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-Algorithm for parsing nested links, images, emphasis, and quotes
-================================================================
-
-When we're parsing inlines and we hit
-
-- a run of `*` or `_` characters
-- a `[` or `![`
-
-we insert a text node with the literal content, and add a pointer
-to this text node to the **delimiter stack.**
-
-The **delimiter stack** is a doubly linked list. Each
-element contains a pointer to a text node, plus information about
-
-- the type of delimiter (`[`, `![`, `*`, `_`)
-- the number of delimiters,
-- whether the delimiter is "active" (all are active to start), and
-- whether the delimiter is a potential opener, a potential closer,
- or both.
-
-When we hit a `]` character, we call the `look_for_link_or_image`
-procedure (see below).
-
-When we hit the end of the input, we call the `process_emphasis`
-procedure (see below), with `stack_bottom` = NULL.
-
-`look_for_link_or_image`
-------------------------
-
-Starting at the top of the delimiter stack, we look backwards
-through the stack for a `[` or `![` delimiter.
-
-If we don't find one, we return a literal text node `]`.
-
-If we do find one, but it's not *active*, we remove the inactive
-delimiter from the stack, and return a literal text node `]`.
-
-If we find one and it's active, then we parse ahead to see if
-we have an inline link/image, reference link/image, compact reference
-link/image, or shortcut reference link/image.
-
-If we don't, then we remove the `[` or `![` delimiter from the
-delimiter stack and return a literal text node `]`.
-
-If we do, then
-
-- We return a link or image node whose children are the inlines
- after the text node pointed to by the opening delimiter.
-
-- We run `process_emphasis` on these inlines, with the `[` opener
- as `stack_bottom`.
-
-- We remove the opening delimiter.
-
-- If we have a link (and not an image), we also set all
- `[` delimiters before the opening delimiter to *inactive*. (This
- will prevent us from getting links within links.)
-
-
-`process_emphasis`
-------------------
-
-Parameter `stack_bottom` sets a lower bound to how far we
-descend in the **delimiter stack**. If it is NULL, we can
-go all the way to the bottom. Otherwise, we stop before
-visiting `stack_bottom`.
-
-Let `current_position` point to the element on the delimiter
-just above `stack_bottom` (or the first element if `stack_bottom`
-is NULL).
-
-We keep track of the `openers_bottom` for each delimiter
-type (`*`, `_`). Initialize this to `stack_bottom`.
-
-Then we repeat the following until we run out of potential
-closers:
-
-- Move `current_position` forward in the delimiter stack (if needed)
- until we find the first potential closer with delimiter `*` or `_`.
- (This will be the potential closer closest
- to the beginning of the input -- the first one in parse order.)
-
-- Now, look back in the stack (staying above `stack_bottom` and
- the `openers_bottom` for this delimiter type) for the
- first matching potential opener ("matching" means same delimiter).
-
-- If one is found:
-
- - Figure out whether we have emphasis or strong emphasis:
- if both closer and opener spans have length >= 2, we have
- strong, otherwise regular.
- - Insert an emph or strong emph node accordingly, after
- the text node corresponding to the opener.
- - Remove delimiters between opener and closer from the delimiter
- stack.
- - Remove 1 (for regular emph) or 2 (for strong emph) delimiters
- from the opening and closing text nodes. If they become empty
- as a result, remove them and remove the corresponding element
- of the delimiter stack. If the closing node is removed, reset
- `current_position` to the next element in the stack.
-
-- If none in found:
-
- - Set `openers_bottom` to the element before `current_position`.
- (We know that there are no openers for this kind of closer up to and
- including this point, so this puts a lower bound on future searches.)
- - If the closer at `current_position` is not a potential opener,
- remove it from the delimiter stack (since we know it can't
- be a closer either).
- - Advance `current_position` to the next element in the stack.
-
-- Repeat.
-
-- After we're done, remove all delimiters above `stack_bottom` from the
- delimiter stack.
-