From d4e62a9768c4ce4964f65cca15b3da0a36b910f1 Mon Sep 17 00:00:00 2001
From: Marc Mutz <marc.mutz@qt.io>
Date: Fri, 7 Oct 2022 16:50:42 +0200
Subject: [PATCH] Short live q20::transform()!
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It just adds constexpr to it (we're ignoring the range version).

Apply it to QOffsetStringArray, where it replaces the copyData()
function.

Pick-to: 6.4
Change-Id: I6caf3b5fd2e60f4fcb0b116684c3ad6a8043f38e
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
---
 src/corelib/global/q20algorithm.h        | 19 ++++++++++++++++++-
 src/corelib/tools/qoffsetstringarray_p.h | 14 +++-----------
 2 files changed, 21 insertions(+), 12 deletions(-)

diff --git a/src/corelib/global/q20algorithm.h b/src/corelib/global/q20algorithm.h
index bdfb3268130..1d2e0117f4c 100644
--- a/src/corelib/global/q20algorithm.h
+++ b/src/corelib/global/q20algorithm.h
@@ -27,13 +27,14 @@
 QT_BEGIN_NAMESPACE
 
 namespace q20 {
-// like std::is_sorted{,_until}, std::copy (ie. constexpr)
+// like std::<algorithm> (ie. not ranged, but constexpr)
 #ifdef __cpp_lib_constexpr_algorithms
 using std::copy;
 using std::copy_if;
 using std::copy_n;
 using std::is_sorted_until;
 using std::is_sorted;
+using std::transform;
 #else
 template <typename InputIterator, typename OutputIterator>
 constexpr OutputIterator
@@ -88,11 +89,27 @@ is_sorted_until(ForwardIterator first, ForwardIterator last, BinaryPredicate p =
     }
     return first;
 }
+
 template <typename ForwardIterator, typename BinaryPredicate = std::less<>>
 constexpr bool is_sorted(ForwardIterator first, ForwardIterator last, BinaryPredicate p = {})
 {
     return q20::is_sorted_until(first, last, p) == last;
 }
+
+template <typename InputIterator, typename OutputIterator, typename UnaryFunction>
+constexpr OutputIterator
+transform(InputIterator first, InputIterator last, OutputIterator dest, UnaryFunction op)
+{
+    while (first != last) {
+        *dest = op(*first);
+        ++first;
+        ++dest;
+    }
+    return dest;
+}
+
+// binary transform missing on purpose (no users)
+
 #endif
 }
 
diff --git a/src/corelib/tools/qoffsetstringarray_p.h b/src/corelib/tools/qoffsetstringarray_p.h
index 40834871751..2a0e6de55b0 100644
--- a/src/corelib/tools/qoffsetstringarray_p.h
+++ b/src/corelib/tools/qoffsetstringarray_p.h
@@ -70,16 +70,6 @@ private:
 };
 
 namespace QtPrivate {
-// std::copy is not constexpr in C++17
-template <typename II, typename OO>
-static constexpr OO copyData(II input, qsizetype n, OO output)
-{
-    using E = decltype(+*output);
-    for (qsizetype i = 0; i < n; ++i)
-        output[i] = E(input[i]);
-    return output + n;
-}
-
 template <size_t Highest> constexpr auto minifyValue()
 {
     if constexpr (Highest <= (std::numeric_limits<quint8>::max)()) {
@@ -137,7 +127,9 @@ constexpr auto makeOffsetStringArray(StringExtractor extractString, const T &...
 
     // prepend zero
     std::array<MinifiedOffsetType, Count + 1> minifiedOffsetList = {};
-    QtPrivate::copyData(fullOffsetList.begin(), Count, minifiedOffsetList.begin() + 1);
+    q20::transform(fullOffsetList.begin(), fullOffsetList.end(),
+                   minifiedOffsetList.begin() + 1,
+                   [] (auto e) { return MinifiedOffsetType(e); });
 
     std::array staticString = QtPrivate::makeStaticString<StringLength>(extractString, entries...);
     return QOffsetStringArray(staticString, minifiedOffsetList);