smd is a renderer for simplified markdown, a variant of markdown that can be processed more quickly and easily than regular markdown.

You can write simplified markdown in such a way that it is compatible with standard markdown, and this intersection captures most of the expressiveness of markdown.

The implementation is ~600 lines of C and compiles to a ~66KB musl-static binary. It processes the input in a single pass, buffering only two lines at a time; it doesn't use malloc at all.

It supports most of the features of markdown including:

• Underline and hash headings (single line only)
• Inline code, indented code, and code fences (backticks only)
• Unordered lists (including nesting, * and - only)
• Ordered lists (including nesting, but must be a single digit)
• Blockquotes (including nesting)
• Links, autolinks, and images (but title field not supported (for tooltips))
• Bold and italic text (but * always means bold and _ always means italic)
• Horizontal rules (--- only)
• Inline HTML (including entities)
• Line breaks (backslash only)

It also supports these extensions:

• Mathjax spans and blocks with $ and $$
• Asterisk footnote links ([^id] and [^id]: footnote)
• Labeled footnote links ([#id])
• Tables (| a | b |) (column alignment colons are ignored)
• Description lists (= name: value or = name then value)
• Blockquote containers (fenced by """)
• Aside containers (fenced by ::: and !!!)
• Collapsible details containers (fenced by +++, first line is summary)
• Figures (image at start of block on its own line and caption on next line)

Markdown features that are not supported:

• Reference-style links (can use footnote links instead)
• Raw HTML blocks

Other implementation differences:

• Inline spans, links, and HTML tags must be contained within a single line
• All inline spans have the same precedence and are processed left to right
• Inline span delimiter runs must be the same length for the start and end
• Leading spaces less than 4 are not removed
• Blockquote > must be followed by space or newline
• List items and blockquotes always contain a paragraph tag
• Tabs are not internally supported for container indentation
  • To use tabs for container indentation, use expand -t 4 <file> | smd
• Spaces are not trimmed inside spans

Other minor/technical differences:

• Lines are limited to 4094 bytes
• Simpler rules for span delimiter adjacencies
• Blocks are delimited by a prefix rather than a pattern
• Trailing characters after code fences and thematic breaks are discarded
• Autolink processing is simplified
• Does not check for non-ASCII whitespace characters

Install by running ./make && sudo ./make install. This requires a C compiler.

Then pass markdown over stdin: smd < filename.md