Move all `startsWith()` funcs to `Util` namespace

The function checks if a string is in the beginning of another. This
is intended for use until the move to C++20 when
`std::basic_string<>::starts_with()` becomes available. Suggested in
https://github.com/Exiv2/exiv2/pull/1777 .

Separate identical definitions have been moved into a single copy.

src/CMakeLists.txt

@@ -38,6 +38,7 @@ add_library( exiv2lib_int OBJECT
     tifffwd_int.hpp
     timegm.h
     unused.h
+    utils.cpp               utils.hpp
 )
 
 set(PUBLIC_HEADERS

src/epsimage.cpp

@@ -33,6 +33,7 @@
 #include "error.hpp"
 #include "futils.hpp"
 #include "version.hpp"
+#include "utils.hpp"
 
 // + standard includes
 #include <algorithm>
@@ -48,6 +49,7 @@
 namespace {
 
     using namespace Exiv2;
+    using namespace Util;
     using Exiv2::byte;
 
     // signature of DOS EPS
@@ -134,12 +136,6 @@ namespace {
         return static_cast<uint32_t>(pos);
     }
 
-    //! Check whether a string has a certain beginning
-    bool startsWith(const std::string& s, const std::string& start)
-    {
-        return s.size() >= start.size() && memcmp(s.data(), start.data(), start.size()) == 0;
-    }
-
     //! Check whether a string contains only white space characters
     bool onlyWhitespaces(const std::string& s)
     {

src/makernote_int.cpp

@@ -32,6 +32,7 @@
 #include "tiffvisitor_int.hpp"
 #include "tiffimage.hpp"
 #include "tiffimage_int.hpp"
+#include "utils.hpp"
 
 // + standard includes
 #include <string>
@@ -1080,14 +1081,6 @@ namespace Exiv2 {
 
 #define NA ((uint32_t)-1)
 
-    // TODO: When moving to C++20, this can be replace with
-    //       std::basic_string<CharT,Traits,Allocator>::starts_with() .
-    //       Suggested in https://github.com/Exiv2/exiv2/pull/1777 .
-    bool startsWith(const std::string& s, const std::string& start)
-    {
-        return s.size() >= start.size() && std::memcmp(s.data(), start.data(), start.size()) == 0;
-    }
-
     //! Nikon binary array version lookup table
     constexpr NikonArrayIdx nikonArrayIdx[] = {
         // NikonSi
@@ -1203,7 +1196,7 @@ namespace Exiv2 {
         // Not valid for models beginning
         std::string model = getExifModel(pRoot);
         for (auto& m : { "SLT-", "HV", "ILCA-" }) {
-            if (startsWith(model, m))
+            if (Util::startsWith(model, m))
                 return -1;
         }
         return 0;

src/sonymn_int.cpp

@@ -27,6 +27,7 @@
 #include "value.hpp"
 #include "exif.hpp"
 #include "i18n.h"                // NLS support.
+#include "utils.hpp"
 
 // + standard includes
 #include <string>
@@ -864,7 +865,7 @@ namespace Exiv2 {
             // Ranges of models that do not support this tag
             std::string model = pos->toString();
             for (auto& m : { "DSC-", "Stellar" }) {
-                if (startsWith(model, m)) {
+                if (Util::startsWith(model, m)) {
                     os << N_("n/a");
                     return os;
                 }
@@ -955,11 +956,6 @@ namespace Exiv2 {
         return tagInfo2010e_;
     }
 
-    bool SonyMakerNote::startsWith(const std::string& s, const std::string& start)
-    {
-        return s.size() >= start.size() && std::memcmp(s.data(), start.data(), start.size()) == 0;
-    }
-
     // https://github.com/Exiv2/exiv2/pull/906#issuecomment-504338797
     static DataBuf sonyTagCipher(uint16_t /* tag */, const byte* bytes, uint32_t size, TiffComponent* const /*object*/, bool bDecipher)
     {

src/sonymn_int.hpp

@@ -76,10 +76,6 @@ namespace Exiv2 {
         static const TagInfo tagInfoSonyMisc1_[];
         static const TagInfo tagInfo2010e_[];
 
-        // TODO: When moving to C++20, this can be replace with
-        //       std::basic_string<CharT,Traits,Allocator>::starts_with() .
-        //       Suggested in https://github.com/Exiv2/exiv2/pull/1777 .
-        static bool startsWith(const std::string& s, const std::string& start);
     }; // class SonyMakerNote
 
     DataBuf sonyTagDecipher(uint16_t, const byte*, uint32_t, TiffComponent* const);

src/utils.cpp

@@ -119,4 +119,10 @@ namespace Util {
         }
     }
 
+    bool startsWith(const std::string& s, const std::string& start)
+    {
+        return s.size() >= start.size() && std::memcmp(s.data(), start.data(), start.size()) == 0;
+    }
+
+
 }                                       // namespace Util

src/utils.hpp

@@ -77,6 +77,15 @@ namespace Util {
      */
     void replace(std::string& text, const std::string& searchText, const std::string& replaceText);
 
+    // TODO: When using C++20, replace with std::basic_string<CharT,Traits,Allocator>::starts_with()
+    //       (suggested in https://github.com/Exiv2/exiv2/pull/1777).
+    /*!
+      @brief Tests if \em start occurs at the beginning of \em s.
+             Returns true if found, else false.
+             When Exiv2 uses C++20, this will be replaced with
+             std::basic_string<CharT,Traits,Allocator>::starts_with().
+     */
+    bool startsWith(const std::string& s, const std::string& start);
 }                                       // namespace Util
 
 #endif                                  // #ifndef UTILS_HPP_