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Published at Oct 10 2020
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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.

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

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

```
(ns largest-series-product-test
(:require [clojure.test :refer [deftest is testing]]
[largest-series-product :as lsp]))
(deftest largest-series-tests
(testing "can find the largest product of 2 with numbers in order"
(is (= 72 (lsp/largest-product 2 "0123456789"))))
(testing "can find the largest product of 2"
(is (= 48 (lsp/largest-product 2 "576802143"))))
(testing "finds the largest product if span equals length"
(is (= 18 (lsp/largest-product 2 "29"))))
(testing "can find the largest product of 3 with numbers in order"
(is (= 504 (lsp/largest-product 3 "0123456789"))))
(testing "can find the largest product of 3"
(is (= 270 (lsp/largest-product 3 "1027839564"))))
(testing "can find the largest product of 5 with numbers in order"
(is (= 15120 (lsp/largest-product 5 "0123456789"))))
(testing "can get the largest product of a big number"
(is (= 23520
(let [ds "73167176531330624919225119674426574742355349194934"]
(lsp/largest-product 6 ds)))))
(testing "can get the largest product of a big number II"
(is (= 28350
(let [ds "52677741234314237566414902593461595376319419139427"]
(lsp/largest-product 6 ds)))))
(testing "can get the largest product of a big number (Project Euler)"
(is (= 23514624000
(let [ds "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450"]
(lsp/largest-product 13 ds)))))
(testing "reports zero if the only digits are zero"
(is (= 0 (lsp/largest-product 2 "0000"))))
(testing "reports zero if all spans include zero"
(is (= 0 (lsp/largest-product 3 "99099"))))
(testing "rejects span longer than string length"
(is (thrown? Throwable (lsp/largest-product 4 "123"))))
(testing "reports 1 for empty string and empty product (0 span)"
(is (= 1 (lsp/largest-product 0 ""))))
(testing "reports 1 for nonempty string and empty product (0 span)"
(is (= 1 (lsp/largest-product 0 "123"))))
(testing "rejects empty string and nonzero span"
(is (thrown? Throwable (lsp/largest-product 1 ""))))
(testing "rejects invalid character in digits"
(is (thrown? Throwable (lsp/largest-product 2 "1234a5"))))
(testing "rejects negative span"
(is (thrown? Throwable (lsp/largest-product -1 "12345")))))
```

```
(ns largest-series-product)
(defn- to-digit [c]
(let [d (Character/digit c 10)]
(if (= d -1)
(throw (Exception. "invalid digit"))
d))
)
(defn largest-product [n ds] ;; <- arglist goes here
(when (> n (count ds)) (throw (Exception. "too big span")))
(if (empty? ds)
1
(->> ds
(map to-digit)
(partition n 1)
(map (partial apply *))
(apply max)))
)
```

A huge amount can be learned from reading other peopleâ€™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?

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