"""List traversal under controlled mutations; not a model of a vendor cursor guarantee."""
import json
initial=[5,4,3,2,1];size=2
first=initial[:size];live=initial.copy();live.remove(5);second=live[2:4]
observed=first+second
assert first==[5,4] and second==[2,1] and sorted(set(initial)-set(observed))==[3]
snapshot=initial.copy();snapshotPages=[snapshot[i:i+size] for i in range(0,len(snapshot),size)]
assert sum(snapshotPages,[])==initial
keyset=[];last=4
while True:
 page=[i for i in live if i<last][:size]
 if not page:break
 keyset+=page;last=page[-1]
assert keyset==[3,2,1]
inserted=[6]+initial;nextInserted=inserted[2:4];assert nextInserted==[4,3]
checkpoint={'account':'A1','filter':'status=open','order':'id descending','snapshot':'S1','nextOffset':2,'committedIds':[5,4]}
report={'originalPopulation':initial,'deleteBeforeNextPage':{'page1':first,'liveAfterDeletion':live,'page2Offset2':second,'observed':observed,'missedOriginal':[3]},'insertBeforeNextPage':{'page1':first,'page2Offset2':nextInserted,'repeatedId':4},'definedSnapshot':snapshotPages,'toyImmutableIdKeysetAfter4':keyset,'checkpointExample':checkpoint,'prematureCheckpoint':{'savedNextOffset':2,'firstPageCommitted':False,'restartOnSnapshot':[3,2,1],'missed':[5,4]},'scope':'Toy algorithms and explicit invented snapshot; not real API behaviour'}
if __name__=='__main__':print(json.dumps(report,indent=2))