pandoc: allow to include other files

The following block will inlcude a second markdown file:

```{.include}
path/to/other/markdown/file.md
```

Move all used pandoc filters from the latex/ base directory into
a new pandoc/ directory.

2 files changed, 140 insertions, 0 deletions

diff --git a/pandoc/center-img.lua b/pandoc/center-img.lua
new file mode 100644
index 00000000..18782725
--- /dev/null
+++ b/pandoc/center-img.lua
@@ -0,0 +1,20 @@
+-- Based on:
+-- https://pandoc.org/lua-filters.html#center-images-in-latex-and-html-output
+-- Licensed under MIT License found at:
+-- https://github.com/TomBener/pandoc-templates/blob/05aba224519d8a2b037108948bde0d820860b786/LICENSE
+
+-- Filter images with this function if the target format is LaTeX.
+if FORMAT:match('beamer') then
+  function Image(elem)
+    -- Surround all images with the 'center' class with image-centering raw LaTeX.
+    if elem.classes[1] == "center" then
+      return {
+        pandoc.RawInline('latex', '\\hfill\\break{\\centering'),
+        elem,
+        pandoc.RawInline('latex', '\\par}')
+      }
+    else
+      return elem
+    end
+  end
+end
diff --git a/pandoc/include-files.lua b/pandoc/include-files.lua
new file mode 100644
index 00000000..f9f208e9
--- /dev/null
+++ b/pandoc/include-files.lua
@@ -0,0 +1,120 @@
+--- include-files.lua – filter to include Markdown files
+---
+--- Copyright: © 2019–2021 Albert Krewinkel
+--- License:   MIT – see LICENSE file for details
+
+-- Module pandoc.path is required and was added in version 2.12
+PANDOC_VERSION:must_be_at_least '2.12'
+
+local List = require 'pandoc.List'
+local path = require 'pandoc.path'
+local system = require 'pandoc.system'
+
+--- Get include auto mode
+local include_auto = false
+function get_vars (meta)
+  if meta['include-auto'] then
+    include_auto = true
+  end
+end
+
+--- Keep last heading level found
+local last_heading_level = 0
+function update_last_level(header)
+  last_heading_level = header.level
+end
+
+--- Update contents of included file
+local function update_contents(blocks, shift_by, include_path)
+  local update_contents_filter = {
+    -- Shift headings in block list by given number
+    Header = function (header)
+      if shift_by then
+        header.level = header.level + shift_by
+      end
+      return header
+    end,
+    -- If image paths are relative then prepend include file path
+    Image = function (image)
+      if path.is_relative(image.src) then
+        image.src = path.normalize(path.join({include_path, image.src}))
+      end
+      return image
+    end,
+    -- Update path for include-code-files.lua filter style CodeBlocks
+    CodeBlock = function (cb)
+      if cb.attributes.include and path.is_relative(cb.attributes.include) then
+        cb.attributes.include =
+          path.normalize(path.join({include_path, cb.attributes.include}))
+        end
+      return cb
+    end
+  }
+
+  return pandoc.walk_block(pandoc.Div(blocks), update_contents_filter).content
+end
+
+--- Filter function for code blocks
+local transclude
+function transclude (cb)
+  -- ignore code blocks which are not of class "include".
+  if not cb.classes:includes 'include' then
+    return
+  end
+
+  -- Markdown is used if this is nil.
+  local format = cb.attributes['format']
+
+  -- Attributes shift headings
+  local shift_heading_level_by = 0
+  local shift_input = cb.attributes['shift-heading-level-by']
+  if shift_input then
+    shift_heading_level_by = tonumber(shift_input)
+  else
+    if include_auto then
+      -- Auto shift headings
+      shift_heading_level_by = last_heading_level
+    end
+  end
+
+  --- keep track of level before recusion
+  local buffer_last_heading_level = last_heading_level
+
+  local blocks = List:new()
+  for line in cb.text:gmatch('[^\n]+') do
+    if line:sub(1,2) ~= '//' then
+      local fh = io.open(line)
+      if not fh then
+        io.stderr:write("Cannot open file " .. line .. " | Skipping includes\n")
+      else
+        -- read file as the given format with global reader options
+        local contents = pandoc.read(
+          fh:read '*a',
+          format,
+          PANDOC_READER_OPTIONS
+        ).blocks
+        last_heading_level = 0
+        -- recursive transclusion
+        contents = system.with_working_directory(
+            path.directory(line),
+            function ()
+              return pandoc.walk_block(
+                pandoc.Div(contents),
+                { Header = update_last_level, CodeBlock = transclude }
+              )
+            end).content
+        --- reset to level before recursion
+        last_heading_level = buffer_last_heading_level
+        blocks:extend(update_contents(contents, shift_heading_level_by,
+                                      path.directory(line)))
+        fh:close()
+      end
+    end
+  end
+  return blocks
+end
+
+return {
+  { Meta = get_vars },
+  { Header = update_last_level, CodeBlock = transclude }
+}