Merge pull request #30685 from sshivensolanki:core-filestorage-unsigned

core: keep large CV_32U values when reading with FileStorage - #30685

Fixes #30684

### Problem

When a `CV_32U` `Mat` was loaded with `FileStorage`, values above `INT_MAX` came back as `0`, in YAML, XML and JSON, with and without base64. The saved file had the correct values, so only reading was affected.

### Fix

- `FileNodeIterator::readRaw` read elements of up to 4 bytes through `(int)node`, so these values turned negative and the `CV_32U` case clamped them to 0. It now always reads the node as `int64_t`. Each case still converts the value to its own type.
- The base64 path stored `getInt32()` as signed, so `CV_32U` now uses a new `Base64Decoder::getUInt32()`, like `getUInt8()` and `getUInt16()`.

`CV_64U` values above `INT64_MAX` are also lost in the text formats, but that has a different cause, in the parsers' `strtoll`, so it is not part of this PR.

### Tests

New `Core_InputOutput.filestorage_32u_large_values` saves values on both sides of `INT_MAX` and loads them back, in YAML, XML and JSON, with and without base64. All 6 combinations fail without the fix, and the test passes with it on Apple M1 Pro.

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake

modules/core/src/persistence.cpp

@@ -1837,6 +1837,10 @@ ushort FileStorage::Impl::Base64Decoder::getUInt16() {
     return val;
 }
 
+unsigned FileStorage::Impl::Base64Decoder::getUInt32() {
+    return (unsigned)getInt32();
+}
+
 int FileStorage::Impl::Base64Decoder::getInt32() {
     size_t sz = decoded.size();
     if (ofs + 4 > sz && !readMore(4))
@@ -1914,7 +1918,7 @@ char *FileStorage::Impl::parseBase64(char *ptr, int indent, FileNode &collection
                         ival = (short) base64decoder.getUInt16();
                         break;
                     case CV_32U:
-                        ival = base64decoder.getInt32();
+                        ival = base64decoder.getUInt32();
                         break;
                     case CV_32S:
                         ival = base64decoder.getInt32();
@@ -2743,7 +2747,7 @@ FileNodeIterator& FileNodeIterator::readRaw( const String& fmt, void* _data0, si
                     FileNode node = *(*this);
                     if( node.isInt() )
                     {
-                        int64_t ival = static_cast<int64_t>(elem_size == 8 ? (int64_t)node : (int)node);
+                        int64_t ival = (int64_t)node;
                         switch( elem_type )
                         {
                         case CV_8U:

modules/core/src/persistence_impl.hpp

@@ -151,6 +151,8 @@ public:
 
         ushort getUInt16();
 
+        unsigned getUInt32();
+
         int getInt32();
 
         int64_t getInt64();

modules/core/test/test_io.cpp

@@ -2662,4 +2662,28 @@ TEST(Core_InputOutput, filestorage_read_uppercase_exponent)
     }
 }
 
+// CV_32U values above INT_MAX must survive saving and loading, in every format
+TEST(Core_InputOutput, filestorage_32u_large_values)
+{
+    const unsigned values[] = { 0u, 1u, 2147483647u, 2147483648u, 4294967295u };
+    const Mat src(1, 5, CV_32U, (void*)values);
+
+    for (const char* ext : { ".yml", ".xml", ".json" })
+    {
+        for (int base64 : { 0, (int)FileStorage::WRITE_BASE64 })
+        {
+            FileStorage writer(ext, FileStorage::WRITE | FileStorage::MEMORY | base64);
+            writer << "m" << src;
+            const std::string text = writer.releaseAndGetString();
+
+            FileStorage reader(text, FileStorage::READ | FileStorage::MEMORY);
+            Mat dst;
+            reader["m"] >> dst;
+
+            EXPECT_EQ(CV_32U, dst.type()) << ext << " base64=" << base64;
+            EXPECT_EQ(0, cv::norm(src, dst, NORM_INF)) << ext << " base64=" << base64;
+        }
+    }
+}
+
 }} // namespace