utf8validator.tl

local function validate_utf8(input: string): boolean, integer
    local _
    local i, len = 1, #input
    local byte, second, third, fourth: integer, integer, integer, integer = 0, 129, 129, 129
    local sbyte = string.byte
    local code_points = 0
    while i <= len do
    byte = sbyte(input, i)
    code_points = code_points + 1

    if byte <= 127 then
        i = i + 1
        --goto none


    elseif byte >= 194 and byte <= 223 then
        second = sbyte(input, i+1)
        i = i + 2
        --goto second

    elseif byte == 224 then
        second = sbyte(input, i+1); third = sbyte(input, i+2)
        -- overlong
        if second ~= nil and  second >= 128 and second <= 159 then return false, code_points end
        i = i + 3
        --goto third

    elseif byte == 237 then
        second = sbyte(input, i+1); third = sbyte(input, i+2)
        -- surrogate
        if second ~= nil and second >= 160 and second <= 191 then return false, code_points end
        i = i + 3
        --goto third

    elseif (byte >= 225 and byte <= 236) or byte == 238 or byte == 239 then
        second = sbyte(input, i+1); third = sbyte(input, i+2)
        i = i + 3
        --goto third

    elseif byte == 240 then
        second = sbyte(input, i+1); third = sbyte(input, i+2); fourth = sbyte(input, i+3)
        -- overlong
        if second ~= nil and second >= 128 and second <= 143 then return false, code_points end
        i = i + 4
        --goto fourth
    
    elseif byte == 241 or byte == 242 or byte == 243 then
        second = sbyte(input, i+1); third = sbyte(input, i+2); fourth = sbyte(input, i+3)
        i = i + 4
        --goto fourth
    
    elseif byte == 244 then
        second = sbyte(input, i+1); third = sbyte(input, i+2); fourth = sbyte(input, i+3)
        -- exceeds max utf-8
        if second ~= nil and second >= 160 and second <= 191 then return false, code_points end
        i = i + 4
        --goto fourth

    else
        -- can't go this high
        return false, code_points
    end

    -- for Lua 5.2+, you can uncomment the goto's above and the lines below for a bit of a speedup
    -- ::fourth:: if fourth == nil or fourth < 128 or fourth > 191 then return false, code_points end
    -- ::third:: if third == nil or third < 128 or third > 191 then return false, code_points end
    -- ::second:: if second == nil or second < 128 or second > 191 then return false, code_points end
    -- ::none::
    
    -- For Lua, but not LuaJIT, you can offload some of the processing to `find` to speed up validation
    -- _, i = input:find(".*", i)
    -- i = i + 1

    -- the below block handles the default use case for Lua 5.1 and LuaJIT
    -- unfinished (nil) or invalid (not in range)
    if second == nil or second < 128 or second > 191 then return false, code_points end
    if third == nil or third < 128 or third > 191 then return false, code_points end
    if fourth == nil or fourth < 128 or fourth > 191 then return false, code_points end

    end
    return true, code_points
end

return validate_utf8