[css-values-5] Define how progress() handles start==end, and abstract a bit to reduce repetition. w3c#10980

Author: tabatkins

css-values-5/Overview.bs

@@ -531,8 +531,29 @@ Interpolation Progress Functional Notations</h2>
 		<var>progress-function</var>() = <var>progress-function</var>( <var>progress value</var> from <var>start value</var> to <var>end value</var> )
 	</pre>
 
-	The resulting ratio is a <<number>>,
-	which can then be input into other calculations,
+	Each resolves to a <<number>>
+	by [=calculating a progress function=].
+
+	<div algorithm>
+		To <dfn export>calculate a progress function</dfn>,
+		given a [=progress value=],
+		[=progress start value=],
+		and [=progress end value=]:
+
+		: If the [=progress start value=] and [=progress end value=] are different values
+		:: <code>([=progress value=] - [=progress start value=]) / ([=progress end value=] - [=progress start value=])</code>.
+		: If the [=progress start value=] and [=progress end value=] are the same value
+		:: 0, -∞, or +∞,
+			depending on whether [=progress value=]
+			is equal to, less than, or greater than
+			the shared value.
+
+		In either case, the result is a <<number>>,
+		[=made consistent=] with the [=consistent type=]
+		of the three arguments.
+	</div>
+
+	The resulting number can then be input into other calculations,
 	such as a [=math function=]
 	or a [=mix notation=].
 
@@ -546,11 +567,6 @@ Calculated Progress Values: the ''progress()'' notation</h3>
 	between two other [=calculations=]
 	(the [=progress start value=]
 	and [=progress end value=]).
-	The argument [=calculations=] can resolve to any <<number>>, <<dimension>>, or <<percentage>>,
-	but must have a [=consistent type=]
-	or else the function is invalid.
-	The result will be a <<number>>,
-	[=made consistent=] with the [=consistent type=] of the arguments.
 
 	The syntax of ''progress()'' is defined as follows:
 
@@ -562,9 +578,12 @@ Calculated Progress Values: the ''progress()'' notation</h3>
 	the [=progress value=], [=progress start value=], and [=progress end value=],
 	respectively.
 
-	The value returned by a valid ''progress()'' notation is
-	(<var>progress value</var> - <var>start value</var>) / (<var>end value</var> - <var>start value</var>),
-	as a <<number>>.
+	The argument [=calculations=] can resolve to any <<number>>, <<dimension>>, or <<percentage>>,
+	but must have a [=consistent type=]
+	or else the function is invalid.
+
+	The result will be a <<number>>,
+	determined by [=calculating a progress function=].
 
 	ISSUE: Do we need a ''percent-progress()'' notation,
 	or do enough places auto-convert that it's not necessary?
@@ -590,20 +609,18 @@ Media Query Progress Values: the ''media-progress()'' notation</h3>
 		<dfn><<media-progress()>></dfn> = media-progress(<<media-feature>> from <<calc-sum>> to <<calc-sum>>)
 	</pre>
 
-	The value returned by a valid ''media-progress()'' notation is
-	<var>progress value</var> / (<var>end value</var> - <var>start value</var>,
-	as a <<number>>.
+	where the value of the <<media-feature>> is the [=progress value=],
+	and the two calculations are the [=progress start value=] and [=progress end value=],
+	respectively.
 
 	The specified [=media query=] must be a valid “range” type query,
-	and its specified [=progress start value=] and [=progress end value=]
+	and the specified [=progress start value=] and [=progress end value=]
 	must be valid values for the specified [=media query=],
+	and must have a [=consistent type=],
 	or else the function is invalid.
 
-	The two input [=calculations=]
-	but must have a [=consistent type=]
-	or else the function is invalid.
 	The result will be a <<number>>,
-	[=made consistent=] with the [=consistent type=] of the arguments.
+	determined by [=calculating a progress function=].
 
 <h3 id="container-progress-func">
 Container Query Progress Values: the ''container-progress()'' notation</h3>
@@ -622,10 +639,15 @@ Container Query Progress Values: the ''container-progress()'' notation</h3>
 	where the optional <<container-name>> component specifies
 	the named containers to consider when selecting a container
 	to resolve against.
+	The value of the <<size-feature>> is the [=progress value=],
+	and the two calculations are the [=progress start value=] and [=progress end value=],
+	respectively.
 
-	The two input [=calculations=]
-	but must have a [=consistent type=]
+	The [=progress start value=] and [=progress end value=]
+	must be valid values for the specified <<size-feature>>,
+	and must have a [=consistent type=],
 	or else the function is invalid.
+
 	The result will be a <<number>>,
 	[=made consistent=] with the [=consistent type=] of the arguments.