problem_008.jl

"""
    Largest product in a series

The four adjacent digits in the 1000-digit number that have the greatest product are 9 × 9 × 8 × 9 = 5832.

Find the thirteen adjacent digits in the 1000-digit number that have the greatest product. What is the value of this product?

# Input parameters:
- `num_str` : The number to find the greatest product in, in string form.
- `n` : Number of consecutive numbers to find the product of.

# Examples/Tests:
```julia
num_str = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450"
problem_008("1234", 2)      # returns 12
problem_008(num_str, 4)     # returns 5832    
problem_008(num_str, 13)    # returns 23514624000
problem_008("12345", 6)     # throws DomainError
```

# Reference
- https://projecteuler.net/problem=8

Contributed by: [Praneeth Jain](https://www.github.com/PraneethJain)
"""
function problem_008(num_str::String, n::Int)
    n > length(num_str) &&
        throw(DomainError("n can't be bigger than length of num_str"))

    return maximum([
        mapreduce(x -> parse(Int64, x), *, num_str[i:i+n-1]) for
        i in 1:length(num_str)-n+1
    ])
end