p8.lua

#!/usr/bin/env lua

--[[
Find the thirteen adjacent digits in the 1,000-digit number that have
the greatest product. What is the value of this product?
]]

local s = [[
73167176531330624919225119674426574742355349194934
96983520312774506326239578318016984801869478851843
85861560789112949495459501737958331952853208805511
12540698747158523863050715693290963295227443043557
66896648950445244523161731856403098711121722383113
62229893423380308135336276614282806444486645238749
30358907296290491560440772390713810515859307960866
70172427121883998797908792274921901699720888093776
65727333001053367881220235421809751254540594752243
52584907711670556013604839586446706324415722155397
53697817977846174064955149290862569321978468622482
83972241375657056057490261407972968652414535100474
82166370484403199890008895243450658541227588666881
16427171479924442928230863465674813919123162824586
17866458359124566529476545682848912883142607690042
24219022671055626321111109370544217506941658960408
07198403850962455444362981230987879927244284909188
84580156166097919133875499200524063689912560717606
05886116467109405077541002256983155200055935729725
71636269561882670428252483600823257530420752963450
]]

--- Compute the largest product of 13 consecutive digits in a string.
---@param s string The string of digits to search over.
local function sum_numbers(s)
    -- Convert string to one line
    s = string.gsub(s, '\n','')
    -- Read in digits of string to a table
    local a = {}
    for i = 1, 1000 do
        a[i] = tonumber(string.sub(s, i, i))
    end
    -- Get max product of 13 adjacent digits
    local max_prod = 0
    for i = 1, 987 do
        local new_prod = 1
        for j = 1, 13 do
            -- Can ignore any window containing a zero
            if a[i + j] == 0 then
                break
            end
            new_prod = new_prod * a[i + j]
        end
        if new_prod > max_prod then
            max_prod = new_prod
        end
    end
    return max_prod
end

-- Compute solution
print(sum_numbers(s)) -- 23,514,624,000