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Fn Selector Encoding

Icon LinkFunction Selector Encoding

To select which function you want to call, first, this function must be in an ABI struct of a Sway program. For instance:

abi MyContract {
    fn foo(a: u64);
    fn bar(a: InputStruct );
} {
    fn baz(a: ()) { }
}

The function selector is the first 4 bytes of the SHA-256 hash function of the signature of the Sway function being called. Then, these 4 bytes are right-aligned to 8 bytes, left-padded with zeroes.

Icon InfoCircle
Note: The word size for the FuelVM is 8 bytes.

Icon LinkFunction Signature

The signature is composed of the function name with the parenthesized list of comma-separated parameter types without spaces. All strings encoded with UTF-8. For custom types such as enum and struct, there is a prefix added to the parenthesized list (see below). Generic struct and enum types also accept a list of comma-separated type arguments in between angle brackets right after the prefix.

For instance, to compute the selector for the following function:

fn entry_one(arg: u64);

we should pass "entry_one(u64)" to the sha256() hashing algorithm. The full digest would be:

0x0c36cb9cb766ff60422db243c4fff06d342949da3c64a3c6ac564941f84b6f06

Then we would get only the first 4 bytes of this digest and left-pad it to 8 bytes:

0x000000000c36cb9c

The table below summarizes how each function argument type is encoded

Type	Encoding
`bool`	`bool`
`u8`	`u8`
`u16`	`u16`
`u32`	`u32`
`u64`	`u64`
`b256`	`b256`
`struct`	`s<<arg1>,<arg2>,...>(<ty1>,<ty2>,...)` where `<ty1>`, `<ty2>`, ... are the encoded types of the struct fields and `<arg1>`, `<arg2>`, ... are the encoded type arguments
`enum`	`e<<arg1>>,<arg_2>,...>(<ty1>,<ty2>,...)` where `<ty1>`, `<ty2>`, ... are the encoded types of the enum variants and `<arg1>`, `<arg2>`, ... are the encoded type arguments
`str[<n>]`	`str[<n>]`
`array`	`a[<ty>;<n>]` where `<ty>` is the encoded element type of the array and `<n>` is its length
`tuple`	`(<ty1>,<ty2>,...)` where `<ty1>`, `<ty2>`, ... are the encoded types of the tuple fields

Icon InfoCircle
Note: Non-generic structs and enums do not require angle brackets.

Icon LinkA Complex Example

enum MyEnum<V> {
    Foo: u64,
    Bar: bool,
}
struct MyStruct<T, U> {
    bim: T,
    bam: MyEnum<u64>,
}
 
struct MyOtherStruct {
    bom: u64,
}
 
fn complex_function(
    arg1: MyStruct<[b256; 3], u8>,
    arg2: [MyStruct<u64, bool>; 4],
    arg3: (str[5], bool),
    arg4: MyOtherStruct,
);

is encoded as:

abi MyContract {
    complex_function(s<a[b256;3],u8>(a[b256;3],e<u64>(u64,bool)),a[s<u64,bool>(u64,e<u64>(u64,bool));4],(str[5],bool),s(u64))
}

which is then hashed into:

51fdfdadc37ff569e281a622281af7ec055f8098c40bc566118cbb48ca5fd28b

and then the encoded function selector is:

0x0000000051fdfdad