ðŸŽ‰ Exercism Research is now launched. Help Exercism, help science and have some fun at research.exercism.io ðŸŽ‰

# JaeHyoLee's solution

## to Largest Series Product in the Lua Track

Published at Jul 13 2018 · 0 comments
Instructions
Test suite
Solution

Given a string of digits, calculate the largest product for a contiguous substring of digits of length n.

For example, for the input `'1027839564'`, the largest product for a series of 3 digits is 270 (9 * 5 * 6), and the largest product for a series of 5 digits is 7560 (7 * 8 * 3 * 9 * 5).

Note that these series are only required to occupy adjacent positions in the input; the digits need not be numerically consecutive.

For the input `'73167176531330624919225119674426574742355349194934'`, the largest product for a series of 6 digits is 23520.

## Running the tests

To run the tests, run the command `busted` from within the exercise directory.

## Further information

For more detailed information about the Lua track, including how to get help if you're having trouble, please visit the exercism.io Lua language page.

## Source

A variation on Problem 8 at Project Euler http://projecteuler.net/problem=8

## Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

### largest-series-product_spec.lua

``````local lsp = require('largest-series-product')

describe('largest-series-product', function()
it('should be able to find the largest product of 2 with ordered digits', function()
assert.equals(72, lsp({ digits = '0123456789', span = 2 }))
end)

it('should be able to find the largest product of 2 with unordered digits', function()
assert.equals(48, lsp({ digits = '576802143', span = 2 }))
end)

it('should be able to find the largest product when the span is the same as the digit count', function()
assert.equals(18, lsp({ digits = '29', span = 2 }))
end)

it('should be able to find the largest product of 3 with ordered digits', function()
assert.equals(504, lsp({ digits = '0123456789', span = 3 }))
end)

it('should be able to find the largest product of 3 with unordered digits', function()
assert.equals(270, lsp({ digits = '1027839564', span = 3 }))
end)

it('should be able to get the largest product with a large span', function()
assert.equals(23520, lsp({
digits = '73167176531330624919225119674426574742355349194934',
span = 6
}))
end)

it('should be able to get the largest product with a really large span', function()
assert.equals(23514624000, lsp({
digits = '7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450',
span = 13
}))
end)

it('should yield a largest product of 0 if all digits are 0', function()
assert.equals(0, lsp({ digits = '0000', span = 2 }))
end)

it('should yield a largest product of 0 if all spans include 0', function()
assert.equals(0, lsp({ digits = '99099', span = 3 }))
end)

it('should yield 1 for no digits and 0 span', function()
assert.equals(1, lsp({ digits = '', span = 0 }))
end)

it('should yield 1 for non-empty digits and 0 span', function()
assert.equals(1, lsp({ digits = '123', span = 0 }))
end)

it('should reject spans larger than the string length', function()
assert.has_error(function()
lsp({ digits = '123', span = 4 })
end)
end)

it('should reject no digits and non-zero span', function()
assert.has_error(function()
lsp({ digits = '', span = 1 })
end)
end)

it('should reject negative spans', function()
assert.has_error(function()
lsp({ digits = '12345', span = -1 })
end)
end)

it('should reject invalid digits', function()
assert.has_error(function()
lsp({ digits = '1234a5', span = 2 })
end)
end)
end)``````
``````return function (input)
if input.span == 0 then return 1 end
if(input.span > #input.digits) or (input.span < 0) then error() end

local max_product = 0
local first = tonumber(input.digits:sub(1,1))
local index

for j = 1, #input.digits - 1 do
local max_product_local = 1
for i = j, j + input.span - 1 do
if i > #input.digits then return max_product end

local n = tonumber(input.digits:sub(i,i))
if n == nil then error() end
if n == 0 then
j = i + input.span
max_product_local = 0
break
else
max_product_local = max_product_local * n
end
end
if max_product < max_product_local then max_product = max_product_local end
end
return max_product
end``````