What is the purpose of the `Bitwise operator |` in the `d3.shuffle` source code and how can it be understood

Question

What is the purpose of the `Bitwise operator |` in the `d3.shuffle` source code and how can it be understood

Learning about the Bitwise operator | can be found in the document here and a helpful video tutorial here. However, understanding the purpose and significance of | may not come easily through basic examples in those resources.

In my exploration of the source code for d3.shuffle, I came across this line utilizing |:

i = Math.random() * m-- | 0;

Based on the source code analysis, it appears that this particular line aims to assign a random index within the range of 0 to m to the variable i.

My inquiry is:

If the objective of this code snippet is to generate a random index between 0 and m, how does the use of | contribute to achieving this outcome?
In simpler terms, what is the practical application and implication of utilizing | in this context, as it currently remains unclear why or when | should be employed elsewhere.
To clarify the latter part: is there a methodology for comprehending all three lines of code together? Or should we rely on a concept of approximation to decipher these following three lines? If so, how? If not, what do the remaining two lines signify?

i = Math.random() * m-- | 0;

i = Math.random() * m-- | 1;

i = Math.random() * m-- | 2;

Thank you!

javascript d3.js

Answer 1

Answer №1

When using the logical OR operator with a float and an integer, the float will be truncated to a 32-bit integer. This technique is explained in detail in this particular answer.

The Mechanism Behind it

By using the | operator, the operands are implicitly coerced into integers because it is designed to work on 32-bit integers specifically.

In addition to this coercion, performing a logical OR operation with 0 results in no change: the bits that were already set remain unaltered.

If you use a non-zero value, such as 1:

x | 1

This action sets the least significant bit of x to 1, making the result always an odd number. Some examples include:

4 | 1 === 5
5 | 1 === 5
6 | 1 === 7 
7.123 | 1 === 7

Another scenario involves using a value like 2:

x | 2

This changes the second least significant bit to 1. Since the value of that bit is 2, the result will either be x itself or x+2:

4 | 2 === 6
5 | 2 === 7
6 | 2 === 6
7 | 2 === 7
8 | 2 === 10

To fully understand the process, observe these numbers in binary form:

left argument:  00000100  (=4)
right argument: 00000010  (=2)
outcome of |:   00000110  (=6)

A bit in the outcome will only be set if that corresponding bit is set in at least one of the arguments.

It's important to note that x | 0 doesn't change x apart from the integer coercion. Therefore, this operator is primarily used for coercion rather than bit-setting.

Implications for Large Numbers

Numbers beyond the range of 2³² (positive or negative) will be mapped within that range, potentially leading to unexpected results.

Answer 2

When using the logical OR operator with a float and an integer, the float will be truncated to a 32-bit integer. This technique is explained in detail in this particular answer.

The Mechanism Behind it

By using the | operator, the operands are implicitly coerced into integers because it is designed to work on 32-bit integers specifically.

In addition to this coercion, performing a logical OR operation with 0 results in no change: the bits that were already set remain unaltered.

If you use a non-zero value, such as 1:

x | 1

This action sets the least significant bit of x to 1, making the result always an odd number. Some examples include:

4 | 1 === 5
5 | 1 === 5
6 | 1 === 7 
7.123 | 1 === 7

Another scenario involves using a value like 2:

x | 2

This changes the second least significant bit to 1. Since the value of that bit is 2, the result will either be x itself or x+2:

4 | 2 === 6
5 | 2 === 7
6 | 2 === 6
7 | 2 === 7
8 | 2 === 10

To fully understand the process, observe these numbers in binary form:

left argument:  00000100  (=4)
right argument: 00000010  (=2)
outcome of |:   00000110  (=6)

A bit in the outcome will only be set if that corresponding bit is set in at least one of the arguments.

It's important to note that x | 0 doesn't change x apart from the integer coercion. Therefore, this operator is primarily used for coercion rather than bit-setting.

Implications for Large Numbers

Numbers beyond the range of 2³² (positive or negative) will be mapped within that range, potentially leading to unexpected results.

Answer 3

Answer №2

When encountering this situation, one could view it as a method to convert a decimal number to a whole number.

In simpler terms, interpret it like so:

i = Math.floor(Math.random() * m--);

The use of an operator may have been favored because:

Possibly for efficiency reasons (although this needs validation)
Simply the personal choice of the individual writing the code

Answer 4

When encountering this situation, one could view it as a method to convert a decimal number to a whole number.

In simpler terms, interpret it like so:

i = Math.floor(Math.random() * m--);

The use of an operator may have been favored because:

Possibly for efficiency reasons (although this needs validation)
Simply the personal choice of the individual writing the code

What is the purpose of the `Bitwise operator |` in the `d3.shuffle` source code and how can it be understood

Answer №1

The Mechanism Behind it

Implications for Large Numbers

Answer №2

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