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# common
[](https://travis-ci.org/ethereum/go-ethereum)
The common package contains the ethereum utility library.
# Installation
As a subdirectory the main go-ethereum repository, you get it with
`go get github.com/ethereum/go-ethereum`.
# Usage
## RLP (Recursive Linear Prefix) Encoding
RLP Encoding is an encoding scheme used by the Ethereum project. It
encodes any native value or list to a string.
More in depth information about the encoding scheme see the
[Wiki](http://wiki.ethereum.org/index.php/RLP) article.
```go
rlp := common.Encode("doge")
fmt.Printf("%q\n", rlp) // => "\0x83dog"
rlp = common.Encode([]interface{}{"dog", "cat"})
fmt.Printf("%q\n", rlp) // => "\0xc8\0x83dog\0x83cat"
decoded := common.Decode(rlp)
fmt.Println(decoded) // => ["dog" "cat"]
```
## Patricia Trie
Patricie Tree is a merkle trie used by the Ethereum project.
More in depth information about the (modified) Patricia Trie can be
found on the [Wiki](http://wiki.ethereum.org/index.php/Patricia_Tree).
The patricia trie uses a db as backend and could be anything as long as
it satisfies the Database interface found in `common/db.go`.
```go
db := NewDatabase()
// db, root
trie := common.NewTrie(db, "")
trie.Put("puppy", "dog")
trie.Put("horse", "stallion")
trie.Put("do", "verb")
trie.Put("doge", "coin")
// Look up the key "do" in the trie
out := trie.Get("do")
fmt.Println(out) // => verb
trie.Delete("puppy")
```
The patricia trie, in combination with RLP, provides a robust,
cryptographically authenticated data structure that can be used to store
all (key, value) bindings.
```go
// ... Create db/trie
// Note that RLP uses interface slices as list
value := common.Encode([]interface{}{"one", 2, "three", []interface{}{42}})
// Store the RLP encoded value of the list
trie.Put("mykey", value)
```
## Value
Value is a Generic Value which is used in combination with RLP data or
`([])interface{}` structures. It may serve as a bridge between RLP data
and actual real values and takes care of all the type checking and
casting. Unlike Go's `reflect.Value` it does not panic if it's unable to
cast to the requested value. It simple returns the base value of that
type (e.g. `Slice()` returns []interface{}, `Uint()` return 0, etc).
### Creating a new Value
`NewEmptyValue()` returns a new \*Value with it's initial value set to a
`[]interface{}`
`AppendList()` appends a list to the current value.
`Append(v)` appends the value (v) to the current value/list.
```go
val := common.NewEmptyValue().Append(1).Append("2")
val.AppendList().Append(3)
```
### Retrieving values
`Get(i)` returns the `i` item in the list.
`Uint()` returns the value as an unsigned int64.
`Slice()` returns the value as a interface slice.
`Str()` returns the value as a string.
`Bytes()` returns the value as a byte slice.
`Len()` assumes current to be a slice and returns its length.
`Byte()` returns the value as a single byte.
```go
val := common.NewValue([]interface{}{1,"2",[]interface{}{3}})
val.Get(0).Uint() // => 1
val.Get(1).Str() // => "2"
s := val.Get(2) // => Value([]interface{}{3})
s.Get(0).Uint() // => 3
```
## Decoding
Decoding streams of RLP data is simplified
```go
val := common.NewValueFromBytes(rlpData)
val.Get(0).Uint()
```
## Encoding
Encoding from Value to RLP is done with the `Encode` method. The
underlying value can be anything RLP can encode (int, str, lists, bytes)
```go
val := common.NewValue([]interface{}{1,"2",[]interface{}{3}})
rlp := val.Encode()
// Store the rlp data
Store(rlp)
```
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