Correct the number of colors returned by quantize

[ijambro]

Closes #9
The number of colors returned by quantize should match maxcolors, including for values > 7.

I've tested this fix using a test.js from a separate project that requires quantize, for many values ranging from 1 to 256, and this fix corrects the off-by-one color count.

A test.js and a Readme update would help to support this Pull Request. Let me know if you're interested.

Curious if anyone else has noticed or investigated this behavior?

[ijambro]

I created Mocha tests for the quantize function, asserting that the format of all returned palettes is valid, for a variety of inputs.

For each of the tests below, I run it using many values of maximumColorCount between 2 and 256:

✓ works on 1-pixel 1-color images
✓ works on 5-pixel 1-color images
✓ works on 1000-pixel 1-color images
✓ works on 5-pixel 5-color images
✓ works on 1000-pixel 5-color images
✓ works on 20-pixel 20-color images
✓ works on 1000-pixel 20-color images

The most interesting results are how the returned ("Actual") count of colors lines up with the requested ("Expected") maximum color count.

Using the current quantize code:
1-pixel: always returns 1 color
More than 1 pixel:
Actual: 3, Expected: 2
Actual: 4, Expected: 3
Actual: 4, Expected: 4
Actual: 5, Expected: 5
Actual: 6, Expected: 6
Actual: 7, Expected: 7
Actual: 7, Expected: 8
Actual: 8, Expected: 9
Actual: 9, Expected: 10
Actual: 10, Expected: 11
Actual: 11, Expected: 12
Actual: 12, Expected: 13
Actual: 13, Expected: 14
Actual: 14, Expected: 15
Actual: 15, Expected: 16
Actual: 31, Expected: 32
Actual: 63, Expected: 64
Actual: 127, Expected: 128
Actual: 255, Expected: 256

Using my proposed change, adding "+ 1" to current quantize code line 474:
1-pixel: always returns 1 color
More than 1 pixel:
Actual: 3, Expected: 2
Actual: 4, Expected: 3
Actual: 4, Expected: 4
Actual: 5, Expected: 5
Actual: 6, Expected: 6
Actual: 7, Expected: 7
Actual: 8, Expected: 8
Actual: 9, Expected: 9
Actual: 10, Expected: 10
Actual: 11, Expected: 11
Actual: 12, Expected: 12
Actual: 13, Expected: 13
Actual: 14, Expected: 14
Actual: 15, Expected: 15
Actual: 16, Expected: 16
Actual: 32, Expected: 32
Actual: 64, Expected: 64
Actual: 128, Expected: 128
Actual: 256, Expected: 256

Two conclusions:

1. For requested ("Expected") maximum color count of 2 and 3, the response includes one too many - exceeding the maximum - in both the original code and my change.
2. For requested ("Expected") maximum color count greater than 7, my change corrects the issue and responds with the exact amount requested.

[ijambro]

After testing AlfredJKwack's change from #6 , I've seen that it returns the exact count of colors requested for all input values between 2 and 256! I've replaced my partial correction with his superior fix.

[Richienb]

// @olivierlesnicki

[alexharris]

+1 for this!

[alexharris]

and, if anyone else stumbles across this I have a fork with this pull request, which seems to fix the issue, implemented here:

https://github.com/alexharris/quantize