@@ -3,13 +3,16 @@ name: Documentation check
 
 on: [pull_request]
 
-jobs:   
+permissions:
+  contents: write
+
+jobs:
   docs-checks:
     name: ${{ matrix.doc-type }}
     strategy:
       matrix:
         doc-type: [html, latex, epub]
-    
+
     runs-on: ubuntu-latest
     steps:
     - name: Checkout
@@ -28,9 +31,62 @@ jobs:
       run: |
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
         pip install -e .
+    - name: Check the example digest is current
+      run: python docs/example_digest.py --check
+
     - name: Build ${{ matrix.doc-type }} documentation
       run: sphinx-build -Wnb ${{ matrix.doc-type }} docs/source/ docs/build-${{ matrix.doc-type }}/
-        
+
+    # Read the Docs stops a build at fifteen minutes and the examples take
+    # about twenty, so it restores what the examples produced here instead
+    # of running them. The archive is named by the digest of the example
+    # inputs and of the library, and Read the Docs only restores one whose
+    # digest matches the commit it is building.
+    - name: Publish the example cache
+      if: matrix.doc-type == 'html' && github.event.pull_request.head.repo.full_name == github.repository
+      env:
+        GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      run: |
+        name=$(python docs/example_cache.py name)
+        python docs/example_cache.py pack "$name"
+        gh release view docs-cache >/dev/null 2>&1 \
+          || gh release create docs-cache --title "Documentation example cache" \
+               --notes "Figures the documentation examples produce, named by the digest from docs/example_digest.py. Read the Docs restores the archive matching the commit it builds. Managed by the documentation workflow." \
+               --latest=false
+        gh release upload docs-cache "$name" --clobber
+
+    # A pull request that changes an example has no cache for its new
+    # digest until the step above finishes, so the Read the Docs build
+    # that started alongside this job ran the examples and timed out.
+    # Build it again now that the archive is published, and it is warm.
+    #
+    # Needs a READTHEDOCS_TOKEN secret, from Settings -> API Tokens on
+    # Read the Docs. Without it this step says so and does nothing, and
+    # the only cost is having to press Rebuild by hand.
+    - name: Rebuild the docs on Read the Docs
+      if: matrix.doc-type == 'html' && github.event.pull_request.head.repo.full_name == github.repository
+      env:
+        RTD_TOKEN: ${{ secrets.READTHEDOCS_TOKEN }}
+        RTD_VERSION: ${{ github.event.pull_request.number }}
+      run: |
+        if [ -z "$RTD_TOKEN" ]; then
+          echo "No READTHEDOCS_TOKEN secret, skipping the rebuild."
+          echo "Press Rebuild on the Read the Docs build to pick up the cache."
+          exit 0
+        fi
+        # A pull request build is a version named after the pull request.
+        code=$(curl -sS -o /tmp/rtd.json -w '%{http_code}' -X POST \
+          -H "Authorization: Token $RTD_TOKEN" \
+          "https://app.readthedocs.org/api/v3/projects/microstructpy/versions/$RTD_VERSION/builds/")
+        echo "Read the Docs answered $code"
+        cat /tmp/rtd.json
+        case "$code" in
+          202) echo "Rebuild triggered." ;;
+          *) echo "Could not trigger a rebuild. The cache is published, so a"
+             echo "rebuild by hand will be warm."; exit 0 ;;
+        esac
+
+
     - name: Prepare documentation artifact
       run: |
         # Define Path to Upload
 
@@ -13,6 +13,25 @@ build:
     python: "3.10"
   apt_packages:
     - freeglut3-dev
+  jobs:
+    # Running every example takes about twenty minutes, and a build here
+    # is stopped at fifteen. The documentation workflow runs them on a
+    # machine without that limit and publishes what they produced, keyed
+    # by a digest of the example inputs and of the library. Restore that
+    # and Sphinx-Gallery skips the examples.
+    #
+    # The archive is only restored when its digest is the digest of this
+    # commit, so an edited example is never served with older figures.
+    # When there is no archive to match, nothing is restored, the
+    # examples run, and the build is likely to time out. That is the
+    # intended failure: publish the cache for this commit, then rebuild.
+    post_install:
+      - python docs/example_cache.py name > /tmp/cache_name
+      - |
+        curl -sSfL -o /tmp/examples.tar.gz \
+          "https://github.com/kip-hart/MicroStructPy/releases/download/docs-cache/$(cat /tmp/cache_name)" \
+          && python docs/example_cache.py restore /tmp/examples.tar.gz \
+          || echo "No example cache for this commit, the examples will be run."
 
 # Build documentation in the "docs/" directory with Sphinx
 sphinx:
 
@@ -0,0 +1,67 @@
+Building the documentation
+==========================
+
+::
+
+    pip install -r docs/requirements.txt
+    pip install -r requirements.txt
+    pip install -e .
+    sphinx-build -Wnb html docs/source docs/build-html
+
+The first build takes about twenty minutes, because
+``docs/source/sphinx_gallery/plot_demos.py`` runs every example in
+``src/microstructpy/examples`` to produce the figures the pages embed.
+Later builds reuse that work and take seconds.
+
+
+The example cache
+-----------------
+
+A build on Read the Docs is stopped at fifteen minutes on the free plan,
+which is less than the examples take. The documentation workflow runs
+them on a machine without that limit and publishes what they produced,
+and the Read the Docs build restores it instead of running them.
+
+Two pieces make that safe.
+
+``docs/example_digest.py`` digests the example inputs and the library
+source, and keeps the result in ``plot_demos.py`` as ``EXAMPLES_DIGEST``.
+Sphinx-Gallery decides whether to run a script by hashing it, and
+``plot_demos.py`` finds the examples with ``glob`` rather than naming
+them, so without this its hash would not move when an example was edited
+or added, and a cached build would serve stale figures. Editing an
+example now changes the digest, which changes the hash, which runs the
+examples again.
+
+``docs/example_cache.py`` packs and restores the figures. An archive
+carries the digest it was built from, and ``restore`` refuses one that
+does not match the working tree. A build that cannot be warm runs the
+examples and may time out, which is intended: it is better to be slow and
+correct than fast and stale.
+
+After editing an example or anything in ``src/microstructpy``::
+
+    python docs/example_digest.py --write
+
+The documentation check runs ``python docs/example_digest.py --check`` and
+fails if the stored digest is out of date.
+
+
+Publishing a cache by hand
+--------------------------
+
+The workflow does this on every pull request, from the ``html`` job. To do
+it manually, after a full build::
+
+    name=$(python docs/example_cache.py name)
+    python docs/example_cache.py pack "$name"
+    gh release upload docs-cache "$name" --clobber
+
+Read the Docs fetches the archive for the digest of the commit it is
+building, so a commit whose examples and library are unchanged reuses the
+archive of an earlier one.
+
+A pull request that changes an example has no cache for its new digest
+until its ``html`` job finishes. A Read the Docs build that starts before
+that will run the examples and time out. Rebuild it once the job has
+published the archive.
@@ -0,0 +1,193 @@
+#!/usr/bin/env python3
+"""Pack and restore the figures the documentation examples produce.
+
+The documentation build runs every example in
+``src/microstructpy/examples``, which takes about twenty minutes and does
+not fit in the fifteen minute limit of a Read the Docs build on the free
+plan.  The work is the same on every build, so this module moves it to a
+machine without a time limit: the documentation workflow packs what the
+examples produced, and the Read the Docs build restores it before Sphinx
+runs.
+
+Two sets of files are needed, and both are in the archive:
+
+* ``docs/source/auto_examples``, the output of Sphinx-Gallery.  It holds
+  the ``.md5`` of ``plot_demos.py``, which is what makes Sphinx-Gallery
+  skip the examples instead of running them.
+* The PNG files under ``src/microstructpy/examples``, which the pages in
+  ``docs/source/examples`` embed with ``figure::``.  The meshes beside
+  them are not in the archive, since no page refers to them.
+
+An archive carries the digest of the inputs it was built from, from
+:mod:`docs.example_digest`.  ``restore`` refuses an archive whose digest
+is not the digest of the working tree, so an edited example cannot be
+served with the figures of an older one.  Refusing leaves the build to
+run the examples itself, which is slow and may time out, and that is the
+intended outcome: a build that cannot be warm should be loud, not stale.
+
+Usage::
+
+    python docs/example_cache.py pack examples.tar.gz
+    python docs/example_cache.py restore examples.tar.gz
+    python docs/example_cache.py name          # archive name for this tree
+
+"""
+
+import argparse
+import json
+import os
+import sys
+import tarfile
+
+HERE = os.path.dirname(os.path.abspath(__file__))
+ROOT = os.path.dirname(HERE)
+
+sys.path.insert(0, HERE)
+from example_digest import digest  # noqa: E402
+
+GALLERY = os.path.join('docs', 'source', 'auto_examples')
+EXAMPLES = os.path.join('src', 'microstructpy', 'examples')
+MANIFEST = 'microstructpy-example-cache.json'
+
+
+def _gallery_members():
+    """Every file of the Sphinx-Gallery output, relative to the root."""
+    members = []
+    root = os.path.join(ROOT, GALLERY)
+    for dirpath, dirnames, filenames in os.walk(root):
+        dirnames.sort()
+        for name in sorted(filenames):
+            path = os.path.join(dirpath, name)
+            members.append(os.path.relpath(path, ROOT))
+    return members
+
+
+def _figure_members():
+    """Every PNG an example wrote, relative to the root.
+
+    Only the files in the output directories of the examples, so that an
+    input such as ``aluminum_micro.png`` is not carried in the archive.
+    """
+    members = []
+    root = os.path.join(ROOT, EXAMPLES)
+    for dirpath, dirnames, filenames in os.walk(root):
+        dirnames.sort()
+        if os.path.abspath(dirpath) == os.path.abspath(root):
+            continue  # the inputs live here, the figures are in subdirs
+        for name in sorted(filenames):
+            if name.endswith('.png'):
+                path = os.path.join(dirpath, name)
+                members.append(os.path.relpath(path, ROOT))
+    return members
+
+
+def pack(filename):
+    """Write an archive of the figures and the gallery output."""
+    gallery = _gallery_members()
+    figures = _figure_members()
+
+    if not gallery:
+        print('No %s to pack. Build the documentation first.' % GALLERY,
+              file=sys.stderr)
+        return 1
+    if not figures:
+        print('No figures under %s to pack.' % EXAMPLES, file=sys.stderr)
+        return 1
+
+    value = digest()
+    manifest = json.dumps({'digest': value,
+                           'gallery': len(gallery),
+                           'figures': len(figures)}, indent=2).encode()
+
+    with tarfile.open(filename, 'w:gz') as tar:
+        info = tarfile.TarInfo(MANIFEST)
+        info.size = len(manifest)
+        tar.addfile(info, _BytesIO(manifest))
+        for rel in gallery + figures:
+            tar.add(os.path.join(ROOT, rel), arcname=rel)
+
+    size = os.path.getsize(filename) / 1048576.0
+    print('packed %d gallery files and %d figures for digest %s '
+          'into %s (%.1f MB)'
+          % (len(gallery), len(figures), value, filename, size))
+    return 0
+
+
+def restore(filename):
+    """Extract an archive, if its digest is the digest of this tree."""
+    if not os.path.exists(filename):
+        print('No archive at %s.' % filename, file=sys.stderr)
+        return 1
+
+    with tarfile.open(filename, 'r:gz') as tar:
+        try:
+            manifest = json.loads(tar.extractfile(MANIFEST).read().decode())
+        except KeyError:
+            print('%s has no %s, refusing to restore it.'
+                  % (filename, MANIFEST), file=sys.stderr)
+            return 1
+
+        current = digest()
+        if manifest.get('digest') != current:
+            print('The archive was built from %s and the examples digest '
+                  'to %s. Refusing to restore it, the examples will be run.'
+                  % (manifest.get('digest'), current), file=sys.stderr)
+            return 1
+
+        members = [m for m in tar.getmembers() if m.name != MANIFEST]
+        for m in members:
+            if m.name.startswith('/') or '..' in m.name.split('/'):
+                print('Refusing a path outside the tree: %s' % m.name,
+                      file=sys.stderr)
+                return 1
+        try:
+            tar.extractall(ROOT, members=members, filter='data')
+        except TypeError:
+            # filter= arrived in Python 3.12. The paths are checked above.
+            tar.extractall(ROOT, members=members)
+
+    print('restored %d files for digest %s' % (len(members), current))
+    return 0
+
+
+def name():
+    """Print the archive name for this working tree."""
+    print('microstructpy-examples-%s.tar.gz' % digest())
+    return 0
+
+
+class _BytesIO(object):
+    """A minimal file object, so tarfile can add bytes already in memory."""
+
+    def __init__(self, data):
+        self._data = data
+        self._pos = 0
+
+    def read(self, size=-1):
+        if size < 0:
+            chunk = self._data[self._pos:]
+        else:
+            chunk = self._data[self._pos:self._pos + size]
+        self._pos += len(chunk)
+        return chunk
+
+
+def main():
+    parser = argparse.ArgumentParser(description=__doc__.split('\n')[0])
+    sub = parser.add_subparsers(dest='command', required=True)
+    p = sub.add_parser('pack', help='write an archive of the figures')
+    p.add_argument('filename')
+    r = sub.add_parser('restore', help='extract an archive of the figures')
+    r.add_argument('filename')
+    sub.add_parser('name', help='the archive name for this working tree')
+
+    args = parser.parse_args()
+    if args.command == 'pack':
+        return pack(args.filename)
+    if args.command == 'restore':
+        return restore(args.filename)
+    return name()
+
+
+if __name__ == '__main__':
+    sys.exit(main())