From 79c2488a0c66486242e1920ffc9e985371b6b7a5 Mon Sep 17 00:00:00 2001
From: Muzaffer Kal <muzaffer@kal.cc>
Date: Tue, 28 Jul 2026 17:20:19 -0700
Subject: [PATCH] Preserve shortreal hierarchy width

Keep 32-bit hierarchy metadata while using eight-byte real storage for promoted shortreal values.
---
 fstcpp/fstcpp_writer.cpp                  | 10 ++++++--
 fstcpp/fstcpp_writer_easy_blocks.test.cpp | 29 +++++++++++++++++++++++
 2 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/fstcpp/fstcpp_writer.cpp b/fstcpp/fstcpp_writer.cpp
index 386a761..129fe7a 100644
--- a/fstcpp/fstcpp_writer.cpp
+++ b/fstcpp/fstcpp_writer.cpp
@@ -126,16 +126,22 @@ Handle Writer::createVar(
 
 	// determine real/string handling like original C implementation
 	bool is_real{false};
+	uint32_t stored_bitwidth{bitwidth};
 	switch (vartype) {
 	case Hierarchy::VarType::VCD_REAL:
 	case Hierarchy::VarType::VCD_REAL_PARAMETER:
 	case Hierarchy::VarType::VCD_REALTIME:
-	case Hierarchy::VarType::SV_SHORTREAL:
 		is_real = true;
 		bitwidth = 8;  // recast to double size
+		stored_bitwidth = bitwidth;
+		break;
+	case Hierarchy::VarType::SV_SHORTREAL:
+		is_real = true;
+		stored_bitwidth = 8;
 		break;
 	case Hierarchy::VarType::GEN_STRING:
 		bitwidth = 0;
+		stored_bitwidth = bitwidth;
 		break;
 	default:
 		break;
@@ -166,7 +172,7 @@ Handle Writer::createVar(
 		// I don't know why the original C implementation encode bitwidth again
 		const uint32_t geom_len{(bitwidth == 0 ? uint32_t(-1) : is_real ? uint32_t(0) : bitwidth)};
 		g.writeLEB128(geom_len);
-		m_value_change_data_.m_variable_infos.emplace_back(bitwidth, is_real);
+		m_value_change_data_.m_variable_infos.emplace_back(stored_bitwidth, is_real);
 	}
 
 	return alias_handle;
diff --git a/fstcpp/fstcpp_writer_easy_blocks.test.cpp b/fstcpp/fstcpp_writer_easy_blocks.test.cpp
index ecae757..d95b116 100644
--- a/fstcpp/fstcpp_writer_easy_blocks.test.cpp
+++ b/fstcpp/fstcpp_writer_easy_blocks.test.cpp
@@ -44,6 +44,10 @@ class WriterTest : public ::testing::Test {
 		);
 	}
 
+	static const VariableInfo &getVariableInfo(Writer &writer, Handle handle) {
+		return writer.m_value_change_data_.m_variable_infos.at(handle - 1);
+	}
+
 	static void writeHeader(const Header &h, ostream &os) { Writer::writeHeader_(h, os); }
 
 	static void appendBlackout(Writer &Writer, std::ostream &os) { Writer.appendBlackout_(os); }
@@ -278,6 +282,31 @@ TEST_F(WriterTest, createVarVcdReal) {
 	EXPECT_EQ(buf, expected);
 }
 
+TEST_F(WriterTest, createVarSvShortreal) {
+	Writer writer;
+	writer.setWriterPackType(WriterPackType::NO_COMPRESSION);
+	const Handle handle{writer.createVar(
+		fst::Hierarchy::VarType::SV_SHORTREAL,
+		fst::Hierarchy::VarDirection::INPUT,
+		/*bitwidth =*/32,
+		"shortreal_val",
+		/*alias handle =*/0
+	)};
+	EXPECT_EQ(handle, 1u);
+
+	// Preserve the 32-bit shortreal width in the hierarchy.
+	const string hierarchy_buf{getHierarchyBuffer(writer)};
+	const string hierarchy_expected{"\x1d\x01shortreal_val\x00\x20\x00"s};
+	EXPECT_EQ(hierarchy_buf, hierarchy_expected);
+
+	// FST stores real values as doubles, including shortreal values promoted to double.
+	const string geometry_buf{getGeometryBuffer(writer)};
+	EXPECT_EQ(geometry_buf, "\x00"s);
+	const VariableInfo &variable_info{getVariableInfo(writer, handle)};
+	EXPECT_TRUE(variable_info.is_real());
+	EXPECT_EQ(variable_info.bitwidth(), 8u);
+}
+
 TEST_F(WriterTest, GeometryBufferNormalVar) {
 	Writer writer;
 	writer.setWriterPackType(WriterPackType::NO_COMPRESSION);