local bigint = require "pluto:bigint"

local country_codes <const> =
    "AD24 AE23 AL28 AT20 AZ28 BA20 BE16 BG22 BH22 BR29 " ..
    "BY28 CH21 CR22 CY28 CZ24 DE22 DK18 DO28 EE20 ES24 " ..
    "FI18 FO18 FR27 GB22 GE22 GI23 GL18 GR27 GT28 HR21 " ..
    "HU28 IE22 IL23 IQ23 IS26 IT27 JO30 KW30 KZ20 LB28 " ..
    "LC32 LI21 LT20 LU20 LV21 MC27 MD24 ME22 MK19 MR27 " ..
    "MT31 MU30 NL18 NO15 PK24 PL28 PS29 PT25 QA29 RO24 " ..
    "RS22 SA24 SC31 SE24 SI19 SK24 SM27 ST25 SV28 TL23 " ..
    "TN24 TR26 UA29 VG24 XK20"

local function is_valid(iban)
    -- Remove spaces from IBAN.
    local s = iban:replace(" ", "")

    -- Check country code and length.
    if not(s:sub(1, 2) .. tostring(#s)) in country_codes then
        return false
    end

    -- Move first 4 characters to the end.
    s = s:sub(5) .. s:sub(1, 4)

    -- Replace A to Z with numbers 10 To 35.
    for i, c in "ABCDEFGHIJKLMNOPQRSTUVWXYZ":split("") do
        s = s:replace(c, tostring(i + 9))
    end

    -- Check whether mod 97 calculation gives a remainder of 1.
    return bigint.new(s) % bigint.new(97) == bigint.new(1)
end

local ibans = { "GB82 WEST 1234 5698 7654 32", "GB82 TEST 1234 5698 7654 32" }
for ibans as iban do
    local valid = is_valid(iban)
    print(iban .. (valid ? " may be valid" : " is not valid"))
end