Calldata format

Calldata is a format that is used within GenVM to exchange data between contracts and VMs. It is designed with following in mind: - be safe to load - be dynamically typed and json-like - be binary and compact - support blockchain specific types

Types

Calldata is one of:

1. arbitrary big integer

2. raw bytes

3. utf8 string

4. array of Calldata

5. mapping from strings to Calldata

6. Address (20 bytes)

Format

“uleb128”

“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

Each calldata value starts with 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 utf8 encoded bytes (difference is that it does not have a type)