ICU-22285 omit the gb2312 & big5han collation tailorings by default

docs/userguide/icu_data/buildtool.md

@@ -78,6 +78,32 @@ To build ICU4J with custom data, you must first build ICU4C with custom data
 and then generate the JAR file.  For more information on building ICU4J, read the
 [ICU4J Readme](../icu4j/).
 
+### Default Configuration
+
+By default (without a configuration file and without option flags),
+the ICU data file includes all of the data in the ICU source tree.
+
+Since ICU 73 (2023q2), there is an exception:
+By default, the "big5han" and "gb2312han" collation tailorings are omitted.
+These mimic the order of their respective charsets, are relatively large, and rarely used.
+(See [ICU-22285](https://unicode-org.atlassian.net/browse/ICU-22285).)
+
+The default configuration is equivalent to a filter file like this:
+
+    {
+      "resourceFilters": [
+        {
+          "categories": [
+            "coll_tree"
+          ],
+          "rules": [
+            "-/collations/big5han",
+            "-/collations/gb2312han"
+          ]
+        }
+      ]
+    }
+
 ### Locale Slicing
 
 The simplest way to slice ICU data is by locale.  The ICU Data Build Tool

icu4c/source/python/icutools/databuilder/__main__.py

@@ -159,6 +159,23 @@ def __init__(self, args):
         if "usePoolBundle" in self.filters_json_data:
             self.use_pool_bundle = self.filters_json_data["usePoolBundle"]
 
+        # By default, exclude collation data that mimics the order of some large legacy charsets.
+        # We do this in "subtractive" strategy by inserting a resourceFilter.
+        # Later rules from an explicit filter file may override this default behavior.
+        # (In "additive" strategy this is unnecessary.)
+        if self.strategy == "subtractive":
+            filters = self.filters_json_data.setdefault("resourceFilters", [])
+            omit_charset_collations = {
+                "categories": [
+                    "coll_tree"
+                ],
+                "rules": [
+                    "-/collations/big5han",
+                    "-/collations/gb2312han"
+                ]
+            }
+            filters.insert(0, omit_charset_collations)
+
     def _parse_filter_file(self, f):
         # Use the Hjson parser if it is available; otherwise, use vanilla JSON.
         try:

icu4j/main/tests/collate/src/com/ibm/icu/dev/test/util/ICUResourceBundleCollationTest.java

@@ -49,19 +49,19 @@ public void TestFunctionalEquivalent(){
            "f",     "zh_MO",                            "zh@collation=stroke", /* alias of zh_Hant_MO */
            "t",     "zh_Hant_MO",                       "zh@collation=stroke",
            "f",     "zh_TW_STROKE",                     "zh@collation=stroke",
-           "f",     "zh_TW_STROKE@collation=big5han",   "zh@collation=big5han",
+           "f",     "zh_TW_STROKE@collation=zhuyin",    "zh@collation=zhuyin",
            "f",     "sv_CN@calendar=japanese",          "sv",
            "t",     "sv@calendar=japanese",             "sv",
-           "f",     "zh_TW@collation=big5han",          "zh@collation=big5han", /* alias of zh_Hant_TW */
-           "t",     "zh_Hant_TW@collation=big5han",     "zh@collation=big5han",
-           "f",     "zh_TW@collation=gb2312han",        "zh@collation=gb2312han", /* alias of zh_Hant_TW */
-           "t",     "zh_Hant_TW@collation=gb2312han",   "zh@collation=gb2312han",
-           "f",     "zh_CN@collation=big5han",          "zh@collation=big5han", /* alias of zh_Hans_CN */
-           "t",     "zh_Hans_CN@collation=big5han",     "zh@collation=big5han",
-           "f",     "zh_CN@collation=gb2312han",        "zh@collation=gb2312han", /* alias of zh_Hans_CN */
-           "t",     "zh_Hans_CN@collation=gb2312han",   "zh@collation=gb2312han",
-           "t",     "zh@collation=big5han",             "zh@collation=big5han",
-           "t",     "zh@collation=gb2312han",           "zh@collation=gb2312han",
+           "f",     "zh_TW@collation=zhuyin",           "zh@collation=zhuyin", /* alias of zh_Hant_TW */
+           "t",     "zh_Hant_TW@collation=zhuyin",      "zh@collation=zhuyin",
+           "f",     "zh_TW@collation=unihan",           "zh@collation=unihan", /* alias of zh_Hant_TW */
+           "t",     "zh_Hant_TW@collation=unihan",      "zh@collation=unihan",
+           "f",     "zh_CN@collation=zhuyin",           "zh@collation=zhuyin", /* alias of zh_Hans_CN */
+           "t",     "zh_Hans_CN@collation=zhuyin",      "zh@collation=zhuyin",
+           "f",     "zh_CN@collation=unihan",           "zh@collation=unihan", /* alias of zh_Hans_CN */
+           "t",     "zh_Hans_CN@collation=unihan",      "zh@collation=unihan",
+           "t",     "zh@collation=zhuyin",              "zh@collation=zhuyin",
+           "t",     "zh@collation=unihan",              "zh@collation=unihan",
            "t",     "hi@collation=standard",            "hi",
            "f",     "hi_AU@collation=standard;currency=CHF;calendar=buddhist",  "hi",
            "f",     "sv_SE@collation=pinyin",           "sv", /* bug 4582 tests */