Programming in Lua(Thrid Edition)笔记---12 Data Files and Persistence

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12 Data File and Persistence

  • 自描述数据格式，数据文件如下：
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    -- data file
    Entry{
        author = "Donald E. Knuth",
        title = "Literate Programming",
        publisher = "CSLI",
        year = 1992
    }
    Entry{
        author = "Jon Bentley",
        title = "More Programming Pearls",
        year = 1990,
        publisher = "Addison-Wesley",
    }

读入程序如下：

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local authors = {} -- a set to collect authors
function Entry (b)
	if b.author then authors[b.author] = true end
end
dofile("data")
for name in pairs(authors) do print(name) end

  • Serialize，无精度丢失的数字：
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    function serialize(o)
    	if type(o) == "number" then
    		io.write(string.format("%a", o))
    	else
    		<other cases>
    	end
    end

安全的字符串，使用string.format()的%q选项，自动转义：

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function serialize(o)
	if type(o) == "number" then
		io.write(o)
	elseif type(o) == "string" then
		io.write(string.format("%q", o))
	else
		<other cases>
	end
end

安全的字符串，使用[=[和]=]构造长字符串，需保证等号数量大于字符串中]=*]形式中等号的数量：

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function quote(s)
	-- find maximum length of sequences of equal signs
	local n = -1
	for w in string.gmatch(s, "]=*]") do
		n = math.max(n, #w - 2) -- -2 to remove the ']'s
	end
	-- produce a string with 'n' plus one equal signs
	local eq = string.rep("=", n + 1)
	-- build quoted string
	return string.format(" [%s[\n%s]%s] ", eq, s, eq)
end

无循环table：

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function serialize(o)
	if type(o) == "number" then
		io.write(o)
	elseif type(o) == "string" then
		io.write(string.format("%q", o))
	elseif type(o) == "table" then
		io.write("{\n")
		for k, v in pairs(o) do
			io.write(" [")
			serialize(k)
			io.write("] = ")
			serialize(v)
			io.write(",\n")
		end
		io.write("}\n")
	else
		error("cannot serialize a " .. type(o))
	end
end

有循环table：

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function baiscSerialize(o)
	if type(o) == "number" then
		return tostring(o)
	else -- assume it is a string
		return string.format("%q", o)
	end
end
function save(name, value, saved)
	saved = saved or {} -- initial value
	io.write(name, " = ")
	if type(value) == "number" or type(value) == "string" then
		io.write(basicSerialize(value), "\n")
	elseif type(value) == "table" then
		if saved[value] then -- value already saved?
			io.write(saved[value], "\n") -- use its previous name
		else
			saved[value] = name -- save name for next time
			io.write("{}\n") -- create a new table
			for k, v in pairs(value) do -- save its fields
				k = basicSerialize(k)
				local fname = string.format("%s[%s]", name, k)
				save(fname, v, saved)
			end
		end
	else
		error("cannot save a " .. type(value))
	end
end
a = {x = 1, y = 2, {3, 4, 5}}
a[2] = a -- cycle
a.z = a[1]
save("a", a)
a = {{"one", "two"}, 3}
b = {k = a[1]}
save("a", a)
save("b", b)
local t = {}
save("a", a, t)
save("b", b, t)