-- 函数定义
function add(a, b)
    return a + b
end

-- 函数是值
local mul = function(a, b) return a * b end
print(mul(3, 4))  -- 12

-- 多重返回值
function minmax(arr)
    local min, max = arr[1], arr[1]
    for _, v in ipairs(arr) do
        if v < min then min = v end
        if v > max then max = v end
    end
    return min, max
end

local lo, hi = minmax({3, 1, 4, 1, 5, 9})
print(lo, hi)  -- 1  9

-- 可变参数
function sum(...)
    local total = 0
    for _, v in ipairs({...}) do
        total = total + v
    end
    return total
end
print(sum(1, 2, 3, 4, 5))  -- 15

-- 命名参数模式
function rename(old, new)
    -- ...
end
rename{old="file.txt", new="backup.txt"}

-- 闭包: 函数 + 它引用的上值 (upvalue)
function newCounter()
    local count = 0
    return function()
        count = count + 1
        return count
    end
end

local c1 = newCounter()
print(c1())  -- 1
print(c1())  -- 2
print(c1())  -- 3

local c2 = newCounter()
print(c2())  -- 1 (独立的计数器)

-- 闭包实现私有成员
function newAccount(initialBalance)
    local balance = initialBalance
    
    local function withdraw(amount)
        if amount > balance then error("insufficient funds") end
        balance = balance - amount
    end
    
    local function deposit(amount)
        balance = balance + amount
    end
    
    local function getBalance()
        return balance
    end
    
    return {
        withdraw = withdraw,
        deposit = deposit,
        getBalance = getBalance
    }
end

local acc = newAccount(100)
acc.deposit(50)
print(acc.getBalance())  -- 150

-- 尾调用: 函数的最后一步是调用另一个函数
function factorial(n, acc)
    acc = acc or 1
    if n <= 1 then return acc end
    return factorial(n - 1, n * acc)  -- 尾调用
end

-- Lua 保证尾调用不会增加调用栈
-- 等价于 goto，可以无限递归

-- 非尾调用
function bad_factorial(n)
    if n <= 1 then return 1 end
    return n * bad_factorial(n - 1)  -- 不是尾调用 (还有乘法)
end

-- 列表的迭代器
function listIterator(list)
    local i = 0
    local n = #list
    return function()
        i = i + 1
        if i <= n then return list[i] end
    end
end

for item in listIterator({"a", "b", "c"}) do
    print(item)
end

-- 无状态迭代器
function rangeIter(from, to)
    return function(_, last)
        local next = last + 1
        if next > to then return nil end
        return next
    end, nil, from - 1
end

for i in rangeIter(1, 5) do
    print(i)  -- 1, 2, 3, 4, 5
end