Initial pure C++ implementation

src/foundation/bit_reader.cpp

@@ -0,0 +1,30 @@
+#include "foundation/bit_reader.h"
+
+namespace joc::bits {
+
+bool variable_bits(BitReader& reader, unsigned width, unsigned max_groups, std::uint32_t* out_value) {
+    std::uint32_t value = 0;
+    for (unsigned group = 0; group < max_groups; ++group) {
+        value += reader.read(width);
+        if (reader.failed()) {
+            return false;
+        }
+        const std::uint32_t more = reader.read(1);
+        if (reader.failed()) {
+            return false;
+        }
+        if (more == 0u) {
+            if (out_value != nullptr) {
+                *out_value = value;
+            }
+            return true;
+        }
+        value = (value + 1u) << width;
+    }
+    // Same failure mode as the reference implementation: an extension chain
+    // that never terminates is a syntax error, not a truncation.
+    reader.fail(JOC_ERR_EMDF_SYNTAX, "variable_bits extension groups exceeded");
+    return false;
+}
+
+}  // namespace joc::bits