local int = require "int"
local fmt = require "fmt"

local limit <const> = 999_999
local c = int.arecomps(limit)
local left_found = false
local right_found = false
print("Largest truncatable primes less than a million:")
for i = limit, 4, -2 do
    if !c[i] then
        if !right_found then
            local p = i // 10
            while p > 0 do
                if p % 2 == 0 or c[p] then break end
                p //= 10
            end
            if p == 0 then
                print($"  Right truncatable prime = {fmt.int(i)}")
                right_found = true
                if left_found then return end
            end
        end
        if !left_found then
            local q = tostring(i):sub(2)
            if !("0" in q) then
                local p = tonumber(q)
                while #q > 0 do
                    if p % 2 == 0 or c[p] then break end
                    q = q:sub(2)
                    p = tonumber(q)
                end
                if q == "" then
                    print($"  Left truncatable prime  = {fmt.int(i)}")
                    left_found = true
                    if right_found then return end
                end
            end
        end
    end
end