goimagehash: Implement PerceptionHashExtend

[TokyoWolFrog]

add a function that returns a hash computation of phash which the size can be set larger than uint64

[coveralls]

Pull Request Test Coverage Report for Build 84

• 76 of 98 (77.55%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-5.08%) to 87.919%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
hashcompute.go	21	23	91.3%
imagehash.go	55	75	73.33%

Totals
Change from base Build 60:	-5.08%
Covered Lines:	262
Relevant Lines:	298

---

💛 - Coveralls

[corona10]

@TokyoWolFrog

Hi, Thank you for the contribution, this PR will be a lot of improvement for goimagehash.

There are some things that I want to achieve with extension hash.

• We need a way to calculate a distance of two hashes which are calculated by PerceptionHashExtend.
• We need a unit test

So my suggestions are these.

• Implement BigImageHash/ExImageHash/(:name what you want) which can compare with other bighash.

// BigImageHash is a struct of hash computation.
type BigImageHash struct {
	hash string
	kind Kind
}

• Add unit tests for PerceptionHashExtend. You can use below codes to check whether bighash left the same result by using the same file on 64bits size case.

goimagehash/hashcompute_test.go

Line 14 in 81672d7

func TestHashCompute(t *testing.T) {

Can you please reflect my comments?
Again thank you!

[TokyoWolFrog]

@corona10
Thanks for reply! It will take some time, but I would like to reflect your comments.

[corona10]

@TokyoWolFrog

Thanks alot!

[corona10]

Oh and the furthermore
Please add your name on https://github.com/corona10/goimagehash/blob/master/AUTHORS.md
and also add on here https://github.com/corona10/goimagehash/blob/master/CODEOWNERS when you implement extension hash.
Thank you!

[golangcibot]

Error return value of h1.Distance is not checked (from errcheck)

[TokyoWolFrog]

@corona10
Hi,

• add distance calculation of two hashes calculated by PerceptionHashExtend.
• add some unit test
  To make the new function more consistent with the original code style, I made some changes of the implement.

[corona10]

@TokyoWolFrog
I left some comments! Most of the things look great!
Most important thing is that we should align the API signature of the hash structure.

[corona10]

Also we need to add func (h *ImageHash) GetHash() []uint64

goimagehash/imagehash.go

Line 53 in 81672d7

func (h *ImageHash) GetHash() uint64 {

to get Hash value([]uint64)

[TokyoWolFrog]

@corona10
Thanks for the review!
I modified the code to align the API signature with #4

[corona10]

@TokyoWolFrog

LGTM to me!!

Just fix the issue which Golang CI mentioned.
#18 (comment)

Everything looks great! Thanks alot