util: Add PruneGBtoMiB() function

This commit does not change behavior.

Author: hebasto

src/qt/optionsmodel.cpp

@@ -240,9 +240,8 @@ void OptionsModel::SetPruneEnabled(bool prune, bool force)
 {
     QSettings settings;
     settings.setValue("bPrune", prune);
-    // Convert prune size from GB to MiB:
-    const uint64_t nPruneSizeMiB = (settings.value("nPruneSize").toInt() * GB_BYTES) >> 20;
-    std::string prune_val = prune ? std::to_string(nPruneSizeMiB) : "0";
+    const int64_t prune_target_mib = PruneGBtoMiB(settings.value("nPruneSize").toInt());
+    std::string prune_val = prune ? std::to_string(prune_target_mib) : "0";
     if (force) {
         m_node.forceSetArg("-prune", prune_val);
         return;

src/qt/optionsmodel.h

@@ -22,6 +22,11 @@ static constexpr unsigned short DEFAULT_GUI_PROXY_PORT = 9050;
  */
 static inline int PruneMiBtoGB(int64_t mib) { return (mib * 1024 * 1024 + GB_BYTES - 1) / GB_BYTES; }
 
+/**
+ * Convert displayed prune target GB to configured MiB. Round down so roundtrip GB -> MiB -> GB conversion is stable.
+ */
+static inline int64_t PruneGBtoMiB(int gb) { return gb * GB_BYTES / 1024 / 1024; }
+
 /** Interface from Qt to configuration data structure for Bitcoin client.
    To Qt, the options are presented as a list with the different options
    laid out vertically.