Ideally then the entire build could be done with a read-only
+  source tree.
+
+* Large file tests fail with linux32 before and after cmake. This was
+  first noticed after 10.6.3. I don't think it's worth fixing.
+
+* Consider updating the fuzzer with code that exercises
+  copyAnnotations, file attachments, and name and number trees. Check
+  fuzzer coverage.
+
+* Add code for creation of a file attachment annotation. It should
+  also be possible to create a widget annotation and a form field.
+  Update the pdf-attach-file.cc example with new APIs when ready.
+
+* Flattening of form XObjects seems like something that would be
+  useful in the library. We are seeing more cases of completely valid
+  PDF files with form XObjects that cause problems in other software.
+  Flattening of form XObjects could be a useful way to work around
+  those issues or to prepare files for additional processing, making
+  it possible for users of the qpdf library to not be concerned about
+  form XObjects. This could be done recursively; i.e., we could have a
+  method to embed a form XObject into whatever contains it, whether
+  that is a form XObject or a page. This would require more
+  significant interpretation of the content stream. We would need a
+  test file in which the placement of the form XObject has to be in
+  the right place, e.g., the form XObject partially obscures earlier
+  code and is partially obscured by later code. Keys in the resource
+  dictionary may need to be changed -- create test cases with lots of
+  duplicated/overlapping keys.
+
+* Part of closed_file_input_source.cc is disabled on Windows because
+  of odd failures. It might be worth investigating so we can fully
+  exercise this in the test suite. That said, ClosedFileInputSource
+  is exercised elsewhere in qpdf's test suite, so this is not that
+  pressing.
+
+* If possible, consider adding CCITT3, CCITT4, or any other easy
+  filters. For some reference code that we probably can't use but may
+  be handy anyway, see
+  http://partners.adobe.com/public/developer/ps/sdk/index_archive.html
+
+* If possible, support the following types of broken files:
+
+   - Files that have no whitespace token after "endobj" such that
+     endobj collides with the start of the next object
+
+   - See ../misc/broken-files
+
+   - See ../misc/bad-files-issue-476. This directory contains a
+     snapshot of the google doc and linked PDF files from issue #476.
+     Please see the issue for details.
+
+* Additional form features
+  * set value from CLI? Specify title, and provide way to
+    disambiguate, probably by giving objgen of field
+
+* Pl_TIFFPredictor is pretty slow.
+
+* Support for handling file names with Unicode characters in Windows
+  is incomplete. qpdf seems to support them okay from a functionality
+  standpoint, and the right thing happens if you pass in UTF-8
+  encoded filenames to QPDF library routines in Windows (they are
+  converted internally to wchar_t*), but file names are encoded in
+  UTF-8 on output, which doesn't produce nice error messages or
+  output on Windows in some cases.
+
+* If we ever wanted to do anything more with character encoding, see
+  ../misc/character-encoding/, which includes machine-readable dump
+  of table D.2 in the ISO-32000 PDF spec. This shows the mapping
+  between Unicode, StandardEncoding, WinAnsiEncoding,
+  MacRomanEncoding, and PDFDocEncoding.
+
+* Some test cases on bad files fail because qpdf is unable to find
+  the root dictionary when it fails to read the trailer. Recovery
+  could find the root dictionary and even the info dictionary in
+  other ways. In particular, issue-202.pdf can be opened by evince,
+  and there's no real reason that qpdf couldn't be made to be able to
+  recover that file as well.
+
+* Audit every place where qpdf allocates memory to see whether there
+  are cases where malicious inputs could cause qpdf to attempt to
+  grab very large amounts of memory. Certainly there are cases like
+  this, such as if a very highly compressed, very large image stream
+  is requested in a buffer. Hopefully normal input to output
+  filtering doesn't ever try to do this. QPDFWriter should be checked
+  carefully too. See also bugs/private/from-email-663916/
+
+* Interactive form modification:
+  https://github.com/qpdf/qpdf/issues/213 contains a good discussion
+  of some ideas for adding methods to modify annotations and form
+  fields if we want to make it easier to support modifications to
+  interactive forms. Some of the ideas have been implemented, and
+  some of the probably never will be implemented, but it's worth a
+  read if there is an intention to work on this. In the issue, search
+  for "Regarding write functionality", and read that comment and the
+  responses to it.
+
+* Look at ~/Q/pdf-collection/forms-from-appian/
+
+* When decrypting files with /R=6, hash_V5 is called more than once
+  with the same inputs.  Caching the results or refactoring to reduce
+  the number of identical calls could improve performance for
+  workloads that involve processing large numbers of small files.
+
+* Consider adding a method to balance the pages tree.  It would call
+  pushInheritedAttributesToPage, construct a pages tree from scratch,
+  and replace the /Pages key of the root dictionary with the new
+  tree.
+
+* Study what's required to support savable forms that can be saved by
+  Adobe Reader. Does this require actually signing the document with
+  an Adobe private key? Search for "Digital signatures" in the PDF
+  spec, and look at ~/Q/pdf-collection/form-with-full-save.pdf, which
+  came from Adobe's example site. See also
+  ../misc/digital-sign-from-trueroad/ and
+  ../misc/digital-signatures/digitally-signed-pdf-xfa.pdf. If digital
+  signatures are implemented, update the docs on crypto providers,
+  which mention that this may happen in the future.
+
+* Qpdf does not honor /EFF when adding new file attachments. When it
+  encrypts, it never generates streams with explicit crypt filters.
+  Prior to 10.2, there was an incorrect attempt to treat /EFF as a
+  default value for decrypting file attachment streams, but it is not
+  supposed to mean that. Instead, it is intended for conforming
+  writers to obey this when adding new attachments. Qpdf is not a
+  conforming writer in that respect.
+
+* The whole xref handling code in the QPDF object allows the same
+  object with more than one generation to coexist, but a lot of logic
+  assumes this isn't the case.  Anything that creates mappings only
+  with the object number and not the generation is this way,
+  including most of the interaction between QPDFWriter and QPDF.  If
+  we wanted to allow the same object with more than one generation to
+  coexist, which I'm not sure is allowed, we could fix this by
+  changing xref_table.  Alternatively, we could detect and disallow
+  that case.  In fact, it appears that Adobe reader and other PDF
+  viewing software silently ignores objects of this type, so this is
+  probably not a big deal.
+
+* From a suggestion in bug 3152169, consider having an option to
+  re-encode inline images with an ASCII encoding.
+
+* From github issue 2, provide more in-depth output for examining
+  hint stream contents. Consider adding on option to provide a
+  human-readable dump of linearization hint tables. This should
+  include improving the 'overflow reading bit stream' message as
+  reported in issue #2. There are multiple calls to stopOnError in
+  the linearization checking code. Ideally, these should not
+  terminate checking. It would require re-acquiring an understanding
+  of all that code to make the checks more robust. In particular,
+  it's hard to look at the code and quickly determine what is a true
+  logic error and what could happen because of malformed user input.
+  See also ../misc/linearization-errors.
+
+* If I ever decide to make appearance stream-generation aware of
+  fonts or font metrics, see email from Tobias with Message-ID
+  <5C3C9C6C.8000102@thax.hardliners.org> dated 2019-01-14.
+
+* Look at places in the code where object traversal is being done and,
+  where possible, try to avoid it entirely or at least avoid ever
+  traversing the same objects multiple times.
+
+----------------------------------------------------------------------
+
+HISTORICAL NOTES
+
+Performance
+===========
+
+As described in https://github.com/qpdf/qpdf/issues/401, there was
+great performance degradation between qpdf 7.1.1 and 9.1.1. Doing a
+bisect between dac65a21fb4fa5f871e31c314280b75adde89a6c and
+release-qpdf-7.1.1, I found several commits that damaged performance.
+I fixed some of them to improve performance by about 70% (as measured
+by saying that old times were 170% of new times). The remaining
+commits that broke performance either can't be correct because they
+would re-introduce an old bug or aren't worth correcting because of
+the high value they offer relative to a relatively low penalty. For
+historical reference, here are the commits. The numbers are the time
+in seconds on the machine I happened to be using of splitting the
+first 100 pages of PDF32000_2008.pdf 20 times and taking an average
+duration.
+
+Commits that broke performance:
+
+* d0e99f195a987c483bbb6c5449cf39bee34e08a1 -- object description and
+  context: 0.39 -> 0.45
+* a01359189b32c60c2d55b039f7aefd6c3ce0ebde (minus 313ba08) -- fix
+  dangling references: 0.55 -> 0.6
+* e5f504b6c5dc34337cc0b316b4a7b1fca7e614b1 -- sparse array: 0.6 -> 0.62
+
+Other intermediate steps that were previously fixed:
+
+* 313ba081265f69ac9a0324f9fe87087c72918191 -- copy outlines into
+  split: 0.55 -> 4.0
+* a01359189b32c60c2d55b039f7aefd6c3ce0ebde -- fix dangling references:
+  4.0 -> 9.0
+
+This commit fixed the awful problem introduced in 313ba081:
+
+* a5a016cdd26a8e5c99e5f019bc30d1bdf6c050a2 -- revert outline
+  preservation: 9.0 -> 0.6
+
+Note that the fix dangling references commit had a much worse impact
+prior to removing the outline preservation, so I also measured its
+impact in isolation.
+
+A few important lessons (in README-maintainer)
+
+* Indirection through PointerHolder<Members> is expensive, and should
+  not be used for things that are created and destroyed frequently
+  such as QPDFObjectHandle and QPDFObject.
+* Traversal of objects is expensive and should be avoided where
+  possible.
+
+Also, it turns out that PointerHolder is more performant than
+std::shared_ptr. (This was true at the time but subsequent
+implementations of std::shared_ptr became much more efficient.)
+
+QPDFPagesTree
+=============
+
+On a few occasions, I have considered implementing a QPDFPagesTree
+object that would allow the document's original page tree structure to
+be preserved. See comments at the top QPDF_pages.cc for why this was
+abandoned.
+
+Partial work is in refs/attic/QPDFPagesTree. QPDFPageTree is mostly
+implemented and mostly tested. There are not enough cases of different
+kinds of operations (pclm, linearize, json, etc.) with non-flat pages
+trees. Insertion is not implemented. Insertion is potentially complex
+because of the issue of inherited objects. We will have to call
+pushInheritedAttributesToPage before adding any pages to the pages
+tree. The test suite is failing on that branch.
+
+Some parts of page tree repair are silent (no warnings). All page tree
+repair should warn. The reason is that page tree repair will change
+object numbers, and knowing that is important when working with JSON
+output.
+
+If we were to do this, we would still need keep a pages cache for
+efficient insertion. There's no reason we can't keep a vector of page
+objects up to date and just do a traversal the first time we do
+getAllPages just like we do now. The difference is that we would not
+flatten the pages tree. It would be useful to go through QPDF_pages
+and reimplement everything without calling flattenPagesTree. Then we
+can remove flattenPagesTree, which is private. That said, with the
+addition of creating non-flat pages trees, there is really no reason
+not to flatten the pages tree for internal use.
+
+In its current state, QPDFPagesTree does not proactively fix /Type or
+correct page objects that are used multiple times. You have to
+traverse the pages tree to trigger this operation. It would be nice if
+we would do that somewhere but not do it more often than necessary so
+isPagesObject and isPageObject are reliable and can be made more
+reliable. Maybe add a validate or repair function? It should also make
+sure /Count and /Parent are correct.
+
+Rejected Ideas
+==============
+
+* Investigate whether there is a way to automate the memory checker
+  tests for Windows.
+
+* Provide support in QPDFWriter for writing incremental updates.
+  Provide support in qpdf for preserving incremental updates.  The
+  goal should be that QDF mode should be fully functional for files
+  with incremental updates including fix_qdf.
+
+  Note that there's nothing that says an indirect object in one
+  update can't refer to an object that doesn't appear until a later
+  update.  This means that QPDF has to treat indirect null objects
+  differently from how it does now.  QPDF drops indirect null objects
+  that appear as members of arrays or dictionaries.  For arrays, it's
+  handled in QPDFWriter where we make indirect nulls direct.  This is
+  in a single if block, and nothing else in the code cares about it.
+  We could just remove that if block and not break anything except a
+  few test cases that exercise the current behavior.  For
+  dictionaries, it's more complicated.  In this case,
+  QPDF_Dictionary::getKeys() ignores all keys with null values, and
+  hasKey() returns false for keys that have null values.  We would
+  probably want to make QPDF_Dictionary able to handle the special
+  case of keys that are indirect nulls and basically never have it
+  drop any keys that are indirect objects.
+
+  If we make a change to have qpdf preserve indirect references to
+  null objects, we have to note this in ChangeLog and in the release
+  notes since this will change output files.  We did this before when
+  we stopped flattening scalar references, so this is probably not a
+  big deal.  We also have to make sure that the testing for this
+  handles non-trivial cases of the targets of indirect nulls being
+  replaced by real objects in an update.  I'm not sure how this plays
+  with linearization, if at all.  For cases where incremental updates
+  are not being preserved as incremental updates and where the data
+  is being folded in (as is always the case with qpdf now), none of
+  this should make any difference in the actual semantics of the
+  files.
+
+* The second xref stream for linearized files has to be padded only
+  because we need file_size as computed in pass 1 to be accurate.  If
+  we were not allowing writing to a pipe, we could seek back to the
+  beginning and fill in the value of /L in the linearization
+  dictionary as an optimization to alleviate the need for this
+  padding.  Doing so would require us to pad the /L value
+  individually and also to save the file descriptor and determine
+  whether it's seekable.  This is probably not worth bothering with.
+
+* Based on an idea suggested by user "Atom Smasher", consider
+  providing some mechanism to recover earlier versions of a file
+  embedded prior to appended sections.
+
+* Consider creating a sanitizer to make it easier for people to send
+  broken files. Now that we have json mode, this is probably no
+  longer worth doing. Here is the previous idea, possibly implemented
+  by making it possible to run the lexer (tokenizer) over a whole
+  file. Make it possible to replace all strings in a file lexically
+  even on badly broken files. Ideally this should work files that are
+  lacking xref, have broken links, duplicated dictionary keys, syntax
+  errors, etc., and ideally it should work with encrypted files if
+  possible. This should go through the streams and strings and
+  replace them with fixed or random characters, preferably, but not
+  necessarily, in a manner that works with fonts. One possibility
+  would be to detect whether a string contains characters with normal
+  encoding, and if so, use 0x41. If the string uses character maps,
+  use 0x01. The output should otherwise be unrelated to the input.
+  This could be built after the filtering and tokenizer rewrite and
+  should be done in a manner that takes advantage of the other
+  lexical features. This sanitizer should also clear metadata and
+  replace images. If I ever do this, the file from issue #494 would
+  be a great one to look at.
+
+* Here are some notes about having stream data providers modify
+  stream dictionaries. I had wanted to add this functionality to make
+  it more efficient to create stream data providers that may
+  dynamically decide what kind of filters to use and that may end up
+  modifying the dictionary conditionally depending on the original
+  stream data. Ultimately I decided not to implement this feature.
+  This paragraph describes why.
+
+  * When writing, the way objects are placed into the queue for
+    writing strongly precludes creation of any new indirect objects,
+    or even changing which indirect objects are referenced from which
+    other objects, because we sometimes write as we are traversing
+    and enqueuing objects. For non-linearized files, there is a risk
+    that an indirect object that used to be referenced would no
+    longer be referenced, and whether it was already written to the
+    output file would be based on an accident of where it was
+    encountered when traversing the object structure. For linearized
+    files, the situation is considerably worse. We decide which
+    section of the file to write an object to based on a mapping of
+    which objects are used by which other objects. Changing this
+    mapping could cause an object to appear in the wrong section, to
+    be written even though it is unreferenced, or to be entirely
+    omitted since, during linearization, we don't enqueue new objects
+    as we traverse for writing.
+
+  * There are several places in QPDFWriter that query a stream's
+    dictionary in order to prepare for writing or to make decisions
+    about certain aspects of the writing process. If the stream data
+    provider has the chance to modify the dictionary, every piece of
+    code that gets stream data would have to be aware of this. This
+    would potentially include end user code. For example, any code
+    that called getDict() on a stream before installing a stream data
+    provider and expected that dictionary to be valid would
+    potentially be broken. As implemented right now, you must perform
+    any modifications on the dictionary in advance and provided
+    /Filter and /DecodeParms at the time you installed the stream
+    data provider. This means that some computations would have to be
+    done more than once, but for linearized files, stream data
+    providers are already called more than once. If the work done by
+    a stream data provider is especially expensive, it can implement
+    its own cache.
+
+  The example examples/pdf-custom-filter.cc demonstrates the use of
+  custom stream filters. This includes a custom pipeline, a custom
+  stream filter, as well as modification of a stream's dictionary to
+  include creation of a new stream that is referenced from
+  /DecodeParms.
+
+* Removal of raw QPDF* from the API. Discussions in #747 and #754.
+  This is a summary of the arguments I put forth in #754. The idea was
+  to make QPDF::QPDF() private and require all QPDF objects to be
+  shared pointers created with QPDF::create(). This would enable us to
+  have QPDFObjectHandle::getOwningQPDF() return a std::weak_ptr<QPDF>.
+  Prior to #726 (QPDFObject/QPDFValue split, released in qpdf 11.0.0),
+  getOwningQPDF() could return an invalid pointer if the owning QPDF
+  disappeared, but this is no longer the case, which removes the main
+  motivation. QPDF 11 added QPDF::create() anyway though.
+
+  Removing raw QPDF* would look something like this. Note that you
+  can't use std::make_shared<T> unless T has a public constructor.
+
+  QPDF_POINTER_TRANSITION = 0 -- no warnings around calling the QPDF constructor
+  QPDF_POINTER_TRANSITION = 1 -- calls to QPDF() are deprecated, but QPDF is still available so code can be backward compatible and use std::make_shared<QPDF>
+  QPDF_POINTER_TRANSITION = 2 -- the QPDF constructor is private; all calls to std::make_shared<QPDF> have to be replaced with QPDF::create
+
+  If we were to do this, we'd have to look at each use of QPDF* in the
+  interface and decide whether to use a std::shared_ptr or a
+  std::weak_ptr. The answer would almost always be to use a
+  std::weak_ptr, which means we'd have to take the extra step of
+  calling lock(), and it means there would be lots of code changes
+  cause people would have to pass weak pointers instead of raw
+  pointers around, and those have to be constructed and locked.
+  Passing std::shared_ptr around leaves the possibility of creating
+  circular references. It seems to be too much trouble in the library
+  and too much toil for library users to be worth the small benefit of
+  not having to call resetObjGen in QPDF's destructor.
+
+* Fix Multiple Direct Object Parent Issue
+
+  This idea was rejected because it would be complicated to implement
+  and would likely have a high performance cost to fix what is not
+  really that big of a problem in practice.
+
+  It is possible for a QPDFObjectHandle for a direct object to be
+  contained inside of multiple QPDFObjectHandle objects or even
+  replicated across multiple QPDF objects. This creates a potentially
+  confusing and unintentional aliasing of direct objects. There are
+  known cases in the qpdf library where this happens including page
+  splitting and merging (particularly with page labels, and possibly
+  with other cases), and also with unsafeShallowCopy. Disallowing this
+  would incur a significant performance penalty and is probably not
+  worth doing. If we were to do it, here are some ideas.
+
+  * Add std::weak_ptr<QPDFObject> parent to QPDFObject. When adding a
+    direct object to an array or dictionary, set its parent. When
+    removing it, clear the parent pointer. The parent pointer would
+    always be null for indirect objects, so the parent pointer, which
+    would reside in QPDFObject, would have to be managed by
+    QPDFObjectHandle. This is because QPDFObject can't tell the
+    difference between a resolved indirect object and a direct object.
+
+  * Phase 1: When a direct object that already has a parent is added
+    to a dictionary or array, issue a warning. There would need to be
+    unsafe add methods used by unsafeShallowCopy. These would add but
+    not modify the parent pointer.
+
+  * Phase 2: In the next major release, make the multiple parent case
+    an error. Require people to create a copy. The unsafe operations
+    would still have to be permitted.
+
+  This approach would allow an object to be moved from one object to
+  another by removing it, which returns the now orphaned object, and
+  then inserting it somewhere else. It also doesn't break the pattern
+  of adding a direct object to something and subsequently mutating it.
+  It just prevents the same object from being added to more than one
+  thing.