Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libutil/json-utils.cc

@@ -0,0 +1,130 @@
+#include "json-utils.hh"
+#include "error.hh"
+#include "types.hh"
+#include <nlohmann/json_fwd.hpp>
+#include <iostream>
+
+namespace nix {
+
+const nlohmann::json * get(const nlohmann::json & map, const std::string & key)
+{
+    auto i = map.find(key);
+    if (i == map.end()) return nullptr;
+    return &*i;
+}
+
+nlohmann::json * get(nlohmann::json & map, const std::string & key)
+{
+    auto i = map.find(key);
+    if (i == map.end()) return nullptr;
+    return &*i;
+}
+
+const nlohmann::json & valueAt(
+    const nlohmann::json::object_t & map,
+    const std::string & key)
+{
+    if (!map.contains(key))
+        throw Error("Expected JSON object to contain key '%s' but it doesn't: %s", key, nlohmann::json(map).dump());
+
+    return map.at(key);
+}
+
+std::optional<nlohmann::json> optionalValueAt(const nlohmann::json::object_t & map, const std::string & key)
+{
+    if (!map.contains(key))
+        return std::nullopt;
+
+    return std::optional { map.at(key) };
+}
+
+
+const nlohmann::json * getNullable(const nlohmann::json & value)
+{
+    return value.is_null() ? nullptr : &value;
+}
+
+/**
+ * Ensure the type of a JSON object is what you expect, failing with a
+ * ensure type if it isn't.
+ *
+ * Use before type conversions and element access to avoid ugly
+ * exceptions, but only part of this module to define the other `get*`
+ * functions. It is too cumbersome and easy to forget to expect regular
+ * JSON code to use it directly.
+ */
+static const nlohmann::json & ensureType(
+    const nlohmann::json & value,
+    nlohmann::json::value_type expectedType
+    )
+{
+    if (value.type() != expectedType)
+        throw Error(
+            "Expected JSON value to be of type '%s' but it is of type '%s': %s",
+            nlohmann::json(expectedType).type_name(),
+            value.type_name(), value.dump());
+
+    return value;
+}
+
+const nlohmann::json::object_t & getObject(const nlohmann::json & value)
+{
+    return ensureType(value, nlohmann::json::value_t::object).get_ref<const nlohmann::json::object_t &>();
+}
+
+const nlohmann::json::array_t & getArray(const nlohmann::json & value)
+{
+    return ensureType(value, nlohmann::json::value_t::array).get_ref<const nlohmann::json::array_t &>();
+}
+
+const nlohmann::json::string_t & getString(const nlohmann::json & value)
+{
+    return ensureType(value, nlohmann::json::value_t::string).get_ref<const nlohmann::json::string_t &>();
+}
+
+const nlohmann::json::number_integer_t & getInteger(const nlohmann::json & value)
+{
+    return ensureType(value, nlohmann::json::value_t::number_integer).get_ref<const nlohmann::json::number_integer_t &>();
+}
+
+const nlohmann::json::boolean_t & getBoolean(const nlohmann::json & value)
+{
+    return ensureType(value, nlohmann::json::value_t::boolean).get_ref<const nlohmann::json::boolean_t &>();
+}
+
+Strings getStringList(const nlohmann::json & value)
+{
+    auto & jsonArray = getArray(value);
+
+    Strings stringList;
+
+    for (const auto & elem : jsonArray)
+        stringList.push_back(getString(elem));
+
+    return stringList;
+}
+
+StringMap getStringMap(const nlohmann::json & value)
+{
+    auto & jsonObject = getObject(value);
+
+    StringMap stringMap;
+
+    for (const auto & [key, value] : jsonObject)
+        stringMap[getString(key)] = getString(value);
+
+    return stringMap;
+}
+
+StringSet getStringSet(const nlohmann::json & value)
+{
+    auto & jsonArray = getArray(value);
+
+    StringSet stringSet;
+
+    for (const auto & elem : jsonArray)
+        stringSet.insert(getString(elem));
+
+    return stringSet;
+}
+}