fix: preserve staff-scoped staff-layout number="N"

src/include/mx/api/SystemLayoutData.h

@@ -16,6 +16,29 @@ namespace mx
 {
 namespace api
 {
+/// A <staff-layout> scoped to one staff of a multi-staff part (MusicXML's number attribute):
+/// the space between the bottom line of the previous staff and the top line of this one.
+/// staffIndex is zero-based; a source with an explicit number attribute always populates
+/// SystemLayoutData::staffDistances with one of these rather than the unscoped
+/// SystemLayoutData::staffDistance, so the number round-trips faithfully.
+class StaffDistanceData
+{
+  public:
+    int staffIndex;
+    double staffDistance;
+
+    explicit inline StaffDistanceData(int inStaffIndex = INDEX_UNSPECIFIED, double inStaffDistance = DOUBLE_UNSPECIFIED)
+        : staffIndex(inStaffIndex), staffDistance(inStaffDistance)
+    {
+    }
+};
+
+MXAPI_EQUALS_BEGIN(StaffDistanceData)
+MXAPI_EQUALS_MEMBER(staffIndex)
+MXAPI_DOUBLES_EQUALS_MEMBER(staffDistance)
+MXAPI_EQUALS_END;
+MXAPI_NOT_EQUALS_AND_VECTORS(StaffDistanceData);
+
 /// Margins and spacing for staff systems.
 class SystemLayoutData
 {
@@ -29,21 +52,27 @@ class SystemLayoutData
     /// Distance from the top margin of the page to the top line of the first staff on the page, in tenths.
     OptionalDouble topSystemDistance;
 
-    /// the space between staves within the same system, in tenths.
+    /// the space between staves within the same system, in tenths. Only populated when the source's
+    /// <staff-layout> had no number attribute (the common single-staff-distance case); a source with
+    /// an explicit number populates staffDistances instead. See StaffDistanceData.
     OptionalDouble staffDistance;
 
+    /// Per-staff overrides of staffDistance for parts with more than one staff-scoped <staff-layout>
+    /// (e.g. an organ's third staff needing its own spacing). Normally empty.
+    std::vector<StaffDistanceData> staffDistances;
+
     /// Returns true if any of the members have values.
     inline bool isUsed() const
     {
-        return margins || systemDistance || topSystemDistance || staffDistance;
+        return margins || systemDistance || topSystemDistance || staffDistance || !staffDistances.empty();
     }
 
     explicit inline SystemLayoutData(std::optional<LeftRight> inMargins = std::nullopt,
                                      OptionalDouble inSystemDistance = std::nullopt,
                                      OptionalDouble inTopSystemDistance = std::nullopt,
                                      OptionalDouble inStaffDistance = std::nullopt)
         : margins(inMargins), systemDistance(inSystemDistance), topSystemDistance{inTopSystemDistance},
-          staffDistance(inStaffDistance)
+          staffDistance(inStaffDistance), staffDistances{}
     {
     }
 };
@@ -53,6 +82,7 @@ MXAPI_EQUALS_MEMBER(margins)
 MXAPI_EQUALS_MEMBER(systemDistance)
 MXAPI_EQUALS_MEMBER(topSystemDistance)
 MXAPI_EQUALS_MEMBER(staffDistance)
+MXAPI_EQUALS_MEMBER(staffDistances)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(SystemLayoutData);
 } // namespace api

src/private/mx/impl/LayoutFunctions.cpp

@@ -236,6 +236,15 @@ void addSystemMargins(const api::DefaultsData &inDefaults, core::ScoreHeaderGrou
         needsDefaults = true;
     }
 
+    for (const auto &staffDistance : inDefaults.systemLayout.staffDistances)
+    {
+        core::StaffLayout staffLayout;
+        staffLayout.setNumber(core::StaffNumber{staffDistance.staffIndex + 1});
+        staffLayout.setStaffDistance(toTenths(staffDistance.staffDistance));
+        layout.addStaffLayout(staffLayout);
+        needsDefaults = true;
+    }
+
     if (needsDefaults)
     {
         layout.setSystemLayout(systemLayout);
@@ -427,15 +436,24 @@ void addStaffLayout(const core::ScoreHeaderGroup &inScoreHeaderGroup, api::Defau
         return;
     }
 
-    const auto &staffLayouts = inScoreHeaderGroup.defaults()->layout().staffLayout();
-    if (staffLayouts.empty())
-    {
-        return;
-    }
-
-    if (staffLayouts.front().staffDistance().has_value())
+    // A source with an explicit number attribute is staff-scoped (api::StaffDistanceData); one
+    // with no number applies to staff 1 by schema default and is captured as the plain
+    // unscoped value. Mirrors ScoreReader::scanForSystemInfo's per-measure handling.
+    for (const auto &staffLayout : inScoreHeaderGroup.defaults()->layout().staffLayout())
     {
-        outDefaults.systemLayout.staffDistance = staffLayouts.front().staffDistance()->value().value();
+        if (!staffLayout.staffDistance().has_value())
+        {
+            continue;
+        }
+        const double distance = staffLayout.staffDistance()->value().value();
+        if (staffLayout.number().has_value())
+        {
+            outDefaults.systemLayout.staffDistances.emplace_back(staffLayout.number()->value() - 1, distance);
+        }
+        else
+        {
+            outDefaults.systemLayout.staffDistance = distance;
+        }
     }
 }
 

src/private/mx/impl/MeasureWriter.cpp

@@ -26,6 +26,7 @@
 #include "mx/core/generated/Repeat.h"
 #include "mx/core/generated/RightLeftMiddle.h"
 #include "mx/core/generated/StaffLayout.h"
+#include "mx/core/generated/StaffNumber.h"
 #include "mx/core/generated/SystemLayout.h"
 #include "mx/core/generated/SystemMargins.h"
 #include "mx/core/generated/Tenths.h"
@@ -255,6 +256,15 @@ void MeasureWriter::writeSystemInfo()
             outLayoutGroup.addStaffLayout(outStaffLayout);
             outPrint.setLayout(outLayoutGroup);
         }
+
+        for (const auto &staffDistance : inSystemLayout.staffDistances)
+        {
+            core::StaffLayout outStaffLayout{};
+            outStaffLayout.setNumber(core::StaffNumber{staffDistance.staffIndex + 1});
+            outStaffLayout.setStaffDistance(core::Tenths{core::Decimal{staffDistance.staffDistance}});
+            outLayoutGroup.addStaffLayout(outStaffLayout);
+            outPrint.setLayout(outLayoutGroup);
+        }
     }
 
     myOutMeasure.addMusicData(core::MusicDataChoice::print(outPrint));

src/private/mx/impl/ScoreReader.cpp

@@ -466,15 +466,23 @@ void ScoreReader::scanForSystemInfo() const
                 }
             }
 
-            // Per-measure <staff-layout> staff-distance. (We model a single
-            // staff-distance, matching how the defaults staff-layout is
-            // mapped; the first staff-layout's distance is used.)
+            // Per-measure <staff-layout> staff-distance. A source with an explicit number
+            // attribute is staff-scoped (api::StaffDistanceData); one with no number applies
+            // to staff 1 by schema default and is captured as the plain unscoped value.
             for (const auto &staffLayout : layoutGroup.staffLayout())
             {
-                if (staffLayout.staffDistance().has_value())
+                if (!staffLayout.staffDistance().has_value())
                 {
-                    systemData.layout.staffDistance = staffLayout.staffDistance()->value().value();
-                    break;
+                    continue;
+                }
+                const double distance = staffLayout.staffDistance()->value().value();
+                if (staffLayout.number().has_value())
+                {
+                    systemData.layout.staffDistances.emplace_back(staffLayout.number()->value() - 1, distance);
+                }
+                else
+                {
+                    systemData.layout.staffDistance = distance;
                 }
             }
 

src/private/mxtest/api/PrintLayoutRoundTripTest.cpp

@@ -71,4 +71,31 @@ TEST(printLayoutRoundTrip, perMeasureStaffDistance)
     CHECK_EQUAL(120.0, sysOut.layout.systemDistance.value());
 }
 
+// A <staff-layout number="N"> is scoped to one staff of a multi-staff part; the writer
+// only ever emitted an unscoped <staff-layout> (matching SystemLayoutData::staffDistance),
+// and the reader dropped the number attribute on the one it read back, silently
+// misattributing the distance to staff 1. Verify a staff-scoped distance survives via
+// SystemLayoutData::staffDistances.
+TEST(printLayoutRoundTrip, staffScopedStaffDistance)
+{
+    auto in = makeScoreWithMeasures(1);
+    auto &staff0 = in.parts.front().measures.front().staves.front();
+    StaffData staff1 = staff0;
+    in.parts.front().measures.front().staves.push_back(staff1);
+
+    SystemData sys{};
+    sys.layout.staffDistances.emplace_back(1, 65.0);
+    in.layout.emplace(0, LayoutData{sys, PageData{}});
+
+    const auto out = mxtest::roundTrip(in);
+
+    const auto it = out.layout.find(0);
+    REQUIRE(it != out.layout.end());
+    const auto &sysOut = it->second.system;
+    CHECK(!static_cast<bool>(sysOut.layout.staffDistance));
+    REQUIRE(1 == sysOut.layout.staffDistances.size());
+    CHECK_EQUAL(1, sysOut.layout.staffDistances.front().staffIndex);
+    CHECK_EQUAL(65.0, sysOut.layout.staffDistances.front().staffDistance);
+}
+
 #endif