[css-easing-2] Rewrite/simplify linear() definition and parsing. Still need to rewrite output algo. w3c#10580

Author: tabatkins

css-easing-2/Overview.bs

@@ -140,107 +140,65 @@ The syntax for specifying an [=easing function=] is as follows:
 A <dfn export>linear easing function</dfn>
 is an [=easing function=]
 that interpolates linearly
-between its [=linear easing function/points=].
+between its [=control points=].
+Each <dfn for="linear easing function">control point</dfn>
+is a pair of numbers,
+associating an [=input progress value=]
+to an [=output progress value=].
 
-A [=linear easing function=] has <dfn for="linear easing function">points</dfn>,
-a [=/list=] of [=linear easing points=].
-Initially a new empty [=/list=].
-
-A <dfn>linear easing point</dfn>
-is a [=/struct=]
-that has:
-
-<dl dfn-for="linear easing point">
-
-: <dfn>input</dfn>
-:: A number or null
-
-      Note: This is only null during [=create a linear easing function=].
-
-: <dfn>output</dfn>
-:: A number
-
-</dl>
 
 ### Syntax ### {#linear-easing-function-syntax}
 
 A [=linear easing function=] has the following syntax:
 
 <pre class="prod">
-  <dfn>&lt;linear-easing-function></dfn> = <dfn function lt="linear()">linear(<<linear-stop-list>>)</dfn>
-  <dfn>&lt;linear-stop-list></dfn> = [ <<linear-stop>> ]#
-  <dfn>&lt;linear-stop></dfn> = <<number>> && <<linear-stop-length>>?
-  <dfn>&lt;linear-stop-length></dfn> = <<percentage>>{1,2}
+  <dfn>linear()</dfn> = linear( [ <<number>> && <<percentage>>{0,2} ]# )
 </pre>
 
 <wpt>
   linear-timing-functions-syntax.html
 </wpt>
 
-''linear()'' is parsed into a [=linear easing function=]
-by calling [=create a linear easing function=],
-passing in its <<linear-stop-list>> as a [=/list=] of <<linear-stop>>s.
-
-### Parsing ### {#linear-easing-function-parsing}
-
-<section algorithm="to create a linear easing function">
-
-To <dfn>create a linear easing function</dfn>
-given a [=/list=] of <<linear-stop>>s |stopList|,
-perform the following.
-It returns a [=linear easing function=] or <i>failure</i>.
-
-1.   Let |function| be a new [=linear easing function=].
-
-1.   Let |largestInput| be negative infinity.
-
-1.   If there are less than two [=list/items=] in |stopList|, then return <i>failure</i>.
-
-1.   [=list/For each=] |stop| in |stopList|:
-
-      1.   Let |point| be a new [=linear easing point=]
-           with its [=linear easing point/output=] set to |stop|'s <<number>> as a number.
-
-      1.   [=list/Append=] |point| to |function|'s [=linear easing function/points=].
-
-      1.   If |stop| has a <<linear-stop-length>>, then:
-
-           1.   Set |point|'s [=linear easing point/input=] to whichever is greater:
-                |stop|'s <<linear-stop-length>>'s first <<percentage>> as a number,
-                or |largestInput|.
-
-           1.   Set |largestInput| to |point|'s [=linear easing point/input=].
-
-           1.   If |stop|'s <<linear-stop-length>> has a second <<percentage>>, then:
-
-                1.   Let |extraPoint| be a new [=linear easing point=]
-                     with its [=linear easing point/output=] set to |stop|'s <<number>> as a number.
-
-                1.   [=list/Append=] |extraPoint| to |function|'s [=linear easing function/points=].
-
-                1.   Set |extraPoint|'s [=linear easing point/input=] to whichever is greater:
-                     |stop|'s <<linear-stop-length>>'s second <<percentage>> as a number,
-                     or |largestInput|.
-
-                1.   Set |largestInput| to |extraPoint|'s [=linear easing point/input=].
-
-      1.   Otherwise, if |stop| is the first [=list/item=] in |stopList|, then:
-
-           1.   Set |point|'s [=linear easing point/input=] to 0.
-
-           1.   Set |largestInput| to 0.
-
-      1.   Otherwise, if |stop| is the last [=list/item=] in |stopList|,
-           then set |point|'s [=linear easing point/input=] to whichever is greater:
-           1 or |largestInput|.
-
-1.   For runs of [=list/items=] in |function|'s [=linear easing function/points=] that have a null [=linear easing point/input=],
-     assign a number to the [=linear easing point/input=] by linearly interpolating between the closest previous and next [=linear easing function/points=]
-     that have a non-null [=linear easing point/input=].
-
-1.   Return |function|.
+Each comma-separated argument defines one or two [=control points=],
+with an [=input progress value=] equal to the specified <<percentage>>
+(converted to a <<number>> between 0 and 1),
+and an [=output progress value=] equal to the specified <<number>>.
+When the argument has two <<percentages>>,
+it defines two [=control points=] in the specified order,
+one per <<percentage>>.
+
+If an argument lacks a <<percentage>>,
+its [=input progress value=] is initially empty,
+but that is immediately corrected by [=linear() canonicalization=] after parsing.
+
+<div algorithm>
+  To <dfn export lt="linear() canonicalization|canonicalize a linear()">canonicalize a linear()</dfn> function's [=control points=],
+  perform the following:
+
+  1. If the first [=control point=] lacks an [=input progress value=],
+    set its [=input progress value=] to 0.
+
+  2. If the last [=control point=] lacks an [=input progress value=],
+    set its [=input progress value=] to 1.
+
+  3. If any [=control point=] has an [=input progress value=]
+    that is less than the [=input progress value=] of any preceding [=control point=],
+    set its [=input progress value=] to the largest [=input progress value=]
+    of any preceding [=control point=].
+
+  4. If any [=control point=] still lacks an [=input progress value=],
+    then for each contiguous run of such [=control points=],
+    set their [=input progress values=]
+    so that they are evenly spaced
+    between the preceding and following [=control points=]
+    with [=input progress values=].
+
+  After canonicalization,
+  every [=control point=] has an [=input progress value=],
+  and the [=input progress values=] are monotonically non-decreasing
+  along the list.
+</div>
 
-</section>
 
 ### Serializing ### {#linear-easing-function-serializing}