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Actually that's not quite correct. Vec::reserve(100) on an empty Vec will give you capacity 100, not 128 even though our amortization is indeed doubling. The rules go roughly like this, suppose length is L, present capacity is C, reserve(N): 1. L + N < C ? Enough capacity already, we're done, return 2. L + N <= C * 2 ? Ordinary doubling, grow to capacity C * 2 3. Otherwise, try to grow to L + N This means we can grow any amount more quickly than the amortized growth strategy or at the same speed - but never less quickly. We can go 100, 250, 600, 1300 and we can go 100, 200, 400, 800, 1600 - but we can''t do 100, 150, 200, 250, 300, 350, 400, 450, 500... |