ICU-23231 ICU 78rc download page

See #3715

Author: markusicu

docs/download/78.md

@@ -16,14 +16,14 @@ License & terms of use: http://www.unicode.org/copyright.html
 ICU is the [premier library for software internationalization](https://icu.unicode.org/#h.i33fakvpjb7o),
 used by a [wide array of companies and organizations](https://icu.unicode.org/#h.f9qwubthqabj).
 
-## Draft page
+## Release Candidate
 
-**This is an early draft of the ICU 78 download page. Nothing is published yet.**
+**This is a release candidate. Please use it for testing, but do not use it in production.**
 
 ## Release Overview
 
-* Download: TODO [releases/tag/release-78-1](https://github.com/unicode-org/icu/releases/tag/release-78-1)
-* TODO [Maven: com.ibm.icu / icu4j / version 78.1](https://mvnrepository.com/artifact/com.ibm.icu/icu4j/78.1)
+* Download: [releases/tag/release-78.1rc](https://github.com/unicode-org/icu/releases/tag/release-78.1rc) <!-- TODO final version -->
+<!-- * TODO [Maven: com.ibm.icu / icu4j / version 78.1](https://mvnrepository.com/artifact/com.ibm.icu/icu4j/78.1) -->
 
 ICU 78 updates to
 [Unicode 17](https://www.unicode.org/versions/Unicode17.0.0/)
@@ -35,7 +35,7 @@ It also updates to
 ([beta blog](https://blog.unicode.org/2025/10/unicode-cldr-48-beta-available-for.html))
 locale data with new locales, and various additions and corrections.
 
-In Java, there is a draft new Segmenter API which is easier and safer to use than BreakIterator.
+In Java, there is a new Segmenter API which is easier and safer to use than BreakIterator.\
 In C++, there is a new set of APIs for Unicode string (UTF-8/16/32) code point iteration
 that works seamlessly with modern C++ iterators and ranges.
 
@@ -53,6 +53,15 @@ For more details, including migration issues, see below.
 Please use the [icu-support mailing list](https://icu.unicode.org/contacts) and/or
 [find/submit error reports](https://icu.unicode.org/bugs).
 
+### Attention: Future Changes
+
+Beginning with CLDR 49 / ICU 79 (2026-mar), CLDR and ICU are planning to make changes in
+time formatting options for the hour cycle (details of 12/24 hour formats),
+make the week-of-year numbering always follow ISO rules,
+and remove the pre-Meiji Japanese eras.
+
+See the [CLDR V49 advance warnings](https://cldr.unicode.org/downloads/cldr-48#v49-advance-warnings).
+
 ### Version Number
 
 The initial release has library version number 78.1.
@@ -83,38 +92,42 @@ Note: There may be additional commits on the [maint/maint-78](https://github.com
       remain recommended for default identifier use.
 * [CLDR 48](https://cldr.unicode.org/downloads/cldr-48)
   ([beta blog](https://blog.unicode.org/2025/10/unicode-cldr-48-beta-available-for.html)):
-  * TODO: old news here
-  * No major data collection for existing locales; focus on bug fixes and structural improvements
-  * New regional variants: English in several European countries, and Cantonese in Macau (`yue_Hant_MO`)
-  * Improved RBNF (number spellout) and transliteration data
-* TODO: old news here
-* Subtle segmentation changes to make ICU fully conform to Unicode 16
-  * Word break: Root tailoring of colon reverted, Swedish & Finnish tailorings removed
-    ([ICU-22941](https://unicode-org.atlassian.net/browse/ICU-22941))
-    * These tailorings were introduced in ICU 72, but feedback has been negative,
-      and the UTC declined to adopt these changes.
-  * Line break: Fixed a bug in the line breaking of obscure sequences
-    ⟨no-break space, combining mark, hyphen, alphabetic character⟩
-    ([ICU-22986](https://unicode-org.atlassian.net/browse/ICU-22986)).
-  * Updated Indic grapheme clusters to use the latest `Indic_Conjunct_Break` data
-    ([ICU-22956](https://unicode-org.atlassian.net/browse/ICU-22956))
+  * Significant data updates across all locales
+  * Locales which are now at modern coverage level: Akan, Bashkir, Chuvash, Kazakh (Arabic), Romansh, Shan, Quechua
+  * Locales which are now at moderate coverage level: Anii, Esperanto
+  * Many new measurement units, for scientific contexts (coulombs, farads, teslas, etc.)
+    and for English systems (fortnights, imperial pints, etc.)
+  * Some measurement unit identifiers changed, see [CLDR 48 Migration](https://cldr.unicode.org/downloads/cldr-48#migration)
 * Time zone data (tzdata) version 2025b (2025-mar).
 
 ## ICU4C Specific Changes
 
-* [API changes since ICU4C 77 (Markdown)](https://github.com/unicode-org/icu/blob/maint/maint-78/icu4c/APIChangeReport.md) / [(HTML)](https://htmlpreview.github.io/?https://github.com/unicode-org/icu/blob/maint/maint-78/icu4c/APIChangeReport.html)
-  * TODO: In C++, there is a new set of APIs for Unicode string (UTF-8/16/32) code point iteration
+* [API changes since ICU4C 77 (Markdown)](https://github.com/unicode-org/icu/blob/maint/maint-78/icu4c/APIChangeReport.md) /
+  [(HTML)](https://htmlpreview.github.io/?https://github.com/unicode-org/icu/blob/maint/maint-78/icu4c/APIChangeReport.html)
+  * The widely used class Locale has been optimized. Locale objects for most common locale IDs now only use 40 bytes
+    (down from at least 224 bytes).
+    ([ICU-20392](https://unicode-org.atlassian.net/browse/ICU-20392))
+  * New set of “C++ Header-Only APIs” for Unicode string (UTF-8/16/32) code point iteration
     that works seamlessly with modern C++ iterators and ranges.
-  * TODO: old news here
-  * New APIs for colloquial C++ use of C USet ([ICU-22876](https://unicode-org.atlassian.net/browse/ICU-22876))
-    and C UCollator ([ICU-22879](https://unicode-org.atlassian.net/browse/ICU-22879))
-    * These were added in ICU 76, but some of the new APIs did not actually compile with `U_SHOW_CPLUSPLUS_API=0`.
-      They have been fixed in ICU 77 and thoroughly tested.
-      USetElementIterator now returns std::u16string instead of icu::UnicodeString,
-      and therefore it and related APIs have been changed to `@draft ICU 77`.
-      ([ICU-22954](https://unicode-org.atlassian.net/browse/ICU-22954))
-    * For details about these APIs and an example see the
-      “C++ Header-Only APIs” section of the [ICU 76 Migration Issues](76.md#migration-issues).
+    As with the existing C macros, there are versions which validate the code unit sequences on the fly,
+    as well as fast but “unsafe” versions which assume & require well-formed strings.
+    ([ICU-23004](https://unicode-org.atlassian.net/browse/ICU-23004)]):\
+    [unicode/utfiterator.h](https://unicode-org.github.io/icu-docs/apidoc/dev/icu4c/utfiterator_8h.html)\
+    (For an introduction to “C++ Header-Only APIs” see
+    this section of the [ICU 76 Migration Issues](76.md#migration-issues).)
+  * Additional Unicode helper APIs
+    ([ICU-23152](https://unicode-org.atlassian.net/browse/ICU-23152)):
+    * ([unicode/utf.h](https://unicode-org.github.io/icu-docs/apidoc/dev/icu4c/utf_8h.html),
+      [unicode/utf8.h](https://unicode-org.github.io/icu-docs/apidoc/dev/icu4c/utf8_8h.html)):
+      U_IS_CODE_POINT(cp), U_IS_SCALAR_VALUE(cp), U8_LENGTH_FROM_LEAD_BYTE\[_UNSAFE\]
+    * ([icu::UnicodeString](https://unicode-org.github.io/icu-docs/apidoc/dev/icu4c/classicu_1_1UnicodeString.html)):
+      begin(), end(), rbegin(), rend(), push_back(c)
+      (a UnicodeString is now a C++ “range” of char16_t code units),
+      toUTF8String() without output string *parameter*
+    * (new [unicode/utfiterator.h](https://unicode-org.github.io/icu-docs/apidoc/dev/icu4c/utfiterator_8h.html)):
+      “Range” classes AllCodePoints & AllScalarValues
+    * (new [unicode/utfstring.h](https://unicode-org.github.io/icu-docs/apidoc/dev/icu4c/utfstring_8h.html)):
+      Functions that write a code point to a string object
 
 ## ICU4J Specific Changes
 
@@ -125,11 +138,23 @@ Note: There may be additional commits on the [maint/maint-78](https://github.com
   Note that [Android desugaring](https://developer.android.com/studio/write/java11-default-support-table)
   supports at least Java 11 since late 2023.
 * [API Changes since ICU4J 77](https://htmlpreview.github.io/?https://github.com/unicode-org/icu/blob/maint/maint-78/icu4j/APIChangeReport.html)
-  * TODO: In Java, there is a draft new Segmenter API which is easier and safer to use than BreakIterator.
+  * New Segmenter API which is easier and safer to use than BreakIterator.
+    The new API builds immutable objects and returns Java Streams of boundaries and segments.
+    ([ICU-22789](https://unicode-org.atlassian.net/browse/ICU-22789))\
+    See [package com.ibm.icu.segmenter](https://unicode-org.github.io/icu-docs/apidoc/dev/icu4j/com/ibm/icu/segmenter/package-summary.html),
+    [interface Segmenter](https://unicode-org.github.io/icu-docs/apidoc/dev/icu4j/com/ibm/icu/segmenter/Segmenter.html)
+    and its implementing classes, etc.
+  * All clone() functions now explicitly return their class types, rather than Object (“covariant return types”),
+    so that call sites no longer need to downcast.
+    ([ICU-23140](https://unicode-org.atlassian.net/browse/ICU-23140))
+  * Additional Unicode helper functions
+    ([class UCharacter](https://unicode-org.github.io/icu-docs/apidoc/dev/icu4j/com/ibm/icu/lang/UCharacter.html)):
+    isNoncharacter(cp), isScalarValue(cp), allCodePoints(), allScalarValuesStream(), etc.
+    ([ICU-23152](https://unicode-org.atlassian.net/browse/ICU-23152))
+    <!-- TODO: after release, change API docs links from dev to released -->
 * We have removed the
-   [ICU4J Locale Service Provider](../userguide/icu4j/locale-service-provider.md)
+   [ICU4J Locale Service Provider](../userguide/icu4j/locale-service-provider.md).
    ([ICU-23071](https://unicode-org.atlassian.net/browse/ICU-23071))\
-   ([Maven: com.ibm.icu / icu4j-localespi / version 77.1](https://mvnrepository.com/artifact/com.ibm.icu/icu4j-localespi/77.1)).
    It had become much less useful than when we added it and had very low usage.
    Projects that used it should call ICU4J directly instead.
 * The Java implementation of the
@@ -169,8 +194,8 @@ ICU4J should work on Android API level 21 and later but may require “[library
 ## Download
 
 ### GitHub
-TODO rc tag:
-Source and binary downloads are available on the git/GitHub tag page: <https://github.com/unicode-org/icu/releases/tag/release-78.1rc>
+Source and binary downloads are available on the git/GitHub tag page:
+<https://github.com/unicode-org/icu/releases/tag/release-78.1rc>
 
 See the [Source Code Setup](../devsetup/source/) page for how to download the ICU file tree directly from GitHub.
 
@@ -183,4 +208,3 @@ ICU locale data was generated from CLDR data equivalent to:
 * TODO: not published yet
 * https://mvnrepository.com/artifact/com.ibm.icu/icu4j/78.1
 * https://mvnrepository.com/artifact/com.ibm.icu/icu4j-charset/78.1
-* https://mvnrepository.com/artifact/com.ibm.icu/icu4j-localespi/78.1