## Truffle Decoding and Encoding

This documentation provides information and complete interface listings for two packages: @truffle/decoder, a high-level library for decoding, and @truffle/codec, a low-level package for encoding, decoding, and data representation.

## Contents

### High-level interface (for common use)

[[@truffle/decoder]] provides a high-level interface for decoding transactions, events, and state variables for Ethereum smart contracts. This package accepts Truffle's contract abstractions as input (or Truffle's artifact files and a provider) and connects to the blockchain to retrieve raw values for decoding.

### Low-level interface (for special-case purposes)

[[@truffle/codec]] provides the underlying low-level interfaces for performing this decoding, as well as interfaces for encoding these values and for decoding of local Solidity variables and other data observable in a debugging trace.

This library is meant to be self-contained, makes no network connections of its own, and seeks to restrict its dependencies to those which provide data structures for high-precision numbers and other special-purpose data utilities.

Although most use cases will not require invoking the codec directly, a few key parts of this library are directly relevant to decoder use. See notes below.

## ℹ️ Important notes

### Output format

Both the decoder and the codec return results as objects containing both type information and either high-precision values or accurate errors in place of values. We recommend familiarizing yourself with the [[Format]] documentation in order to use these packages.

### Decoding modes

The decoder operates in either of two modes:

1. Full mode (`"decodingMode": "full"`), which uses source code information to return types and values to match the original Solidity.
2. ABI mode (`"decodingMode": "abi"`), which uses only information from the contract ABI and returns less information, e.g. returning [[Format.Values.UintResult|`UintResult`]]s in place of [[Format.Values.EnumResult|`EnumResult`]]s and [[Format.Values.AddressResult|`AddressResult`]]s instead of [[Format.Values.ContractResult|`ContractResult`]]s

By default, the decoder uses full mode, but for technical reasons, this may not be reliable. If decoding fails in full mode, it falls back to ABI mode.

Returned decodings (i.e. [[CalldataDecoding]] and [[LogDecoding]]) indicate which mode was used via the `"decodingMode"` field.

To ensure full mode works:

- Use Solidity v0.4.12 or higher;
- Ensure all contracts in your projects have distinct names;
- Compile all your contracts at the same time;
- Ensure all custom data types are declared in a file with at least one contract.

(Our apologies for these technical limitations, but we are working to address these last three problems.)

If you can't use full mode or don't want to deal with the distinction, the decoder provides [[ProjectDecoder.abifyCalldataDecoding|`decoder.abifyCalldataDecoding`]] and [[ProjectDecoder.abifyLogDecoding|`decoder.abifyLogDecoding`]] methods, which accept decodings in either mode and always return ABI mode.

#### Additional notes on decoding modes

- Full mode may reject certain decodings (e.g. out of range enums) that are fine in ABI mode.

- ABI-fied full mode decodings may contain extra information that regular ABI mode results do not.

- Decoding mode applies to the entire decoding, but objects that contain multiple decodings (e.g. [[DecodedLog]]) may contain decodings in different modes.

- You can only decode state variables in full mode. If full mode fails while decoding a state variable, it will throw an exception.

- If a contract `Base` declares an event `Event` and a contract `Derived` inheriting from `Base` overrides `Event`, if `Derived` then emits `Base.Event`, ABI mode may not be able to decode it.

#### Additional notes on decoding state variables

- While internal function pointers can only be decoded in full mode, full mode still may not be able to determine all the information about them. Thus, for internal function pointers, you may get a bare-bones decoding, or you may get a decoding with more information.

- Solidity 0.6.5 contains a bug that may cause some state variables to decode incorrectly if there is an immutable state variable which is written to but never read from.

- In any version of Solidity, it is impossible to decode an immutable state variable which is written to but never read from; these will decode to an error.

- Not all constant state variables can presently be decoded; some of these may simply decode to an error.

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