Merchants in first-visit order
Requirement
From a month of transactions, list each merchant once, in the order it was first visited — repeat visits must not move a merchant later in the list. The data is in the code below; both versions must print the line shown under Expected output.
Expected output
Cafe Aroma, Metro Card, Green Grocer, Noodle Bar
Side by side
Native Dart
FxDart
Why they differ
The tempting native one-liner is toSet().toList() — and it
would even print the right thing, because Dart's default set happens to
be insertion-ordered. But the Iterable.toSet contract
promises no order at all, so code whose requirement is
first-visit order shouldn't lean on it; the honest native version is a
seen-set loop with two collections and an if. FxDart's
uniq makes the guarantee part of the name: it lazily keeps
the first occurrence of each element, by contract, as one chain step
after map.
Benchmark
N = 100
Time Tie
Peak memory Tie
N = 10,000
Time Tie
Peak memory Tie
N = 1,000,000
Time Native wins
Peak memory Tie
Bars are medians of repeated timed iterations in fresh processes per side (small N is batched for timer resolution). Sides within 5% of each other — or within 0.6 ms, a difference no person can perceive — count as a tie; close relative races are re-measured up to 5 runs. In an app, anything under a few milliseconds is invisible to the user regardless of which bar is shorter. Memory is peak process RSS. The Dart VM and the dataset are identical on both sides, so the difference between the two bars is what the pipeline itself holds onto.