feat(core): Extend SU.collatedSort for complex table sorting

core/utilities/sorting.lua

@@ -1,21 +1,30 @@
 --- Table sorting with language-dependent collation.
 -- @module SU.sorting
+--
 
 local icu = require("justenoughicu")
 
 local collatedSort = {
    -- No ICU for language "und", fallback to 'natural' table.sort
-   und = function (t, _)
-      table.sort(t)
+   und = function (t, _, comparator)
+      if comparator then
+         table.sort(t, function (e1, e2)
+            return comparator(e1, e2, function (s1, s2)
+               return s1 < s2 and -1 or s1 > s2 and 1 or 0
+            end)
+         end)
+      else
+         table.sort(t)
+      end
    end,
 }
 
 setmetatable(collatedSort, {
-   __call = function (self, t, options)
+   __call = function (self, t, options, comparator)
       local lang = SILE.settings:get("document.language")
       if self[lang] and type(self[lang]) == "function" then
          -- Allow overriding ICU for some languages, typically "und"
-         return self[lang](t, options)
+         return self[lang](t, options, comparator)
       end
 
       if self[lang] and type(self[lang]) == "table" then
@@ -25,8 +34,17 @@ setmetatable(collatedSort, {
       -- Be efficient: create the collator once before sorting.
       -- I don't think we need to cache it, still.
       local collator = icu.collation_create(lang, options or {})
-      table.sort(t, function (s1, s2)
+
+      local stringCompareClosure = function (s1, s2)
          return icu.compare(collator, s1, s2)
+      end
+      table.sort(t, function (e1, e2)
+         -- Allow custom comparison function, notably for complex objects
+         -- Pass the stringCompare function so that it can be used.
+         if comparator then
+            return comparator(e1, e2, stringCompareClosure)
+         end
+         return stringCompareClosure(e1, e2) < 0
       end)
       icu.collation_destroy(collator)
    end,