# Tendermint Encoding

## Binary Serialization (TMBIN)

Tendermint aims to encode data structures in a manner similar to how the corresponding Go structs
are laid out in memory.
Variable length items are length-prefixed.
While the encoding was inspired by Go, it is easily implemented in other languages as well given its
intuitive design.

XXX: This is changing to use real varints and 4-byte-prefixes.
See https://github.com/tendermint/go-wire/tree/sdk2.

### Fixed Length Integers

Fixed length integers are encoded in Big-Endian using the specified number of bytes.
So `uint8` and `int8` use one byte, `uint16` and `int16` use two bytes,
`uint32` and `int32` use 3 bytes, and `uint64` and `int64` use 4 bytes.

Negative integers are encoded via twos-complement.

Examples:

```go
encode(uint8(6))    == [0x06]
encode(uint32(6))   == [0x00, 0x00, 0x00, 0x06]

encode(int8(-6))    == [0xFA]
encode(int32(-6))   == [0xFF, 0xFF, 0xFF, 0xFA]
```

### Variable Length Integers

Variable length integers are encoded as length-prefixed Big-Endian integers.
The length-prefix consists of a single byte and corresponds to the length of the encoded integer.

Negative integers are encoded by flipping the leading bit of the length-prefix to a `1`.

Zero is encoded as `0x00`. It is not length-prefixed.

Examples:

```go
encode(uint(6))     == [0x01, 0x06]
encode(uint(70000)) == [0x03, 0x01, 0x11, 0x70]

encode(int(-6))     == [0xF1, 0x06]
encode(int(-70000)) == [0xF3, 0x01, 0x11, 0x70]

encode(int(0))      == [0x00]
```

### Strings

An encoded string is a length prefix followed by the underlying bytes of the string.
The length-prefix is itself encoded as an `int`.

The empty string is encoded as `0x00`. It is not length-prefixed.

Examples:

```go
encode("")      == [0x00]
encode("a")     == [0x01, 0x01, 0x61]
encode("hello") == [0x01, 0x05, 0x68, 0x65, 0x6C, 0x6C, 0x6F]
encode("¥")     == [0x01, 0x02, 0xC2, 0xA5]
```

### Arrays (fixed length)

An encoded fix-lengthed array is the concatenation of the encoding of its elements.
There is no length-prefix.

Examples:

```go
encode([4]int8{1, 2, 3, 4})     == [0x01, 0x02, 0x03, 0x04]
encode([4]int16{1, 2, 3, 4})    == [0x00, 0x01, 0x00, 0x02, 0x00, 0x03, 0x00, 0x04]
encode([4]int{1, 2, 3, 4})      == [0x01, 0x01, 0x01, 0x02, 0x01, 0x03, 0x01, 0x04]
encode([2]string{"abc", "efg"}) == [0x01, 0x03, 0x61, 0x62, 0x63, 0x01, 0x03, 0x65, 0x66, 0x67]
```

### Slices (variable length)

An encoded variable-length array is a length prefix followed by the concatenation of the encoding of
its elements.
The length-prefix is itself encoded as an `int`.

An empty slice is encoded as `0x00`. It is not length-prefixed.

Examples:

```go
encode([]int8{})                == [0x00]
encode([]int8{1, 2, 3, 4})      == [0x01, 0x04, 0x01, 0x02, 0x03, 0x04]
encode([]int16{1, 2, 3, 4})     == [0x01, 0x04, 0x00, 0x01, 0x00, 0x02, 0x00, 0x03, 0x00, 0x04]
encode([]int{1, 2, 3, 4})       == [0x01, 0x04, 0x01, 0x01, 0x01, 0x02, 0x01, 0x03, 0x01, 0x4]
encode([]string{"abc", "efg"})  == [0x01, 0x02, 0x01, 0x03, 0x61, 0x62, 0x63, 0x01, 0x03, 0x65, 0x66, 0x67]
```

### BitArray

BitArray is encoded as an `int` of the number of bits, and with an array of `uint64` to encode
value of each array element.

```go
type BitArray struct {
    Bits  int
    Elems []uint64
}
```

### Time

Time is encoded as an `int64` of the number of nanoseconds since January 1, 1970,
rounded to the nearest millisecond.

Times before then are invalid.

Examples:

```go
encode(time.Time("Jan 1 00:00:00 UTC 1970"))            == [0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]
encode(time.Time("Jan 1 00:00:01 UTC 1970"))            == [0x00, 0x00, 0x00, 0x00, 0x3B, 0x9A, 0xCA, 0x00] // 1,000,000,000 ns
encode(time.Time("Mon Jan 2 15:04:05 -0700 MST 2006"))  == [0x0F, 0xC4, 0xBB, 0xC1, 0x53, 0x03, 0x12, 0x00]
```

### Structs

An encoded struct is the concatenation of the encoding of its elements.
There is no length-prefix.

Examples:

```go
type MyStruct struct{
    A int
    B string
    C time.Time
}
encode(MyStruct{4, "hello", time.Time("Mon Jan 2 15:04:05 -0700 MST 2006")}) ==
    [0x01, 0x04, 0x01, 0x05, 0x68, 0x65, 0x6C, 0x6C, 0x6F, 0x0F, 0xC4, 0xBB, 0xC1, 0x53, 0x03, 0x12, 0x00]
```

## Merkle Trees

Simple Merkle trees are used in numerous places in Tendermint to compute a cryptographic digest of a data structure.

RIPEMD160 is always used as the hashing function.

The function `SimpleMerkleRoot` is a simple recursive function defined as follows:

```go
func SimpleMerkleRoot(hashes [][]byte) []byte{
    switch len(hashes) {
    case 0:
        return nil
    case 1:
        return hashes[0]
    default:
        left := SimpleMerkleRoot(hashes[:(len(hashes)+1)/2])
        right := SimpleMerkleRoot(hashes[(len(hashes)+1)/2:])
        return RIPEMD160(append(left, right))
    }
}
```

Note we abuse notion and call `SimpleMerkleRoot` with arguments of type `struct` or type `[]struct`.
For `struct` arguments, we compute a `[][]byte` by sorting elements of the `struct` according to
field name and then hashing them.
For `[]struct` arguments, we compute a `[][]byte` by hashing the individual `struct` elements.

## JSON (TMJSON)

Signed messages (eg. votes, proposals) in the consensus are encoded in TMJSON, rather than TMBIN.
TMJSON is JSON where `[]byte` are encoded as uppercase hex, rather than base64.

When signing, the elements of a message are sorted by key and the sorted message is embedded in an
outer JSON that includes a `chain_id` field.
We call this encoding the CanonicalSignBytes. For instance, CanonicalSignBytes for a vote would look
like:

```json
{"chain_id":"my-chain-id","vote":{"block_id":{"hash":DEADBEEF,"parts":{"hash":BEEFDEAD,"total":3}},"height":3,"round":2,"timestamp":1234567890, "type":2}
```

Note how the fields within each level are sorted.

## Other

### MakeParts

TMBIN encode an object and slice it into parts.

```go
MakeParts(object, partSize)
```

### Part

```go
type Part struct {
    Index int
    Bytes byte[]
    Proof byte[]
}
```
