Added utility file for operations on selectors

cascade.ml

@@ -0,0 +1,82 @@
+open Types
+
+(* This file defines functions that construct an element tree with applied
+ * stylesheet based on:
+ * - http://www.w3.org/TR/CSS2/cascade.html#specificity
+ *)
+
+let is_pseudo_class = function
+  | Pseudo (_, ("link" | "hover" | "visited" | "active"), None) -> true
+  | _ -> false
+
+(* Specificity (a, b, c, d):
+ * a = 0 (1 if in style="" definition which is always false in a stylesheet)
+ * b = number of ID attributes
+ * c = number of other (pseudo-)attributes
+ * d = number of (pseudo-)elements
+ *)
+let rec specificity =
+  let add (a, b, c, d) (e, f, g, h) = (a + e, b + f, c + g, d + h) in
+  function
+  | No_element | All_elements ->
+    (0, 0, 0, 0)
+  | Element _ ->
+    (0, 0, 0, 1)
+  | Id (base, _) ->
+    add (0, 1, 0, 0) (specificity base)
+  | Class (base, _) | Attribute (base, _, _) ->
+    add (0, 0, 1, 0) (specificity base)
+  | Pseudo (base, _, _) as addon when is_pseudo_class addon ->
+    add (0, 0, 1, 0) (specificity base)
+  | Pseudo (base, _, _) ->
+    add (0, 0, 0, 1) (specificity base)
+  | Combinator (left, _, right) ->
+    add (specificity left) (specificity right)
+
+let precedes (a, b, c, d) (e, f, g, h) =
+  let rec loop = function
+    | []                -> true
+    | 0 :: tl           -> loop tl
+    | n :: _ when n > 0 -> true
+    | _                 -> false
+  in
+  loop [a - e; b - f; c - g; d - h]
+
+let overwrites selector1 selector2 =
+  precedes (specificity selector1) (specificity selector2)
+
+let can_match_same selector1 selector2 =
+  let unfold =
+    let rec loop classes ids pseudos attrs = function
+      | No_element ->
+        ("", classes, ids, pseudos, attrs)
+      | All_elements ->
+        ("*", classes, ids, pseudos, attrs)
+      | Element elem ->
+        (elem, classes, ids, pseudos, attrs)
+      | Id (base, id) ->
+        loop classes (id :: ids) pseudos attrs base
+      | Class (base, cls) ->
+        loop (cls :: classes) ids pseudos attrs base
+      | Pseudo (base, f, arg) ->
+        (* XXX: what about :not(...) ? *)
+        loop classes ids ((f, arg) :: pseudos) attrs base
+      | Attribute (base, attr, _) ->
+        loop classes ids pseudos (attr :: attrs) base
+      | Combinator (_, _, right) ->
+        loop classes ids pseudos attrs right
+    in
+    loop [] [] [] []
+  in
+  let rec intersect l = function
+    | [] -> false
+    | hd :: _ when List.mem hd l -> true
+    | _ :: tl -> intersect l tl
+  in
+  let elem1, classes1, ids1, pseudos1, attrs1 = unfold selector1 in
+  let elem2, classes2, ids2, pseudos2, attrs2 = unfold selector2 in
+  elem1 = "*" || elem2 = "*" || elem1 = elem2 && elem1 <> "" ||
+  intersect classes1 classes2 ||
+  intersect ids1 ids2 ||
+  intersect pseudos1 pseudos2 ||
+  intersect attrs1 attrs2