Lua-Playground/sets.lua at master · ztmark/Lua-Playground · GitHub

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local Set = {}

local mt = {}

function Set.new(l)
    local set = {}
    setmetatable(set, mt)
    for _, v in ipairs(l) do
        set[v] = true
    end
    return set
end

function Set.union(a, b)
    local res = Set.new{}
    for k in pairs(a) do
        res[k] = true
    end
    for k in pairs(b) do
        res[k] = true
    end
    return res
end

function Set.intersection( a, b )
    local res = Set.new{}
    for k in pairs(a) do
        res[k] = b[k]
    end
    return res
end

function Set.tostring( set )
    local l = {}
    for e in pairs(set) do
        l[#l + 1] = tostring(e)
    end
    return "{" .. table.concat(l, ", ") .. "}"
end

mt.__add = Set.union
mt.__mul = Set.intersection

mt.__le = function ( a, b )
    for k in pairs(a) do
        if not b[k] then
            return false
        end
    end
    return true
end

mt.__lt = function ( a, b )
    return a <= b and not (b <= a)
end

mt.__eq = function ( a, b )
    return a <= b and b <= a
end

mt.__tostring = Set.tostring

mt.__metatable = "not your business"

return Set