pseq: Add format_utils namespace

Add a namespace containing utility functions for formatting data.

The initial utility function converts a span of container elements into
a string.

This function is a temporary solution until g++ implements the C++23
std::format() support for formatting ranges.

fmt::format() supports formatting spans, but the OpenBMC project is
converting to use of std::format() instead.

Tested:
* Ran all new and existing gtests.

Change-Id: Ie84b59db35bb49e3f1bb529490aaa05af434b6fd
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
diff --git a/phosphor-power-sequencer/src/format_utils.hpp b/phosphor-power-sequencer/src/format_utils.hpp
new file mode 100644
index 0000000..327e518
--- /dev/null
+++ b/phosphor-power-sequencer/src/format_utils.hpp
@@ -0,0 +1,58 @@
+/**
+ * Copyright © 2024 IBM Corporation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#pragma once
+
+#include <format>
+#include <span>
+#include <string>
+
+/**
+ * @namespace format_utils
+ *
+ * Contains utility functions for formatting data.
+ */
+namespace phosphor::power::sequencer::format_utils
+{
+
+/**
+ * Returns a string containing the elements in the specified container span.
+ *
+ * The string starts with "[", ends with "]", and the elements are separated by
+ * ", ".  The individual elements are formatted using std::format.
+ *
+ * This function is a temporary solution until g++ implements the C++23
+ * std::format() support for formatting ranges.
+ *
+ * @param span span of elements to format within a container
+ * @return formatted string containing the specified elements
+ */
+template <typename T>
+std::string toString(const std::span<T>& span)
+{
+    std::string str{"["};
+    for (auto it = span.cbegin(); it != span.cend(); ++it)
+    {
+        str += std::format("{}", *it);
+        if (std::distance(it, span.cend()) > 1)
+        {
+            str += ", ";
+        }
+    }
+    str += "]";
+    return str;
+}
+
+} // namespace phosphor::power::sequencer::format_utils