improve patch perf

[obust]

looking at the benchmark

name	met (ms)	iters	throughput (GElems/s)	min (ms)	mean (ms)	max (ms)	duration (ms)
diff_append_row[1000+1]	2.533135881104034	471	0.00039516237856286157	2.533135881104034	2.533135881104034	2.533135881104034	1193.107
diff_append_row[1000+10]	2.6789449339207048	454	0.00037701409506832946	2.6789449339207048	2.6789449339207048	2.6789449339207048	1216.241
diff_append_row[1000+100]	3.3380165745856356	362	0.00032953700960473766	3.338016574585635	3.3380165745856356	3.338016574585635	1208.362
diff_append_row[1000+1000]	11.11425	100	0.00017994916436106802	11.11425	11.11425	11.11425	1111.425

looking at diff_append_row[1000+N]:

• diffing 1000 rows takes ~2.5ms
• the more we append rows the more the timing increase. that is because each Patch is copying the dst element Patch.__init__(out self, owned action: String, path: List[Int], value: Element):. Ideally the Patch should just take a reference (Pointer) to the destination element to avoid copy. I could confirm that diff_append_row[1000+1000] goes to ~2.5ms when using
  • var value: Optional[UnsafePointer[Element]]: could run this and diff_append_row[1000+1000] gets to ~2.5ms. However tests crash. I think because some referenced elements are moving underneath the pointer.
  • var value: Optional[Pointer[Element, origin]]: could not get the origin to propagate to ListPatch