30. Designing a small class — Homework solutions

local Counter = {}
Counter.__index = Counter

function Counter.new()
    return setmetatable({ count = 0 }, Counter)
end
function Counter:increment() self.count = self.count + 1 end
function Counter:get() return self.count end
function Counter:reset() self.count = 0 end

local c = Counter.new()
c:increment(); c:increment(); c:increment()
print(c:get())   -- 3
c:reset()
print(c:get())   -- 0

local Health = {}
Health.__index = Health

function Health.new(max)
    return setmetatable({ max = max, current = max }, Health)
end
function Health:heal(n)
    self.current = self.current + n
    if self.current > self.max then self.current = self.max end
end
function Health:damage(n)
    self.current = self.current - n
    if self.current < 0 then self.current = 0 end
end
function Health:get() return self.current end

local h = Health.new(100)
h:damage(30); print(h:get())   -- 70
h:heal(1000); print(h:get())   -- 100  (capped at max)
h:damage(9999); print(h:get()) -- 0    (floored at 0)

local Switch = {}
Switch.__index = Switch

function Switch.new()
    return setmetatable({ on = false }, Switch)
end
function Switch:toggle() self.on = not self.on end
function Switch:isOn() return self.on end

local s = Switch.new()
print(s:isOn())   -- false
s:toggle(); print(s:isOn())   -- true
s:toggle(); print(s:isOn())   -- false

local Stack = {}
Stack.__index = Stack

function Stack.new()
    return setmetatable({ items = {} }, Stack)
end
function Stack:push(v) table.insert(self.items, v) end
function Stack:pop() return table.remove(self.items) end
function Stack:size() return #self.items end

local s = Stack.new()
s:push("a"); s:push("b")
print(s:size())   -- 2
print(s:pop())    -- b
print(s:pop())    -- a
print(s:pop())    -- nil   (empty, but no crash)
print(s:size())   -- 0