encoding - 豆子笔记

包 Encoding 定义了其他包共享的接口，这些包将数据转换为字节级和文本表示形式。检查这些接口的包包括 encoding / gob，encoding / json 和 encoding / xml。因此，实现一次接口可以使类型在多种编码中有用。实现这些接口的标准类型包括 time.Time 和 net.IP. 接口成对出现，包含生成和使用编码数据。

包 gob 管理 gobs 流 - 在编码器（发送器）和解码器（接收器）之间交换的二进制值。典型的用途是传输远程过程调用（RPC）的参数和结果，例如包“net / rpc”提供的参数和结果。

该实现为流中的每种数据类型编译自定义编解码器，并且当使用单个编码器传输值流时，最有效，分摊编译成本。

package main

import (
	"bytes"
	"encoding/gob"
	"fmt"
	"log"
)

type P struct {
	X, Y, Z int
	Name    string
}

type Q struct {
	X, Y *int32
	Name string
}

// This example shows the basic usage of the package: Create an encoder,
// transmit some values, receive them with a decoder.
func main() {
	// Initialize the encoder and decoder. Normally enc and dec would be
	// bound to network connections and the encoder and decoder would
	// run in different processes.
	var network bytes.Buffer        // Stand-in for a network connection
	enc := gob.NewEncoder(&network) // Will write to network.
	dec := gob.NewDecoder(&network) // Will read from network.

	// Encode (send) some values.
	err := enc.Encode(P{3, 4, 5, "Pythagoras"})
	if err != nil {
		log.Fatal("encode error:", err)
	}
	err = enc.Encode(P{1782, 1841, 1922, "Treehouse"})
	if err != nil {
		log.Fatal("encode error:", err)
	}

	// Decode (receive) and print the values.
	var q Q
	err = dec.Decode(&q)
	if err != nil {
		log.Fatal("decode error 1:", err)
	}
	fmt.Printf("%q: {%d, %d}\n", q.Name, *q.X, *q.Y)
	err = dec.Decode(&q)
	if err != nil {
		log.Fatal("decode error 2:", err)
	}
	fmt.Printf("%q: {%d, %d}\n", q.Name, *q.X, *q.Y)

}

包 json 实现了 RFC 7159 中定义的 JSON 编码和解码.JSON 和 Go 值之间的映射在 Marshal 和 Unmarshal 函数的文档中描述。

package main

import (
	"encoding/json"
	"fmt"
	"log"
	"strings"
)

type Animal int

const (
	Unknown Animal = iota
	Gopher
	Zebra
)

func (a *Animal) UnmarshalJSON(b []byte) error {
	var s string
	if err := json.Unmarshal(b, &s); err != nil {
		return err
	}
	switch strings.ToLower(s) {
	default:
		*a = Unknown
	case "gopher":
		*a = Gopher
	case "zebra":
		*a = Zebra
	}

	return nil
}

func (a Animal) MarshalJSON() ([]byte, error) {
	var s string
	switch a {
	default:
		s = "unknown"
	case Gopher:
		s = "gopher"
	case Zebra:
		s = "zebra"
	}

	return json.Marshal(s)
}

func main() {
	blob := `["gopher","armadillo","zebra","unknown","gopher","bee","gopher","zebra"]`
	var zoo []Animal
	if err := json.Unmarshal([]byte(blob), &zoo); err != nil {
		log.Fatal(err)
	}

	census := make(map[Animal]int)
	for _, animal := range zoo {
		census[animal] += 1
	}

	fmt.Printf("Zoo Census:\n* Gophers: %d\n* Zebras:  %d\n* Unknown: %d\n",
		census[Gopher], census[Zebra], census[Unknown])

}

Package xml 实现了一个简单的 XML 1.0 解析器，它可以理解 XML 名称空间。

package main

import (
	"encoding/xml"
	"fmt"
	"log"
	"strings"
)

type Animal int

const (
	Unknown Animal = iota
	Gopher
	Zebra
)

func (a *Animal) UnmarshalXML(d *xml.Decoder, start xml.StartElement) error {
	var s string
	if err := d.DecodeElement(&s, &start); err != nil {
		return err
	}
	switch strings.ToLower(s) {
	default:
		*a = Unknown
	case "gopher":
		*a = Gopher
	case "zebra":
		*a = Zebra
	}

	return nil
}

func (a Animal) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
	var s string
	switch a {
	default:
		s = "unknown"
	case Gopher:
		s = "gopher"
	case Zebra:
		s = "zebra"
	}
	return e.EncodeElement(s, start)
}

func main() {
	blob := `
	<animals>
		<animal>gopher</animal>
		<animal>armadillo</animal>
		<animal>zebra</animal>
		<animal>unknown</animal>
		<animal>gopher</animal>
		<animal>bee</animal>
		<animal>gopher</animal>
		<animal>zebra</animal>
	</animals>`
	var zoo struct {
		Animals []Animal `xml:"animal"`
	}
	if err := xml.Unmarshal([]byte(blob), &zoo); err != nil {
		log.Fatal(err)
	}

	census := make(map[Animal]int)
	for _, animal := range zoo.Animals {
		census[animal] += 1
	}

	fmt.Printf("Zoo Census:\n* Gophers: %d\n* Zebras:  %d\n* Unknown: %d\n",
		census[Gopher], census[Zebra], census[Unknown])

}

包 csv 读取和写入逗号分隔值（CSV）文件。CSV 文件有很多种; 此程序包支持 RFC 4180 中描述的格式。

csv 文件包含每条记录一个或多个字段的零个或多个记录。每个记录由换行符分隔。最终记录可以可选地后跟换行符。

package main

import (
	"encoding/csv"
	"fmt"
	"io"
	"log"
	"strings"
)

func main() {
	in := `first_name,last_name,username
"Rob","Pike",rob
Ken,Thompson,ken
"Robert","Griesemer","gri"
`
	r := csv.NewReader(strings.NewReader(in))

	for {
		record, err := r.Read()
		if err == io.EOF {
			break
		}
		if err != nil {
			log.Fatal(err)
		}

		fmt.Println(record)
	}
}