Calldata Encoding

Calldata is a format used within GenVM to exchange data between contracts and VMs. It is designed to be safe to load, dynamically typed and JSON-like, binary and compact, and supports blockchain specific types.

Types

Calldata is one of:

1. Arbitrary big integer

2. Raw bytes

3. UTF-8 string

4. Array of Calldata

5. Mapping from strings to Calldata

6. Address (20 bytes)

Format

ULEB128 Encoding

“Unsigned little endian base 128” is a variable-length code compression used to store arbitrarily large integers.

Encoding: Split number into groups of 7 bits, little-endian, zero extend the biggest one. For each except the biggest one (rightmost), set 8th bit to one and concatenate.

Examples:

• 0 ↔ 0x00

• 1 ↔ 0x01

• 128 ↔ 0x80 0x01

Calldata Encoding

Each calldata value starts with a ULEB128 number, which is treated as follows:

least significant 3 bits	interpreted as type	number shifted by this 3 bits	followed by
0	atom	0 ⇒ null 1 ⇒ false 2 ⇒ true 3 ⇒ followed by address _ ⇒ reserved for future use	nothing nothing nothing 20 bytes of address reserved for future use
1	positive int or 0	value	nothing
2	negative int	abs(value) - 1	nothing
3	bytes	length	bytes[length]
4	string	length	bytes[length] of utf8 encoded string
5	array	length	calldata[length]
6	map	length	Pair(FastString, calldata)[length] sorted by keys
7	reserved for future use

FastString is encoded as ULEB128 length followed by UTF-8 encoded bytes (difference is that it does not have a type).