Levenshtein distance - Lua implementation

--[[
    Levenshtein distance implemented in lua
    Made by darraghd493
]]

local module = {}

module.distance = function(s1, s2)
    s1 = string.lower(s1)
    s2 = string.lower(s2)

    local costs = {}

    for i=1,#s1 do
        local last = i;
        for j=1,#2 do
            if i == 1 then
                costs[j] = j
            elseif j > 1 then
                local newValue = costs[j-1]
                if s1:sub(i,i) ~= s2:sub(j,j) then
                    newValue = math.min(math.min(newValue, last), costs[j]) + 1
                end
                costs[j-1] = last
                last = newValue
            end
        end
    end

    return costs[#costs]
end

module.similarity = function(s1, s2)
    local longer = if #s1 > #s2 then s1 else s2
    local shorter = if #s1 < #s2 then s1 else s2

    if #longer == 0 then return 1 end

    return (#longer - module.distance(longer, shorter)) / #longer
end

return module