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JSON Parser in Rust

A simple, lightweight, from-scratch JSON parser written in Rust. This project reads JSON files, tokenizes the input strings, and parses them into an Abstract Syntax Tree (AST) representation using custom tokens.

Features

  • Lexical Analysis (Lexer): Tokenizes incoming streams of characters into logical JSON elements (Strings, Numbers, Booleans, Punctuation, Null).
  • Parsing (Parser): Validates and constructs an in-memory representation of the JSON data structure. Handles nested Objects and Arrays.
  • Robust Error Handling: Uses thiserror to provide descriptive errors indicating where parsing or syntax failures happened.
  • Order Preservation: Uses indexmap internally to retain the insertion order of object keys, making debugging and structured output easier.

Project Structure

  • src/main.rs: The entry point that reads a file, tokenizes it, and runs the parsing logic.
  • src/lexer.rs: Contains the Lexer implementation to convert raw text into Token variants.
  • src/parser.rs: Contains the Parser implementation to construct the parsed object tree.
  • src/token.rs: Defines the Token enums.
  • src/error.rs: Custom error definitions (Lexer and Parser errors).
  • src/constants.rs: Helpful generic constants for parsing (e.g., punctuation marks like {, }, [, ]).

Getting Started

Prerequisites

You need Rust and Cargo installed. If you haven't installed them, follow the instructions at rustup.rs.

Running the Parser

By default, the program looks for a file passed as the first argument. If no arguments are passed, it defaults to evaluating tests/default_test.json.

# Run with the default test file
cargo run

# Run with a specific JSON file
cargo run -- tests/extreme_test.json

Example Output

Given a file like:

{
  "key": "value",
  "number": 123
}

The output will display:

  1. The raw file content
  2. The list of generated tokens
  3. The beautifully formatted, parsed representation of the JSON tree

Development and TODOs

  • Handle escape characters in strings
  • Handle complex numbers better
  • Handle errors better instead of panicking
  • Add more test cases for edge cases and error cases
  • Refactor Constants
  • Iterator implementation for Lexer (in progress!)

Dependencies

  • indexmap: A hash table with consistent order. Used over HashMap to retain JSON key order.
  • thiserror: A popular crate for handling and deriving Error types cleanly in Rust.

License

This project is Public domain and available under the 'Unlicense' License.