Base-64 encoding

A little encoder (expanding every 3 bytes to 4 printable chars), useful for mime / email / http uploads.

  #!/usr/bin/env lux

  -- Base 64 encoding
  -- 10/02/2001 jcw@equi4.com

  if not tester then dofile('tester.lua') end --XXX
  if not join then dofile('util.lua') end --XXX

  base64={}

  function base64.encode(s)
    local m={} -- this could be inited once instead of every time
    local z='ABCDEFGHIJKLMNOPQRSTUVWXYZ'
          ..'abcdefghijklmnopqrstuvwxyz0123456789+/'
    for i=1,strlen(z) do m[i-1]=strsub(z,i,i) end
    local f=function (n,b) return %m[band(rshift(n,b),63)] end
    -- processing starts here
    local t,e={},strlen(s)
    for i=1,e,45 do
      local o=''
      for j=1,45,3 do
        if i+j-1>e then break end
        local n=0
        for k=-1,1 do
          n=n*256
          if i+j+k<=e then n=n+strbyte(s,i+j+k) end
        end
        o=o..f(n,18)..f(n,12)..f(n,6)..f(n,0)
        if i+j+1>e then
          o=strsub(o,1,-2)..'='
          if i+j>e then o=strsub(o,1,-3)..'==' end
        end
      end
      tinsert(t,o)
    end
    return join(t,'\n')
  end

  tester [[
                   base64.encode('Hi') == SGk=
                  base64.encode('abc') == YWJj
           base64.encode('1234567890') == MTIzNDU2Nzg5MA==
  ]]

  base64.x='This is a test, this is only a test, what a boring test...'
  base64.y=[[VGhpcyBpcyBhIHRlc3QsIHRoaXMgaXMgb25seSBhIHRlc3QsIHdoYXQgYSBi
  b3JpbmcgdGVzdC4uLg==]]
  base64.z=base64.encode(base64.x)
  assert(base64.z == base64.y)

The test code is based on A little test harness for Lua, it can be removed. The "join" proc is as follows:

  function join(v,s)
    s=s or " "
    local r=""
    for i=1,getn(v) do
      if i>1 then r=r..s end
      r=r..(v[i] or "nil")
    end
    return r
  end