ABI Encoding & Decoding

Understanding Ethereum ABI encoding and decoding mechanics

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Overview

ABI encoding serializes typed data into byte sequences for EVM consumption. Solidity ABI specification defines encoding rules for all Ethereum types.

Encoding Rules

Static types (fixed size):

• uint<N>, int<N>, address, bool, bytes<N> (N ≤ 32)
• Encoded in-place as 32-byte words

Dynamic types (variable size):

• bytes, string, arrays (T[], T[k])
• Encoded with offset pointer + length + data

Function Encoding

Function calls encode as: selector (4 bytes) + params (ABI-encoded)

// transfer(address,uint256)
// Selector: 0xa9059cbb
// Params: [recipient, amount]
const data = Abi.Function.encodeParams(fn, [recipient, amount]);

Event Encoding

Events encode indexed parameters as topics, non-indexed as data.

// Transfer(address indexed from, address indexed to, uint256 value)
// topic0: event selector
// topic1: from address
// topic2: to address
// data: ABI-encoded value
const topics = Abi.Event.encodeTopics(event, { from, to });

Topic limits: Max 3 indexed parameters (topic0 is selector)

Error Encoding

Errors encode like functions: selector (4 bytes) + params

// InsufficientBalance(uint256 requested, uint256 available)
const data = Abi.Error.encodeParams(error, [requested, available]);

Constructor Encoding

Constructor parameters append to contract bytecode during deployment.

// constructor(string name, uint8 decimals)
const params = Abi.Constructor.encodeParams(ctor, [name, decimals]);
// Deploy: bytecode + params

Tuple Encoding

Tuples (structs) encode components sequentially.

// struct Point { uint256 x; uint256 y; }
// Encodes as: [x, y] (two 32-byte words)

Array Encoding

Fixed-size arrays: Encode elements sequentially Dynamic arrays: Encode length + elements

// uint256[3] - Fixed (3 * 32 = 96 bytes)
// uint256[] - Dynamic (32-byte length + N * 32 bytes)

Packed Encoding

Standard ABI uses 32-byte words. Packed encoding removes padding (used in abi.encodePacked for signatures/hashing).

Tevm uses standard ABI encoding. For packed encoding, use manual byte manipulation.

Decoding

Decoding reverses encoding process, extracting typed values from byte sequences.

// Decode function parameters
const params = Abi.Function.decodeParams(fn, data);

// Decode function results
const result = Abi.Function.decodeResult(fn, returnData);

// Decode event log
const values = Abi.Event.decodeLog(event, log.data, log.topics);

Specification

Full encoding rules: Solidity ABI Specification

See Also

• Function encoding - Encode function parameters
• Event encoding - Encode event topics
• Selectors - Selector generation