uint256 bits() seems to be scanning from right to left rather than left to right #4972

issue donovanhide opened this issue on September 24, 2014

donovanhide commented at 4:38 PM on September 24, 2014: none
https://github.com/bitcoin/bitcoin/blob/master/src/uint256.h#L258-L259 https://github.com/bitcoin/bitcoin/blob/master/src/uint256.cpp#L204-L216

I might be going a little insane, but I'd assume to find the highest set bit in a uint256 you'd need to start at the highest bit of the leftmost uint32 and progress right. That is:
```
    for (int pos = 0; pos < WIDTH; pos++) {
```
theuni commented at 5:17 PM on September 24, 2014: member

Least significant limbs come first. So you're looking for the highest bit at the last non-zero position.
donovanhide commented at 5:22 PM on September 24, 2014: none

I can see that fits with the compact format, but does it make sense in the context of the division operator?

https://github.com/bitcoin/bitcoin/blob/master/src/uint256.cpp#L95-L96
sipa commented at 5:25 PM on September 24, 2014: member

The number represented by a uint256 with uint32_t pn[8] = {a,b,c,d,e,f,g,h} represents value: a + 2^32_b + 2^64_c + 2^96_d + 2^128_e + 2^160_f + 2^192_g + 2^224*h.

All uint256 operators work on this format.
laanwj closed this on Sep 25, 2014
laanwj commented at 7:37 AM on September 25, 2014: member

Not an issue, closing.
bitcoin locked this on Sep 8, 2021

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