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hegel-cpp v0.13.1: generators::text with an alphabet doesn't restrict the generated characters #146
Description
Summary
TextParams::alphabet is documented as a "fixed set of allowed characters", but strings drawn with it contain characters outside the set. build_text_handle (src/generators.cpp) implements alphabet as an empty category list plus include_characters. string_generator_text (src/engine.cpp:343) passes categories.data() to the engine, and data() of an empty std::vector is nullptr with libstdc++, which the engine reads as "no category restriction".
Reproduction
auto s = tc.draw(gs::text({.alphabet = std::string("ab ~.-_%")}));
// drawn: "/" (not in the alphabet)
auto k = tc.draw(gs::text({.alphabet = std::string(" \tabcXYZ-")}));
// drawn: ":" (not in the alphabet)
Suggested fix
Pass a non-null pointer for an empty category list, or add an explicit "no categories" flag to the C ABI.
Seen with hegel-cpp v0.13.1 fetched by CMake, on Linux (Ubuntu 24.04, GCC 13.3, libstdc++).
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