A high-performance, memory-free JSON parser for MQL5, based on a flat tape architecture.
Single-pass iterative state machine (as is, without extra loops, "pure" o(n)): no recursion, no dynamic memory fragmentation, no overhead from function calls.

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Main Features

• Tape-based zero-alloc model: the entire JSON is parsed into a single contiguous long[] array
• Single flat loop: one switch over a token enum, no helper function calls during parsing, strictly O(n)
• FNV-1a key hashing: key lookup in objects without string comparison
• Handle-based navigation: CJsonNode is a lightweight struct (pointer + two ints), copying is free
• In-place mutation: SetInt, SetDbl, SetBool modify the tape directly without re-parsing
• Iterator support: CJsonIteratorObj and CJsonIteratorArray for clean traversal

Parse and navigate

#include "Src\\JsonNode.mqh"

TSN::CJsonParser parser;
parser.Assing("{\"symbol\":\"EURUSD\",\"bid\":1.2345,\"ask\":1.2347,\"active\":true}");
parser.Parse();

TSN::CJsonNode root = parser.GetRoot();
string symbol = root["symbol"].ToString();
double bid    = root["bid"].ToDouble(0.0);
bool   active = root["active"].ToBool(false);

Iterate an object

TSN::CJsonIteratorObj it = root.BeginObj();
while(it.IsValid())
 {
  PrintFormat("%s : %s", it.Key(), it.Val().ToString());
  it.Next();
 }

Iterate an array

TSN::CJsonNode arr = root["prices"];
TSN::CJsonIteratorArray it = arr.BeginArr();
while(it.IsValid())
 {
  Print(it.Val().ToDouble(0.0));
  it.Next();
 }

Parse from file

TSN::CJsonParser parser;
parser.AssingFile("data.json", false);
parser.Parse();

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Performance

Benchmark: twitter.json (616 KB), 1000 iterations, MetaTrader 5 x64 build 5836 started for MetaQuotes Ltd. (Laptop Lenovo (approximately from 2017), Terminal Windows 10 build 19045, 8 x Intel Core i5-8250U @ 1.60GHz, AVX2, 3 / 7 Gb memory, 34 / 222 Gb disk SSD, UAC, GMT-5)

Parser	Time (ms)
JsonParserByLeo	1094
fast_json v3.4 (One copy of array)	1265

The gap comes from architecture, not micro-optimizations: one flat loop vs. a state machine with separate helper calls and per-iteration reserve checks.

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Repository Structure

JsonParserByLeo/
├── Src/ # Full code (Defines, Node, Parser)
└── Test/ # Test and Benchmarks (vs)

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License

Read Full License
By downloading or using this repository, you accept the license terms.

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Requirements

See dependencies.json for the full list.

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Installation

cd "C:\Users\YOUR_USER\AppData\Roaming\MetaQuotes\Terminal\YOUR_ID\MQL5\Shared Projects"
tsndep install "https://forge.mql5.io/nique_372/JsonParserByLeo.git"

Requires the tsndep package, available on PyPI. It automatically downloads and installs all declared dependencies.

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Quick Start

1. Include the library:

#include "..\\JsonParserByLeo\\Src\\JsonNode.mqh"

2. Parse and access:

TSN::CJsonParser parser;
parser.Assing(my_json_string);
if(parser.Parse())
 {
  TSN::CJsonNode root = parser.GetRoot();
  double price = root["price"].ToDouble(0.0);
 }

3. Re-parse without re-copying (benchmark pattern):

parser.Assing(raw_string);   // copy once
for(int i = 0; i < 1000; i++)
  parser.Parse();             // re-parse in-place

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Contact

• Platform: MQL5 Community
• Profile: https://www.mql5.com/es/users/nique_372
• Articles: https://www.mql5.com/es/users/nique_372/publications