Lua 短域名算法

结合MD5的实现算法：

1. 将网址md5后生成32位串，分4段， 每段8字节
2. 对这四段循环处理，和0x3FFFFFFF与运算，即保留30位数据
3. 根据字母表长度取索引值，如a-zA-Z0-9，就有62位，取0x3D （下标从0开始）
4. 将前面的30位数据分成6段，每5位再和0x3D与运算，算得字母表下标，取到6位字符串 这种算法，会生成4个6位字符串。

lua版的算法如下：

local md5 = require "md5.core"
function md5.sumhexa(k)
    k = md5.sum(k)
    return (string.gsub(k, ".", function (c)
        return string.format("%02x", string.byte(c))
    end))
end

local SHORT_BASE = {
    "a" , "b" , "c" , "d" , "e" , "f" , "g" , "h" , "i" , "j" , "k" , "l" ,
    "m" , "n" , "o" , "p" , "q" , "r" , "s" , "t" , "u" , "v" , "w" , "x" ,
    "y" , "z" , "0" , "1" , "2" , "3" , "4" , "5" , "6" , "7" , "8" , "9" ,
    "A" , "B" , "C" , "D" , "E" , "F" , "G" , "H" , "I" , "J" , "K" , "L" ,
    "M" , "N" , "O" , "P" , "Q" , "R" , "S" , "T" , "U" , "V" , "W" , "X" ,
    "Y" , "Z"
}
local SHORT_PRE = "short_pre"

function short_url(str)
    local m = md5.sumhexa(SHORT_PRE..str)
    local ret = {}
    for k=0,3 do
            local sub = string.sub(m, k*8+1, k*8+8)
            local int =  0x3fffffff & tonumber(sub, 16)
            local out = ''
            for j=1,6 do
                    local v = ( 0x3d & int) + 1
                    out = out .. SHORT_BASE[v]
                    int = int >> 5
            end
            table.insert(ret, out)
    end
    return ret
end

local ret = short_url("hello world")
for k,v in pairs(ret) do
    print("short_url", v)
end

另外，再提供可让用户输入的短地址版本。就是只保留大写字母和数字，同时去掉了容易混淆的O0I1 。算法类似，可做激活码。

local md5 = require "md5.core"
function md5.sumhexa(k)
    k = md5.sum(k)
    return (string.gsub(k, ".", function (c)
        return string.format("%02x", string.byte(c))
    end))
end

local SHORT_BASE = {
    'A','B','C','D','E','F','G','H',
    'J','K','L','M','N','P','Q','R',
    'S','T','U','V','W','X','Y','Z',
    '2','3','4','5','6','7','8','9',
}
local SHORT_PRE = "short_pre"

function short_url(str)
    local m = md5.sumhexa(SHORT_PRE..str)
    local ret = {}
    for k=0,3 do
            local sub = string.sub(m, k*8+1, k*8+8)
            local int =  0x3fffffff & tonumber(sub, 16)
            local out = ''
            for j=1,6 do
                    local v = ( 0x1f & int) + 1
                    out = out .. SHORT_BASE[v]
                    int = int >> 5
            end
            table.insert(ret, out)
    end
    return ret
end

local ret = short_url("hello world")
for k,v in pairs(ret) do
    print("short_url", v)
end