fast_json v3.7.0 — High-Performance MQL5 JSON Library

"If your JSON parser needs recursion, you're doing it wrong."

Overview

fast_json is a production-grade, high-performance JSON library for MQL5, engineered for scenarios where I/O throughput and memory efficiency are paramount.

Unlike standard libraries that fragment the heap with thousands of small object allocations, fast_json employs a tape-based zero-allocation memory model. It parses JSON directly into a contiguous integer array (the "tape"), enabling it to process gigabytes of data with minimal overhead and zero garbage collection pressure.

Key Features

• 🚀 Zero-Alloc Tape Architecture: Parses the entire JSON into a single contiguous integer array. No objects are created during parsing.
• ⚡ SWAR Acceleration: Uses Simulated 64-bit SWAR (SIMD within a Register) to scan 8 bytes at a time for structural characters and backslash detection.
• 🔢 Decomposed Number Parser: XOR-first pattern with single-pass integer accumulation (fxsaber architecture). Extended Pow10 tables cover full DBL_MAX_10_EXP — zero MathPow calls at runtime.
• 🛡️ Stack-Safe Iterative Parser: Finite state machine. Zero recursion. Immune to stack overflow.
• 🔧 Handle-Based Navigation: Lightweight CJsonNode structs that point to the tape. Copying nodes is free.
• 📡 Direct Buffer Parsing: ParseBuffer(uchar&[]) accepts raw byte arrays — zero StringToCharArray overhead for WebSocket/WebRequest data.
• ⚙️ Branchless Internals: HexToDec lookup table, packed key format with embedded length, direct byte serialization for literals.

Performance

Metric	fast_json	Standard Libs
Allocation Strategy	1 contiguous block (Tape)	N objects per node
Recursion Depth	Unlimited (Heap stack)	Limited (Call stack)
Read/Write Speed	EXTREME (300MB/s+ Est.)	LOW
Memory Fragmentation	Near Zero	High
Number Parsing	XOR-first + Pow10 tables	StringToDouble
Key Lookup	FNV-1a hash (24-bit + length)	O(N) linear scan

Usage

Parsing from string

#include <fast_json.mqh>

CJson toolkit;
string json = "{\"price\": 1.2345, \"currency\": \"EURUSD\"}";

if (toolkit.Parse(json)) {
    double price = toolkit["price"].ToDouble();
    string curr  = toolkit["currency"].ToString();
    Print(curr, ": ", price);
} 

Parsing from uchar buffer (WebSocket/WebRequest)

uchar data[];
int data_len = /* receive from WebSocket */;

CJson toolkit;
if (toolkit.ParseBuffer(data, data_len)) {
    double bid = toolkit["bid"].ToDouble();
}

Building & Serializing

CJsonBuilder builder;
builder.Obj()
       .Key("signal").Val("buy")
       .Key("volume").Val(0.1)
       .EndObj();

string payload = builder.Build();
// Result: {"signal":"buy","volume":0.1}

Why fast_json?

Because typical MQL5 libraries were designed for configuration files, not high-throughput data streams. fast_json bridges that gap.

Credits

• fxsaber — XOR-first decomposed number parser architecture
• Sunriser — Bug reports #1 (boolean encoding) and #2 (ToString type safety)

License

MIT.