A program small enough for zeno build to compile today

# primes.zeno: a program small enough for `zeno build` to compile today.
#
#     zeno run examples/primes.zeno      run it with the interpreter
#     zeno build examples/primes.zeno    compile it to primes.c and ./primes
#     zeno build --llvm examples/primes.zeno    the same through LLVM IR (primes.ll)
#
# All three must print exactly the same thing. No types are written anywhere:
# the checker works them out, and the C that comes out is fully typed.

function is_prime(n) {
    if n < 2 {
        return false
    }
    var d = 2
    while d * d <= n {
        if n % d == 0 {
            return false
        }
        d += 1
    }
    return true
}

function count_primes(limit) {
    var count = 0
    for n in range(limit) {
        if is_prime(n) {
            count += 1
        }
    }
    return count
}

function average_gap(limit) {
    return limit / count_primes(limit)
}

let limit = 10000
print(f"primes below {limit}: {count_primes(limit)}")
print(f"one in every {average_gap(limit):.2f} numbers")

var largest = limit
while not is_prime(largest) {
    largest -= 1
}
print(f"the largest is {largest}, or {largest:x} in hex")

primes below 10000: 1229
one in every 8.14 numbers
the largest is 9973, or 26f5 in hex