libcamera: utils: Add string join function

Add a utils::join() function to join elements of a container into a
string, with a separator and an optional conversion function if the
elements are not implicitly convertible to std::string.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

src/libcamera/include/utils.h

@@ -11,6 +11,7 @@
 #include <chrono>
 #include <memory>
 #include <ostream>
+#include <sstream>
 #include <string>
 #include <string.h>
 #include <sys/time.h>
@@ -109,6 +110,49 @@ inline _hex hex<uint64_t>(uint64_t value, unsigned int width)
 
 size_t strlcpy(char *dst, const char *src, size_t size);
 
+#ifndef __DOXYGEN__
+template<typename Container, typename UnaryOp>
+std::string join(const Container &items, const std::string &sep, UnaryOp op)
+{
+	std::ostringstream ss;
+	bool first = true;
+
+	for (typename Container::const_iterator it = std::begin(items);
+	     it != std::end(items); ++it) {
+		if (!first)
+			ss << sep;
+		else
+			first = false;
+
+		ss << op(*it);
+	}
+
+	return ss.str();
+}
+
+template<typename Container>
+std::string join(const Container &items, const std::string &sep)
+{
+	std::ostringstream ss;
+	bool first = true;
+
+	for (typename Container::const_iterator it = std::begin(items);
+	     it != std::end(items); ++it) {
+		if (!first)
+			ss << sep;
+		else
+			first = false;
+
+		ss << *it;
+	}
+
+	return ss.str();
+}
+#else
+template<typename Container, typename UnaryOp>
+std::string join(const Container &items, const std::string &sep, UnaryOp op = nullptr);
+#endif
+
 namespace details {
 
 class StringSplitter

src/libcamera/utils.cpp

@@ -297,6 +297,22 @@ details::StringSplitter::iterator details::StringSplitter::end() const
 	return iterator(this, std::string::npos);
 }
 
+/**
+ * \fn template<typename Container, typename UnaryOp> \
+ * std::string utils::join(const Container &items, const std::string &sep, UnaryOp op)
+ * \brief Join elements of a container in a string with a separator
+ * \param[in] items The container
+ * \param[in] sep The separator to add between elements
+ * \param[in] op A function that converts individual elements to strings
+ *
+ * This function joins all elements in the \a items container into a string and
+ * returns it. The \a sep separator is added between elements. If the container
+ * elements are not implicitly convertible to std::string, the \a op function
+ * shall be provided to perform conversion of elements to std::string.
+ *
+ * \return A string that concatenates all elements in the container
+ */
+
 /**
  * \fn split(const std::string &str, const std::string &delim)
  * \brief Split a string based on a delimiter