验证码代码解读

下面解读的验证码代码来自： http://www.oschina.net/code/snippet_173630_12006

这个验证码的效果类似如下下图：

这个验证码包含下面三层元素

• 随机大小和颜色的10个点
• 4位数字的验证码（随机偏转方向、每个点间距随机）
• 一条类似删除线的干扰线

对应的带代码注释的源文件如下：

package main

import (
    crand "crypto/rand"
    "fmt"
    "image"
    "image/color"
    "image/png"
    "io"
    "math/rand"
    "net/http"
    "strconv"
    "time"
)

const (
    stdWidth  = 100 // 固定图片宽度
    stdHeight = 40  // 固定图片高度
    maxSkew   = 2
)

// 字体常量信息
const (
    fontWidth  = 5 // 字体的宽度
    fontHeight = 8 // 字体的高度
    blackChar  = 1
)

// 简化期间使用的字体库
var font = [][]byte{
    { // 0
        0, 1, 1, 1, 0,
        1, 0, 0, 0, 1,
        1, 0, 0, 0, 1,
        1, 0, 0, 0, 1,
        1, 0, 0, 0, 1,
        1, 0, 0, 0, 1,
        1, 0, 0, 0, 1,
        0, 1, 1, 1, 0},
    { // 1
        0, 0, 1, 0, 0,
        0, 1, 1, 0, 0,
        1, 0, 1, 0, 0,
        0, 0, 1, 0, 0,
        0, 0, 1, 0, 0,
        0, 0, 1, 0, 0,
        0, 0, 1, 0, 0,
        1, 1, 1, 1, 1},
    { // 2
        0, 1, 1, 1, 0,
        1, 0, 0, 0, 1,
        0, 0, 0, 0, 1,
        0, 0, 0, 1, 1,
        0, 1, 1, 0, 0,
        1, 0, 0, 0, 0,
        1, 0, 0, 0, 0,
        1, 1, 1, 1, 1},
    { // 3
        1, 1, 1, 1, 0,
        0, 0, 0, 0, 1,
        0, 0, 0, 1, 0,
        0, 1, 1, 1, 0,
        0, 0, 0, 1, 0,
        0, 0, 0, 0, 1,
        0, 0, 0, 0, 1,
        1, 1, 1, 1, 0},
    { // 4
        1, 0, 0, 1, 0,
        1, 0, 0, 1, 0,
        1, 0, 0, 1, 0,
        1, 0, 0, 1, 0,
        1, 1, 1, 1, 1,
        0, 0, 0, 1, 0,
        0, 0, 0, 1, 0,
        0, 0, 0, 1, 0},
    { // 5
        1, 1, 1, 1, 1,
        1, 0, 0, 0, 0,
        1, 0, 0, 0, 0,
        1, 1, 1, 1, 0,
        0, 0, 0, 0, 1,
        0, 0, 0, 0, 1,
        0, 0, 0, 0, 1,
        1, 1, 1, 1, 0},
    { // 6
        0, 0, 1, 1, 1,
        0, 1, 0, 0, 0,
        1, 0, 0, 0, 0,
        1, 1, 1, 1, 0,
        1, 0, 0, 0, 1,
        1, 0, 0, 0, 1,
        1, 0, 0, 0, 1,
        0, 1, 1, 1, 0},
    { // 7
        1, 1, 1, 1, 1,
        0, 0, 0, 0, 1,
        0, 0, 0, 0, 1,
        0, 0, 0, 1, 0,
        0, 0, 1, 0, 0,
        0, 1, 0, 0, 0,
        0, 1, 0, 0, 0,
        0, 1, 0, 0, 0},
    { // 8
        0, 1, 1, 1, 0,
        1, 0, 0, 0, 1,
        1, 0, 0, 0, 1,
        0, 1, 1, 1, 0,
        1, 0, 0, 0, 1,
        1, 0, 0, 0, 1,
        1, 0, 0, 0, 1,
        0, 1, 1, 1, 0},
    { // 9
        0, 1, 1, 1, 0,
        1, 0, 0, 0, 1,
        1, 0, 0, 0, 1,
        1, 1, 0, 0, 1,
        0, 1, 1, 1, 1,
        0, 0, 0, 0, 1,
        0, 0, 0, 0, 1,
        1, 1, 1, 1, 0},
}

type Image struct {
    *image.NRGBA
    color   *color.NRGBA
    width   int //a digit width
    height  int //a digit height
    dotsize int
}

func init() {
    // 打乱随机种子
    rand.Seed(int64(time.Second))

}

// 产生指定长宽的图片
func NewImage(digits []byte, width, height int) *Image {
    img := new(Image)
    r := image.Rect(img.width, img.height, stdWidth, stdHeight)
    img.NRGBA = image.NewNRGBA(r)
    img.color = &color.NRGBA{
        uint8(rand.Intn(129)),
        uint8(rand.Intn(129)),
        uint8(rand.Intn(129)),
        0xFF}

    img.calculateSizes(width, height, len(digits))

    // Draw background (10 random circles of random brightness)
    // 画背景， 10个随机亮度的园
    img.fillWithCircles(10, img.dotsize)
    maxx := width - (img.width+img.dotsize)*len(digits) - img.dotsize
    maxy := height - img.height - img.dotsize*2
    x := rnd(img.dotsize*2, maxx)
    y := rnd(img.dotsize*2, maxy)

    // Draw digits. 画验证码
    for _, n := range digits {
        img.drawDigit(font[n], x, y)
        x += img.width + img.dotsize // 下一个验证码字符的起始位置
    }

    // Draw strike-through line. 画类似删除线的干扰线
    img.strikeThrough()
    return img

}

func (img *Image) WriteTo(w io.Writer) (int64, error) {
    return 0, png.Encode(w, img)

}

// 计算几个要显示字符的尺寸，没有开始绘画。
func (img *Image) calculateSizes(width, height, ncount int) {

    // Goal: fit all digits inside the image.
    var border int // 边距
    if width > height {
        border = height / 5
    } else {
        border = width / 5
    }
    // Convert everything to floats for calculations.
    w := float64(width - border*2)  //  100 - 8*2=84
    h := float64(height - border*2) //  40 - 8*2 = 24
    fmt.Println("ddd%v;%v", w, h)

    // fw takes into account 1-dot spacing between digits.
    fw := float64(fontWidth) + 1 // 6
    fh := float64(fontHeight)    // 8
    nc := float64(ncount)        // 4
    fmt.Println("eee%v;%v;%v", fw, fh, nc)

    // Calculate the width of a single digit taking into account only the
    // width of the image.
    nw := w / nc //  84/ 4 = 21

    // Calculate the height of a digit from this width.
    nh := nw * fh / fw //  21*8/6 = 28

    // Digit too high?
    if nh > h {
        // Fit digits based on height.
        nh = h            // nh = 24
        nw = fw / fh * nh // 6 / 8 * 24 = 18
    }

    // Calculate dot size.
    img.dotsize = int(nh / fh) // 24/8 = 3

    // Save everything, making the actual width smaller by 1 dot to account
    // for spacing between digits.
    img.width = int(nw)                // 18
    img.height = int(nh) - img.dotsize // 24 - 3 = 21

    fmt.Printf("format:%v;%v;%v/r/n", img.dotsize, img.width, img.height)
}

// 随机画指定个数个圆点
func (img *Image) fillWithCircles(n, maxradius int) {
    color := img.color
    maxx := img.Bounds().Max.X
    maxy := img.Bounds().Max.Y
    for i := 0; i < n; i++ {
        setRandomBrightness(color, 255) // 随机颜色亮度
        r := rnd(1, maxradius)          // 随机大小
        img.drawCircle(color, rnd(r, maxx-r), rnd(r, maxy-r), r)
    }

}

// 画 水平线
func (img *Image) drawHorizLine(color color.Color, fromX, toX, y int) {
    // 遍历画每个点
    for x := fromX; x <= toX; x++ {
        img.Set(x, y, color)
    }

}

// 画指定颜色的实心圆
func (img *Image) drawCircle(color color.Color, x, y, radius int) {
    f := 1 - radius
    dfx := 1
    dfy := -2 * radius
    xx := 0
    yy := radius
    img.Set(x, y+radius, color)
    img.Set(x, y-radius, color)
    img.drawHorizLine(color, x-radius, x+radius, y)
    for xx < yy {
        if f >= 0 {
            yy--
            dfy += 2
            f += dfy
        }
        xx++
        dfx += 2
        f += dfx
        img.drawHorizLine(color, x-xx, x+xx, y+yy)
        img.drawHorizLine(color, x-xx, x+xx, y-yy)
        img.drawHorizLine(color, x-yy, x+yy, y+xx)
        img.drawHorizLine(color, x-yy, x+yy, y-xx)
    }

}

// 画一个随机干扰线
func (img *Image) strikeThrough() {
    r := 0
    maxx := img.Bounds().Max.X
    maxy := img.Bounds().Max.Y
    y := rnd(maxy/3, maxy-maxy/3)
    for x := 0; x < maxx; x += r {
        r = rnd(1, img.dotsize/3)
        y += rnd(-img.dotsize/2, img.dotsize/2)
        if y <= 0 || y >= maxy {
            y = rnd(maxy/3, maxy-maxy/3)
        }
        img.drawCircle(img.color, x, y, r)
    }

}

// 画指定的验证码其中一个字符
func (img *Image) drawDigit(digit []byte, x, y int) {
    // 随机偏转方向
    skf := rand.Float64() * float64(rnd(-maxSkew, maxSkew))
    xs := float64(x)
    minr := img.dotsize / 2               // minumum radius
    maxr := img.dotsize/2 + img.dotsize/4 // maximum radius
    y += rnd(-minr, minr)
    for yy := 0; yy < fontHeight; yy++ {
        for xx := 0; xx < fontWidth; xx++ {
            if digit[yy*fontWidth+xx] != blackChar {
                continue
            }
            // Introduce random variations.
            // 引入一些随机变化，不过这里变化量非常小
            or := rnd(minr, maxr)
            ox := x + (xx * img.dotsize) + rnd(0, or/2)
            oy := y + (yy * img.dotsize) + rnd(0, or/2)
            img.drawCircle(img.color, ox, oy, or)
        }
        xs += skf
        x = int(xs)
    }

}

// 设置随机颜色亮度
func setRandomBrightness(c *color.NRGBA, max uint8) {
    minc := min3(c.R, c.G, c.B)
    maxc := max3(c.R, c.G, c.B)
    if maxc > max {
        return
    }
    n := rand.Intn(int(max-maxc)) - int(minc)
    c.R = uint8(int(c.R) + n)
    c.G = uint8(int(c.G) + n)
    c.B = uint8(int(c.B) + n)

}

// 三个数中的最小数
func min3(x, y, z uint8) (o uint8) {
    o = x
    if y < o {
        o = y
    }
    if z < o {
        o = z
    }
    return

}

// 三个数的最大数
func max3(x, y, z uint8) (o uint8) {
    o = x
    if y > o {
        o = y
    }
    if z > o {
        o = z
    }
    return

}

// rnd returns a random number in range [from, to].
// 然会指定范围的随机数
func rnd(from, to int) int {
    //println(to+1-from)
    return rand.Intn(to+1-from) + from

}

const (
    // Standard length of uniuri string to achive ~95 bits of entropy.
    StdLen = 16
    // Length of uniurl string to achive ~119 bits of entropy, closest
    // to what can be losslessly converted to UUIDv4 (122 bits).
    UUIDLen = 20
)

// Standard characters allowed in uniuri string.
// 验证码中标准的字符 大小写与数字
var StdChars = []byte("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789")

// New returns a new random string of the standard length, consisting of
// standard characters.
func New() string {
    return NewLenChars(StdLen, StdChars)

}

// NewLen returns a new random string of the provided length, consisting of standard characters.
// 返回指定长度的随机字符串
func NewLen(length int) string {
    return NewLenChars(length, StdChars)

}

// NewLenChars returns a new random string of the provided length, consisting
// of the provided byte slice of allowed characters (maximum 256).
// 返回指定长度，指定候选字符的随机字符串（最大256）
func NewLenChars(length int, chars []byte) string {
    b := make([]byte, length)
    r := make([]byte, length+(length/4)) // storage for random bytes. 随机字节的存储空间, 多读几个以免

    clen := byte(len(chars))
    maxrb := byte(256 - (256 % len(chars))) // 问题， 为什么要申请这么长的数组？ 看下面循环的 continue 条件

    i := 0
    for {
        // rand.Read() 和 io.ReadFull(rand.Reader) 的区别?
        // http://www.cnblogs.com/ghj1976/p/3435940.html
        if _, err := io.ReadFull(crand.Reader, r); err != nil {
            panic("error reading from random source: " + err.Error())
        }
        for _, c := range r {
            if c >= maxrb {
                // Skip this number to avoid modulo bias.
                // 跳过 maxrb， 以避免麻烦,这也是随机数要多读几个的原因。
                continue
            }
            b[i] = chars[c%clen]
            i++
            if i == length {
                return string(b)
            }
        }
    }
    panic("unreachable")

}

func pic(w http.ResponseWriter, req *http.Request) {
    // 产生验证码byte数组
    d := make([]byte, 4)
    s := NewLen(4)
    d = []byte(s)

    // 把验证码变成需要显示的字符串
    ss := ""
    for v := range d {
        d[v] %= 10
        ss += strconv.FormatInt(int64(d[v]), 32)
    }

    // 图片流方式输出
    w.Header().Set("Content-Type", "image/png")
    NewImage(d, 100, 40).WriteTo(w)

    // 打印出这次使用的验证码
    fmt.Println(ss)
}

func index(w http.ResponseWriter, req *http.Request) {
    str := "<meta charset="utf-8"><h3>golang 图片验证码例子</h3><img border="1" src="/pic" alt="图片验证码" onclick="this.src='/pic'" />"
    w.Header().Set("Content-Type", "text/html")
    w.Write([]byte(str))

}

func main() {
    http.HandleFunc("/pic", pic)
    http.HandleFunc("/", index)
    s := &http.Server{
        Addr:           ":8080",
        ReadTimeout:    30 * time.Second,
        WriteTimeout:   30 * time.Second,
        MaxHeaderBytes: 1 << 20}
    s.ListenAndServe()

}