A simple UTF-8 validator in (tex)lua

validate-utf8.lua

#!/usr/bin/env texlua

-- A simple UTF-8 validator in Lua. (Tested only with texlua.)
-- Manuel Pégourié-Gonnard, 2009, WTFPL v2.

-- returns true if s is a valid utf-8 sequence according to rfc3629
function is_valid_utf8(str) 
    local len = string.len(str)
    local not_cont = function(b) return b == nil or b < 128 or b >= 192 end
    local i = 0
    local next_byte = function()
        i = i + 1
        return string.byte(str, i)
    end
    while i < len do
        local seq = {}
        seq[1] = next_byte()
        if seq[1] >= 245 then 
            return false, 'Illegal byte '..seq[1]..' at byte '..i
        end
        if seq[1] >= 128 then                   
            local offset -- non-coding bits of the 1st byte
            for l, threshold in ipairs{[2] = 192, 224, 240} do
                if seq[1] >= threshold then     -- >= l byte sequence
                    seq[l] = next_byte()
                    if not_cont(seq[l]) then 
                        return false, 'Illegal continuation byte '..
                        seq[l]..' at byte '..i
                    end
                    offset = threshold
                end
            end
            if offset == nil then
                return false, 'Illegal first byte '..seq[1]..' at byte '..i
            end
            -- compute the code point for some verifications
            local code_point = seq[1] - offset
            for j = 2, #seq do
                code_point = code_point * 64 + seq[j] - 128
            end
            local n -- nominal length of the bytes sequence
            if     code_point <= 0x00007F then n = 1
            elseif code_point <= 0x0007FF then n = 2
            elseif code_point <= 0x00FFFF then n = 3
            elseif code_point <= 0x10FFFF then n = 4
            end
            if n == nil then
                return false, 
                'Code point '..code_point..' too large at byte '..i
            end
            if n ~= #seq then
                return false, 'Overlong encoding at byte '..i
            end
            if code_point >= 0xD800 and code_point <= 0xDFFF then
                return false,  'Code point '..code_point..
                ' reserved for utf-16 surrogate pairs at byte '..i
            end
        end -- if seq[0] >= 128
    end
    return true
end