JaeHyoLee's solution to Largest Series Product on the Lua track

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

Community comments

Find this solution interesting? Ask the author a question to learn more.

What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

What compromises have been made?
Are there new concepts here that you could read more about to improve your understanding?

JaeHyoLee's solution

to Largest Series Product in the Lua Track