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