simdjson Contribution: Implement Pretty Print of DOM

Preface

Years ago, I was finding some projects related to SIMD for taking a break after making contributions to sse2neon. At the same time, I was working in a big tech company, and one of my colleague, a senior C++ developer, said the magic of simdjson, which can parse gigabytes of JSON per second ¹.

In order to make some remarks in this world, I started to find whether there are any issues that I can take to sharpen my skills on SIMD and C++. Luckily, I spotted this issue: “Pretty print for DOM” ², and I thought it was a great starting point.

A Brief Introduction to simdjson

simdjson is a C++ based library that parses gigabytes of JSON per second. It also provides a strict validation on JSON and UTF-8, and it applies a two stage algorithm which utilizes SIMD instructions ³. Because of its superior performance, it is widely adopted on many projects including the Node.js runtime.

What I’m Going to Do

• Create a indented JSON string.
• Performance priority #1.

The Evolution of My Implementation

At first, I implemented in an inheritance way:

class mini_formatter {
  mini_formatter() = default;
  virtual ~mini_formatter() = default;
};

class pretty_formatter : public mini_formatter {
  pretty_formatter() {}
  ~pretty_formatter() {}
};

Suggested by a reviewer ⁴, I altered my implementation into a CRTP ⁵ solution:

template<class formatter>
class base_formatter {  
  simdjson_inline void call_print_newline() {
      this->print_newline();
  }

  simdjson_inline void call_print_indents(size_t depth) {
      this->print_indents(depth);
  }

  simdjson_inline void call_print_space() {
      this->print_space();
  }
};

class mini_formatter : public base_formatter<mini_formatter> {
public:
  simdjson_inline void print_newline();

  simdjson_inline void print_indents(size_t depth);

  simdjson_inline void print_space();
};

class pretty_formatter : public base_formatter<pretty_formatter> {
public:
  simdjson_inline void print_newline();

  simdjson_inline void print_indents(size_t depth);

  simdjson_inline void print_space();

protected:
  int indent_step = 4;
};

After applying CRTP, my implementation achieved speed improvement up to 10x (from 2s to 0.2s) on the testing dataset.

For the entire PR, you can visit here: https://github.com/simdjson/simdjson/pull/2026.

Conclusion and Future Aspect

In this PR, I not only implemented the JSON indentation feature, but also proved that C++ templating can enhance performance.

For the future aspect, I am going to add the indentation spaces feature (i.e., user can set 3 space indentation).

References

1. https://github.com/simdjson/simdjson ↩

2. https://github.com/simdjson/simdjson/issues/1329 ↩

3. https://branchfree.org/2019/02/25/paper-parsing-gigabytes-of-json-per-second/ ↩

4. https://github.com/simdjson/simdjson/pull/2026#discussion_r1252196548 ↩

5. https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern ↩