On Tue, Jun 21, 2022 at 06:20:54PM +0530, Abhradeep Chakraborty wrote: > Taylor Blau <me@xxxxxxxxxxxx> wrote: > > > I'm not sure if I remember why `table[i] - selected->xor_offset` is > > right and not `i - selected->xor_offset`. > > Even I myself got confused! Before sending the patch to the mailing > list, I was clear about that. That's why I didn't catch the so called > mistake I have been notifying till now. Thanks Taylor for asking > the question! > > I should add a comment before the line so that people can understand it. > Let us parse `table_inv[table[i] - selected->xor_offset]` - > > Suppose bitmap entries be like - > > Bitmap 0 (for commit 0) > Bitmap 1 (for commit 1) > Bitmap 2 (for commit 2) > Bitmap 3 (for commit 3) > . > . > . > Bitmap 20 (for commit 20) > > These bitmaps are ordered by the date of their corresponding commit. > `table` array maps commit's lexicographic order to its bitmap order. > `table_inv` stores the reverse (i.e. it maps bitmap order to lexicographic > order). Say for example, if commit 4 is lexicographically first among all the > Commits then `table[0]` is 4. Similarly `table[1]`=2, table[2]=1 etc. > `table_inv[4]` is 0, table_inv[2]=1 etc. > > Now suppose commit 4's bitmap has xor-relation with commit 2's bitmap. > So, xor-offset for bitmap 4 is 2. And `table[0] - selected->xor_offset` > is equal to 4-2 = 2. It is pointing to the commit 2. Now, 2 is in bitmap > Order. We need to convert it into lexicographic order. So, table_inv[2] > gives us the lexicographic order position of commit 2 I.e. 1. > > Long story short, there is no issue regarding xor_offset. This xor_offset > is not relative to the current commit. It is absolute. > > Sorry for the initial claim :) Ahhhhh. Makes perfect sense. Thanks! Thanks, Taylor
