yagagagaga commented on PR #880:
URL:
https://github.com/apache/incubator-baremaps/pull/880#issuecomment-2173976507
As all we known, Long is a 64-bits length data type with 63-bits represents
the numerical value and 1-bit represents positive or negative.
The `key` in `MemoryAlignedDataMap` needs to consider the following
limitations:
1. `value`'s data size. If the `value` occupies 4 bytes, it means that
2-bits (in 63-bits) are needed to represent.
2. The calculation of `segmentIndex` depends on the size of `segmentSize`.
If the `segmentSize` is 1024, it means that 10-bits (in 61-bits) are needed to
represent and the remaining 51-bits represent the number of segments.
Considering that `segmentIndex` is an Integer field, it will cause precision
loss when the `segmentIndex` is greater than 32-bits. There are two cases to
discuss:
1. `segmentIndex` will not loss precision if `segmentSize` greater than
32-bits.
```java
value data size = 2-bits segmentSize > 32-bits
/\ /~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~\
0111111111111111111111111111111111111111111111111111111111111111
^ \__________________________/
| v
| segmentIndex <= 30-bits
| \___________________________________________________________/
| v
-/+ key's range
```
2. `segmentIndex` will loss precision if `segmentSize` less than
32-bits. The range of `segmentIndex` must be limited in order to be represented
as an integer.
```java
segmentIndex segmentSize
/~~~~~~~~~~~~~~~^~~~~~~~~~~~~~\/~~~~^~~~\
0111111111111111111111111111111111111111111111111111111111111111
^ \/
| alue data size = 2-bits
| \_____________________________________/
| v
-/+ key's range
```
If following your implement, some wrong will happened:
```java
// this.upperBoundary = ((long) Integer.MAX_VALUE) / (long) dataType.size()
* ((long) memory.segmentSize());
var map = new MemoryAlignedDataMap<>(new IntegerDataType(), new
OnHeapMemory(1024));
map.put(549755813887L, 1); // error will happen, but as the matter of fact,
you can calculate segmentIndex and segmentOffset with this number
var map = new MemoryAlignedDataMap<>(new IntegerDataType(), new
OnHeapMemory(1L << 34)); // if could be, the upperBoundary will numeric overflow
```
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