CODEWITHSUNDEEP
javahashintbit-manipulationbit-shift
Lucien
Lucien

Reputation: 896

Creating a unique integer based on 3 different integers

I'd like to create an int called hash in Java, from the coordinates x, y and w.

The values of x and y are signed and often negative.

The size of an int is 32 bits.

I'd like to assign 4 bits to w (or 3 if the most significant bit isn't usable), 14 bits to x and 14 bits to y.

I have tried the following method and I do not understand why the values clash at all: w + x << 4 + y << 18.

For example, x = 1 clashes with y = 1 when w == 0.

The advantages of doing this are as follows:

Upvotes: 0

Views: 223

Answers (3)

user555045
user555045

Reputation: 64904

The only problem here is operator precedence. + goes before <<, so you have to write it like this:

w + (x << 4) + (y << 18)

This doesn't confine w or x to their allotted fields, but that doesn't do bad things with the hash value. If you had used | to combine them, it would be a bad hash when w or x get negative, but it's fine with +.

Upvotes: 2

WalterM
WalterM

Reputation: 2706

int hash =  w << 28 | (x & 0x3FFF) << 14 | y & 0x3FFF;

+ is not a bitwise operator. This will make it look like this:

11110000000000000011111111111111
w -^ X          -^ Y          -^

Upvotes: 1

Bohemian
Bohemian

Reputation: 425073

Your problem is operator precedence: + takes precedence over <<, so your expression

w + x << 4 + y << 18

is equivalent to

((w + x) << (4 + y)) << 18

Try this:

w + (x << 4) + (y << 18)

Upvotes: 1

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