mul

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This is a problem from the Gopher Slack room

Problem:

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

The number is:

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

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

Results

florin@sniper [09:10:45] [~/golang/src/github.com/dlsniper/mul/mul] [master]
-> % go test -bench=.                                 
testing: warning: no tests to run
PASS
BenchmarkMul1-8     2000            684466 ns/op
BenchmarkMul2-8     5000            457176 ns/op
BenchmarkMul3-8     5000            212010 ns/op
BenchmarkMul4-8    10000            150611 ns/op
BenchmarkMul5-8    10000            153173 ns/op
BenchmarkMul6-8     3000            976646 ns/op
ok      github.com/dlsniper/mul/mul     10.907s

florin@sniper [09:09:58] [~/golang/src/github.com/dlsniper/mul/mul] [master *]
-> % go test -benchmem -test.memprofilerate=1 -bench=.
testing: warning: no tests to run
PASS
BenchmarkMul1-8     2000            735461 ns/op           17335 B/op          5 allocs/op
BenchmarkMul2-8     3000            455565 ns/op           24584 B/op          2 allocs/op
BenchmarkMul3-8     5000            210202 ns/op           16384 B/op          1 allocs/op
BenchmarkMul4-8    10000            149474 ns/op           16384 B/op          1 allocs/op
BenchmarkMul5-8    10000            152326 ns/op           16384 B/op          1 allocs/op
BenchmarkMul6-8     2000           1764112 ns/op           26337 B/op        112 allocs/op
ok      github.com/dlsniper/mul/mul     10.744s

License

Original problem asked by @mhatch. The code in mul1 belongs to him.

The rest is under MIT.