Question

（也发布在Lua邮件列表上）

所以我一直在编写深拷贝算法，我想测试它们是否按照我希望的方式工作。虽然我可以访问原始＆gt;复制映射，但我想要一个通用的深度比较算法，它必须能够比较表键（表作为键？）。

我的深拷贝算法在这里是可用的：https://gist.github.com/SoniEx2/fc5d3614614e4e3fe131（它不是很有条理，但有3个，一个使用递归调用，另一个使用todo表，以及另一个模拟调用堆栈（以非常丑陋但5.1兼容的方式））

递归版：

local function deep(inp,copies)
  if type(inp) ~= "table" then
    return inp
  end
  local out = {}
  copies = (type(copies) == "table") and copies or {}
  copies[inp] = out -- use normal assignment so we use copies' metatable (if any)
  for key,value in next,inp do -- skip metatables by using next directly
    -- we want a copy of the key and the value
    -- if one is not available on the copies table, we have to make one
    -- we can't do normal assignment here because metatabled copies tables might set metatables

    -- out[copies[key] or deep(key,copies)]=copies[value] or deep(value,copies)
    rawset(out,copies[key] or deep(key,copies),copies[value] or deep(value,copies))
  end
  return out
end

修改：我发现这样的内容并不能真正处理表格作为键：http://snippets.luacode.org/snippets/Deep_Comparison_of_Two_Values_3（下面的代码段副本）

function deepcompare(t1,t2,ignore_mt)
  local ty1 = type(t1)
  local ty2 = type(t2)
  if ty1 ~= ty2 then return false end
  -- non-table types can be directly compared
  if ty1 ~= 'table' and ty2 ~= 'table' then return t1 == t2 end
  -- as well as tables which have the metamethod __eq
  local mt = getmetatable(t1)
  if not ignore_mt and mt and mt.__eq then return t1 == t2 end
  for k1,v1 in pairs(t1) do
    local v2 = t2[k1]
    if v2 == nil or not deepcompare(v1,v2) then return false end
  end
  for k2,v2 in pairs(t2) do
    local v1 = t1[k2]
    if v1 == nil or not deepcompare(v1,v2) then return false end
  end
  return true
end

序列化也不是一种选择，因为序列化的顺序是＆＃34;随机＆＃34;。

Answer 1

正如其他人所说，这在很大程度上取决于你对等价的定义。如果你想这是真的：

local t1 = {[{}] = {1}, [{}] = {2}}
local t2 = {[{}] = {1}, [{}] = {2}}
assert( table_eq(t1, t2) )

如果你这样做，那么每当t1中的键是一个表时，你就必须这样做检查它与t2中每个表键的等价性，然后逐个尝试一。这是一种方法（为了便于阅读而省略了可用的东西）。

function table_eq(table1, table2)
   local avoid_loops = {}
   local function recurse(t1, t2)
      -- compare value types
      if type(t1) ~= type(t2) then return false end
      -- Base case: compare simple values
      if type(t1) ~= "table" then return t1 == t2 end
      -- Now, on to tables.
      -- First, let's avoid looping forever.
      if avoid_loops[t1] then return avoid_loops[t1] == t2 end
      avoid_loops[t1] = t2
      -- Copy keys from t2
      local t2keys = {}
      local t2tablekeys = {}
      for k, _ in pairs(t2) do
         if type(k) == "table" then table.insert(t2tablekeys, k) end
         t2keys[k] = true
      end
      -- Let's iterate keys from t1
      for k1, v1 in pairs(t1) do
         local v2 = t2[k1]
         if type(k1) == "table" then
            -- if key is a table, we need to find an equivalent one.
            local ok = false
            for i, tk in ipairs(t2tablekeys) do
               if table_eq(k1, tk) and recurse(v1, t2[tk]) then
                  table.remove(t2tablekeys, i)
                  t2keys[tk] = nil
                  ok = true
                  break
               end
            end
            if not ok then return false end
         else
            -- t1 has a key which t2 doesn't have, fail.
            if v2 == nil then return false end
            t2keys[k1] = nil
            if not recurse(v1, v2) then return false end
         end
      end
      -- if t2 has a key which t1 doesn't have, fail.
      if next(t2keys) then return false end
      return true
   end
   return recurse(table1, table2)
end

assert( table_eq({}, {}) )
assert( table_eq({1,2,3}, {1,2,3}) )
assert( table_eq({1,2,3, foo = "fighters"}, {["foo"] = "fighters", 1,2,3}) )
assert( table_eq({{{}}}, {{{}}}) )
assert( table_eq({[{}] = {1}, [{}] = {2}}, {[{}] = {1}, [{}] = {2}}) )
assert( table_eq({a = 1, [{}] = {}}, {[{}] = {}, a = 1}) )
assert( table_eq({a = 1, [{}] = {1}, [{}] = {2}}, {[{}] = {2}, a = 1, [{}] = {1}}) )

assert( not table_eq({1,2,3,4}, {1,2,3}) )
assert( not table_eq({1,2,3, foo = "fighters"}, {["foo"] = "bar", 1,2,3}) )
assert( not table_eq({{{}}}, {{{{}}}}) )
assert( not table_eq({[{}] = {1}, [{}] = {2}}, {[{}] = {1}, [{}] = {2}, [{}] = {3}}) )
assert( not table_eq({[{}] = {1}, [{}] = {2}}, {[{}] = {1}, [{}] = {3}}) )

Answer 2

您可以尝试序列化每个表并比较序列化结果，而不是直接比较。有几个序列化程序将表作为键处理，可以序列化深层和递归结构。例如，您可以尝试Serpent（作为独立模块提供，也包含在Mobdebug中）：

local dump = pcall(require, 'serpent') and require('serpent').dump
  or pcall(require, 'mobdebug') and require('mobdebug').dump
  or error("can't find serpent or mobdebug")
local a = dump({a = 1, [{}] = {}})
local b = dump({[{}] = {}, a = 1})
print(a==b)

这为我返回true（Lua 5.1和Lua 5.2）。其中一个警告是序列化结果取决于键序列化的顺序。默认情况下，表作为键是基于它们的tostring值进行排序，这意味着顺序取决于它们的分配地址，并且不难想出在Lua 5.2下失败的示例：

local dump = pcall(require, 'serpent') and require('serpent').dump
  or pcall(require, 'mobdebug') and require('mobdebug').dump
  or error("can't find serpent or mobdebug")
local a = dump({a = 1, [{}] = {1}, [{}] = {2}})
local b = dump({[{}] = {2}, a = 1, [{}] = {1}})
print(a==b) --<-- `false` under Lua 5.2

防止这种情况的一种方法是在密钥比较中始终表示表格;例如，您可能希望序列化表及其值并根据该值对其进行排序（serpent允许custom handler for sortkeys），而不是默认tostring，这将使排序更加健壮并且序列化结果更稳定。

如何深入比较2个Lua表，这些表可能有也可能没有表作为键？

2 个答案: