minor API changes and additional docs

Author: tingerrr

README.md

@@ -4,13 +4,13 @@ document. By default, it will assume that it is used in the header of your docum
 the last heading if and only if it is numbered and the next heading is not the first on the current
 page.
 
-By default hdyra assumes that you use `a4` page size, see the FAQ if you use different page size or
-margins.
+By default hydra also assumes that you use `a4` page size, see the FAQ if you use different page
+size or margins.
 
 ## Example
 ```typst
 #import "@preview/hydra:0.0.1": hydra
-#set page(header: hydra())
+#set page(header: #[hydra() #line(length: 100%)])
 #set heading(numbering: "1.1")
 #show heading.where(level: 1): it => pagebreak(weak: true) + it
 
@@ -32,20 +32,41 @@ margins.
 ![example2][example2]
 
 ## Non-default behavior
+Changing the default behavior can be done using its keyword arguments:
+```typst
+#let hydra(
+  sel: heading,                     // the elements to consider
+  getter: default.get-adjacent,     // gets the neighboring elements according to sel
+  prev-filter: default.prev-filter, // checks if the last element is valid
+  next-filter: default.next-filter, // checks if the next element is valid
+  display: default.display,         // displays the last element
+  resolve: default.resolve,         // contains the glue code combining the other given args
+  is-footer: false,                 // whether this is used from a footer
+) = {
+  ...
+}
+```
+These functions generally take a queried element and sometimes the current location, see the source
+for more info. The defaults assume only headings and fail if another element type is provided.
+
+The `sel` argument can be an element function or selector, or either an array containing either
+of those and an addiitonal filter function. The additional filter function is applied before the
+adjacent arguments are selected from the result of the queries.
+
 ### Configuring filter and display
-By default it will hydra will display `[#numbering #body]` of the heading and this reject unnumbered
-ones. This filtering can be configured using `prev-filter` and `next-filter`, if those are changed
-to include unnumbered headings `display` be changed too.
+By default hydra will display `[#numbering #body]` of the heading and this reject unnumbered
+ones. This filtering can be configured using `prev-filter` and `next-filter`.
 ```typst
-#set page(header: hydra(prev-filter: _ => true, display: h => h.body))
+#set page(header: hydra(prev-filter: (_, _) => true))
 ```
 
 Keep in mind that `next-filter` is also responsible for checking that the next heading is on the
 current page.
 
 ### In the footer
 To use the hydra functon in the footer of your doc, pass `is-footer: true` and place a
-`#metadata(()) <hydra>` somewhere in your header.
+`#metadata(()) <hydra>` somewhere in your header, or before your headings. Hydra will use the
+location of this label to search for the correct headings instead of searching from the footer.
 
 ```typst
 #set page(header: [#metadata(()) <hydra>], footer: hydra(is-footer: true))
@@ -62,7 +83,7 @@ specific levels of headings, pass the appropriate selector.
 #set page(header: hydra(sel: heading.where(level: 1)))
 
 // only consider level 1 - 3
-#set page(header: hydra(sel: (heading, h => h.level <= 3)))
+#set page(header: hydra(sel: (heading, (h, _) => h.level <= 3)))
 
 // consider also figures with this kind, must likely override all default functions other than
 // resolve, or resolve directly, see source

src/default.typ

@@ -7,8 +7,8 @@
     loc = query(selector(<hydra>).before(loc), loc).last().location()
   }
 
-  let prev = query(sel.before(loc), loc).filter(filter)
-  let next = query(sel.after(loc), loc).filter(filter)
+  let prev = query(sel.before(loc), loc).filter(x => filter(x, loc))
+  let next = query(sel.after(loc), loc).filter(x => filter(x, loc))
 
   let prev = if prev != () { prev.last() }
   let next = if next != () { next.first() }
@@ -36,8 +36,10 @@
 #let display(element, loc) = {
   import "/src/util.typ": assert-element
   assert-element(element, heading)
-  numbering(element.numbering, ..counter(heading).at(element.location()))
-  [ ]
+  if element.numbering != none {
+    numbering(element.numbering, ..counter(heading).at(element.location()))
+    [ ]
+  }
   element.body
 }
 

src/util.typ

@@ -7,7 +7,7 @@
 // split a selector into a pair of selector and post filter
 #let into-sel-filter-pair(sel) = {
   if type(sel) in (selector, function) {
-    (selector(sel), _ => true)
+    (selector(sel), (_, _) => true)
   } else if (
     type(sel) == array and sel.len() == 2
     and type(sel.at(0)) in (selector, function) and type(sel.at(1)) == function