1 files changed, 101 insertions, 0 deletions
diff --git a/doc/manual/substitute.py b/doc/manual/substitute.py
new file mode 100755
index 000000000..18b7bed68
--- /dev/null
+++ b/doc/manual/substitute.py
@@ -0,0 +1,101 @@
+#!/usr/bin/env python3
+
+from pathlib import Path
+import json
+import os, os.path
+import sys
+
+name = 'substitute.py'
+
+def log(*args, **kwargs):
+    kwargs['file'] = sys.stderr
+    return print(f'{name}:', *args, **kwargs)
+
+def do_include(content: str, relative_md_path: Path, source_root: Path, search_path: Path):
+    assert not relative_md_path.is_absolute(), f'{relative_md_path=} from mdbook should be relative'
+
+    md_path_abs = source_root / relative_md_path
+    var_abs = md_path_abs.parent
+    assert var_abs.is_dir(), f'supposed directory {var_abs} is not a directory (cwd={os.getcwd()})'
+
+    lines = []
+    for l in content.splitlines(keepends=True):
+        if l.strip().startswith("{{#include "):
+            requested = l.strip()[11:][:-2]
+            if requested.startswith("@generated@/"):
+                included = search_path / Path(requested[12:])
+                requested = included.relative_to(search_path)
+            else:
+                included = source_root / relative_md_path.parent / requested
+                requested = included.resolve().relative_to(source_root)
+            assert included.exists(), f"{requested} not found at {included}"
+            lines.append(do_include(included.read_text(), requested, source_root, search_path) + "\n")
+        else:
+            lines.append(l)
+    return "".join(lines)
+
+def recursive_replace(data, book_root, search_path):
+    match data:
+        case {'sections': sections}:
+            return data | dict(
+                sections = [recursive_replace(section, book_root, search_path) for section in sections],
+            )
+        case {'Chapter': chapter}:
+            path_to_chapter = Path(chapter['path'])
+            chapter_content = chapter['content']
+
+            return data | dict(
+                Chapter = chapter | dict(
+                    # first process includes. this must happen before docroot processing since
+                    # mdbook does not see these included files, only the final agglomeration.
+                    content = do_include(
+                        chapter_content,
+                        path_to_chapter,
+                        book_root,
+                        search_path
+                    ).replace(
+                        '@docroot@',
+                        ("../" * len(path_to_chapter.parent.parts) or "./")[:-1]
+                    ),
+                    sub_items = [
+                        recursive_replace(sub_item, book_root, search_path)
+                        for sub_item in chapter['sub_items']
+                    ],
+                ),
+            )
+
+        case rest:
+            assert False, f'should have been called on a dict, not {type(rest)=}\n\t{rest=}'
+
+def main():
+
+    if len(sys.argv) > 1 and sys.argv[1] == 'supports':
+        return 0
+
+    # mdbook communicates with us over stdin and stdout.
+    # It splorks us a JSON array, the first element describing the context,
+    # the second element describing the book itself,
+    # and then expects us to send it the modified book JSON over stdout.
+
+    context, book = json.load(sys.stdin)
+
+    # book_root is the directory where book contents leave (ie, src/)
+    book_root = Path(context['root']) / context['config']['book']['src']
+
+    # includes pointing into @generated@ will look here
+    search_path = Path(os.environ['MDBOOK_SUBSTITUTE_SEARCH'])
+
+    # Find @var@ in all parts of our recursive book structure.
+    replaced_content = recursive_replace(book, book_root, search_path)
+
+    replaced_content_str = json.dumps(replaced_content)
+
+    # Give mdbook our changes.
+    print(replaced_content_str)
+
+try:
+    sys.exit(main())
+except AssertionError as e:
+    print(f'{name}: INTERNAL ERROR in mdbook preprocessor', file=sys.stderr)
+    print(f'this is a bug in {name}', file=sys.stderr)
+    raise