CLDR-18948 Investigate coverage

common/pathCoverage/README.md

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+# Path Coverage Data
+These are **draft** data files that can be used to find the coverage levels for each path in each locale file.
+The format is a series of lines, where blank lines and lines starting with '#' are ignored, and lines are trimmed.
+
+The lines have the following format:
+
+## Variables
+Variables can be assigned for use in later rules.
+
+```
+variableAssignment := variable '=' values
+variable := '%' cp+
+cp := [\-_A-Za-z0-9]
+values := value (',' value)*
+value := cp+ // a list of cldr attribute values
+```
+
+## Rules
+Rules are used to find the coverage for a path. 
+The structure is the following.
+
+A _chassis_ is an XPath in CLDR, where the attribute values have been removed, along with the preceding '='.
+A rule for a chassis is of the following form:
+
+```
+rule := 'path=' chassis levelTest* \n elseLevel
+
+levelTest := (attributesMatch* 'level=' level)* \n
+attributesMatch := attribute ('='|'≠') variable | values \n
+attribute := 'attr' attributeNumber
+attributeNumber := \d
+
+level := 'core'|'basic'|'moderate'|'modern'|'comprehensive'
+elseLevel := 'elseLevel=' level \n
+```
+
+[ wfc: The chassis within an attributesMatch must be unique and in ascending order, and must be valid according to CLDR ]
+[ wfc: The attributeNumbers within an attributesMatch must be unique and in ascending order ]
+[ wfc: The levels within a level test must be in non-descending order (eg, moderate..moderate is ok, but not moderate..basic ]
+
+Example:
+
+```
+path=//ldml/dates/calendars/calendar[@type]/dateTimeFormats/intervalFormats/intervalFormatItem[@id]/greatestDifference[@id]
+ attr0=gregorian
+ attr1=%intervalFormatItem31
+  level=moderate
+ attr0=generic
+ attr1=%intervalFormatItem23
+  level=moderate
+ attr0=generic,gregorian
+ attr1=Bh,Bhm
+  level=modern
+  elseLevel=comprehensive
+```
+Notice that levels can occur multiple times with different conditions, as with level=moderate above.
+
+Logically, this is read into a main map from chassis to a submap from attributeMatchers to levels.
+Thus the above corresponds to:
+
+```
+chassis → 
+   attr0 ∈ {gregorian} AND attr1 ∈ {value(%intervalFormatItem31)}
+     → moderate
+   attr0 ∈ {generic} AND attr1 ∈ {value(%intervalFormatItem23)}
+     → moderate
+   attr0 ∈ {generic,gregorian} AND attr1 ∈ {Bh,Bhm}
+     → modern
+   ELSE --> comprehensive
+```
+
+The expression attrN=!… is equivalent to attrN ∉ {…}
+
+To use that information to get a level from a path, 
+that path is first converted to a chassis plus an attribute map from attributeNumber to a set of attributeValues 
+(or a variable that resolves to a set of attributeValues).
+
+1. The chassis is looked up in the main map to get the submap. 
+2. If there is none, the resulting level is `comprehensive`.
+3. Then the path's attributeMap is checked against the attributeMatchers, until a match is found.
+4. When a match is found, the previous level is returned
+4. If there no match, the elseLevel is returned
+
+Notes: 
+- While the value associated with an attrN is logically a set,
+it could be transformed by an implementation into another format, such as a regex.
+- No set of attribute conditions need have all of the possible attributes.
+For the second set of attribute conditions in the following example, the attr1 is missing: that means that _any_ attr1 matches.
+
+```
+ attr0=gregorian
+ attr1=%intervalFormatItem31
+  level=moderate
+ attr0=generic
+  level=moderate
+```