[C++] Support resetting group count to current index to allow for streaming operations where the group count is difficult to determine upfront but can be limited and reset down to current index value when group is complete.

Author: mjpt777

sbe-samples/src/main/cpp/GeneratedStubExample.cpp

@@ -126,7 +126,6 @@ std::size_t encodeCar(Car &car, char *buffer, std::uint64_t offset, std::uint64_
     return car.encodedLength();
 }
 
-
 const char *format(BooleanType::Value value)
 {
     if (value == BooleanType::T)
@@ -176,8 +175,12 @@ const char *format(bool value)
 }
 
 std::size_t decodeCar(
-    Car &car, char *buffer, std::uint64_t offset, std::uint64_t actingBlockLength,
-    std::uint64_t actingVersion, std::uint64_t bufferLength)
+    Car &car,
+    char *buffer,
+    std::uint64_t offset,
+    std::uint64_t actingBlockLength,
+    std::uint64_t actingVersion,
+    std::uint64_t bufferLength)
 {
     car.wrapForDecode(buffer, offset, actingBlockLength, actingVersion, bufferLength);
 
@@ -252,7 +255,7 @@ std::size_t decodeCar(
 
     char tmp[1024];
     std::uint64_t bytesCopied = engine.getFuel(tmp, sizeof(tmp));
-    std::cout << "\ncar.engine.effifiency=" << (int)engine.efficiency();
+    std::cout << "\ncar.engine.efficiency=" << (int)engine.efficiency();
     std::cout << "\ncar.engine.boosterEnabled=" << format(engine.boosterEnabled());
     std::cout << "\ncar.engine.fuelLength=" << bytesCopied;
     std::cout << "\ncar.engine.fuel=" << std::string(tmp, bytesCopied);

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/cpp/CppGenerator.java

@@ -192,15 +192,16 @@ private static void generateGroupClassHeader(
         final int blockLength = tokens.get(index).encodedLength();
         final Token blockLengthToken = Generators.findFirst("blockLength", tokens, index);
         final Token numInGroupToken = Generators.findFirst("numInGroup", tokens, index);
-        final String cppTypeForBlockLength = cppTypeName(blockLengthToken.encoding().primitiveType());
-        final String cppTypeForNumInGroup = cppTypeName(numInGroupToken.encoding().primitiveType());
+        final String cppTypeBlockLength = cppTypeName(blockLengthToken.encoding().primitiveType());
+        final String cppTypeNumInGroup = cppTypeName(numInGroupToken.encoding().primitiveType());
 
         new Formatter(sb).format("\n" +
             indent + "class %1$s\n" +
             indent + "{\n" +
             indent + "private:\n" +
             indent + "    char *m_buffer = nullptr;\n" +
             indent + "    std::uint64_t m_bufferLength = 0;\n" +
+            indent + "    std::uint64_t m_initialPosition = 0;\n" +
             indent + "    std::uint64_t *m_positionPtr = nullptr;\n" +
             indent + "    std::uint64_t m_blockLength = 0;\n" +
             indent + "    std::uint64_t m_count = 0;\n" +
@@ -228,8 +229,9 @@ private static void generateGroupClassHeader(
             indent + "        m_bufferLength = bufferLength;\n" +
             indent + "        m_blockLength = dimensions.blockLength();\n" +
             indent + "        m_count = dimensions.numInGroup();\n" +
-            indent + "        m_index = std::numeric_limits<std::uint64_t>::max();\n" +
+            indent + "        m_index = 0;\n" +
             indent + "        m_actingVersion = actingVersion;\n" +
+            indent + "        m_initialPosition = *pos;\n" +
             indent + "        m_positionPtr = pos;\n" +
             indent + "        *m_positionPtr = *m_positionPtr + %1$d;\n" +
             indent + "    }\n",
@@ -262,14 +264,18 @@ private static void generateGroupClassHeader(
             indent + "        %7$s dimensions(buffer, *pos, bufferLength, actingVersion);\n" +
             indent + "        dimensions.blockLength((%1$s)%2$d);\n" +
             indent + "        dimensions.numInGroup((%3$s)count);\n" +
-            indent + "        m_index = std::numeric_limits<std::uint64_t>::max();\n" +
+            indent + "        m_index = 0;\n" +
             indent + "        m_count = count;\n" +
             indent + "        m_blockLength = %2$d;\n" +
             indent + "        m_actingVersion = actingVersion;\n" +
+            indent + "        m_initialPosition = *pos;\n" +
             indent + "        m_positionPtr = pos;\n" +
             indent + "        *m_positionPtr = *m_positionPtr + %4$d;\n" +
             indent + "    }\n",
-            cppTypeForBlockLength, blockLength, cppTypeForNumInGroup, dimensionHeaderLength,
+            cppTypeBlockLength,
+            blockLength,
+            cppTypeNumInGroup,
+            dimensionHeaderLength,
             minCheck,
             numInGroupToken.encoding().applicableMaxValue().longValue(),
             dimensionsClassName);
@@ -311,25 +317,41 @@ private static void generateGroupClassHeader(
 
             indent + "    SBE_NODISCARD inline bool hasNext() const SBE_NOEXCEPT\n" +
             indent + "    {\n" +
-            indent + "        return m_index + 1 < m_count;\n" +
+            indent + "        return m_index < m_count;\n" +
             indent + "    }\n\n" +
 
             indent + "    inline %3$s &next()\n" +
             indent + "    {\n" +
+            indent + "        if (m_index >= m_count)\n" +
+            indent + "        {\n" +
+            indent + "            throw std::runtime_error(\"index >= count [E108]\");\n" +
+            indent + "        }\n" +
             indent + "        m_offset = *m_positionPtr;\n" +
             indent + "        if (SBE_BOUNDS_CHECK_EXPECT(((m_offset + m_blockLength) > m_bufferLength), false))\n" +
             indent + "        {\n" +
-            indent + "            throw std::runtime_error(\"" +
-            "buffer too short to support next group index [E108]\");\n" +
+            indent + "            throw std::runtime_error(\"buffer too short for next group index [E108]\");\n" +
             indent + "        }\n" +
             indent + "        *m_positionPtr = m_offset + m_blockLength;\n" +
             indent + "        ++m_index;\n\n" +
 
             indent + "        return *this;\n" +
             indent + "    }\n",
-            dimensionHeaderLength, blockLength, formatClassName(groupName));
+            dimensionHeaderLength,
+            blockLength,
+            formatClassName(groupName));
+
+        sb.append("\n")
+            .append(indent).append("    inline std::uint64_t resetCountToIndex() SBE_NOEXCEPT\n")
+            .append(indent).append("    {\n")
+            .append(indent).append("        m_count = m_index;\n")
+            .append(indent).append("        ").append(dimensionsClassName)
+            .append(" dimensions(m_buffer, m_initialPosition, m_bufferLength, m_actingVersion);\n")
+            .append(indent).append("        dimensions.numInGroup((").append(cppTypeNumInGroup).append(")m_count);\n")
+            .append(indent).append("        return m_count;\n")
+            .append(indent).append("    }\n");
 
-        sb.append(indent).append("#if __cplusplus < 201103L\n")
+        sb.append("\n")
+            .append(indent).append("#if __cplusplus < 201103L\n")
             .append(indent).append("    template<class Func> inline void forEach(Func& func)\n")
             .append(indent).append("    {\n")
             .append(indent).append("        while (hasNext())\n")