rosetta_luhn.jsonnet

local toInt(x) = if x > (std.floor(x) + 0.5) then std.floor(x) + 1 else std.floor(x);
local sum(xs) = std.foldl(function(a,b) a+b, xs, 0);
local headn(x) = std.floor(x/10);
local tailn(x) = toInt(((x/10) - headn(x)) * 10);
local sumdigits(x) = if x < 10 then x else sumdigits(headn(x)) + tailn(x);
local splitrev(x) = if x<10 then [x] else [tailn(x)] + splitrev(headn(x));
local luhn(x) =
  local xs = splitrev(x);
  (sum(xs[::2]) + sum([sumdigits(n*2) for n in xs[1::2]])) % 10 == 0;

local ccnums = [ 49927398716, 49927398717, 1234567812345678, 1234567812345670 ];

std.assertEqual([luhn(x) for x in ccnums], [true, false, false, true])