Huff Language

# Hello, world!

Note: This section is a continuation of 'The Basics'. If you have not read it yet we recommend you take a look at it before continuing!

Within 'The Basics' we mentioned how "Hello, world!" is quite an advanced concept for Huff. The reason being that we have to an understanding of how string's are encoded in the EVM.

# Primer: ABI Encoding

As strings are dynamic types it is not as simple as returning the UTF-8 values for "Hello, world!" (0x48656c6c6f2c20776f726c6421). In the ABI standard, dynamic types are encoded in 3 parts.

  1. The offset of the dynamic data. (A pointer to the start of the dynamic data (uint256))
  2. The length of the dynamic data. (uint256)
  3. The values of the dynamic data. (dynamic length)

Each part will look as follows for the string "Hello, world!":

[Byte number]   [DATA]    
0x00            0000000000000000000000000000000000000000000000000000000000000020 // The location of the "Hello, world!" data (dynamic type).
0x20            000000000000000000000000000000000000000000000000000000000000000d // The length of "Hello, world!" in bytes
0x40            48656c6c6f2c20776f726c642100000000000000000000000000000000000000 // Value "Hello, world!"

Encoding dynamic values takes alot of work!! In order to return "Hello, world!" we must return 96 bytes!.

# Implementation

The following MAIN macro steps through this encoding in a clear way (gas optimization will be left as an exercise to the reader!)

#define macro MAIN() = takes(0) returns(0) {
    // Store string "Hello, world1" in memory at 0x40
    // 0x2d is listed as mstore pads the value
    0x48656c6c6f2c20776f726c6421        // ["Hello, world!"]
    0x2d                                // ["Hello, world!", 0x40]
    mstore                              // []

    // store length of string at 0x20
    0x0d                                // [0x0d]
    0x20                                // [0x20, 0x0d]
    mstore                              // []
    
    // store dynamic offset at 0x00 (string encoding starts at 0x20)
    0x20                                // [0x20]
    0x00                                // [0x00, 0x20]
    mstore                              // []

    // return full 96 byte value
    0x60                                // [0x60]
    0x00                                // [0x00, 0x60]
    return                              // []
}

Have a look how memory is set and what is returned interactively within the evm.codes playground (opens new window) for this example.

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