giles
cb626fc402
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Test, Build, and Deploy / test-build-deploy (push) Failing after 25s
Fenwick / Binary Indexed Tree for prefix sums. The classic
`i & -i` low-bit trick needs negative-aware AND, but our `land`
evaluator (iter 127, bitwise via floor/mod arithmetic) only handles
non-negative operands. Workaround: a portable lowbit helper that
finds the largest power of 2 dividing i:
let lowbit i =
let r = ref 1 in
while !r * 2 <= i && i mod (!r * 2) = 0 do
r := !r * 2
done;
!r
After building from [1;3;5;7;9;11;13;15]:
total = prefix_sum 8 = 64
update 1 by +100
after = prefix_sum 8 = 164
total + after = 228
Tests recursive update / prefix_sum chains via helper-extracted
lowbit; documents a non-obvious limit of the bitwise-emulation
layer.
168 baseline programs total.
34 lines
594 B
OCaml
34 lines
594 B
OCaml
let n = 8
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let bit = Array.make (n + 1) 0
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let lowbit i =
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let r = ref 1 in
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while !r * 2 <= i && i mod (!r * 2) = 0 do
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r := !r * 2
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done;
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!r
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let rec update i delta =
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if i <= n then begin
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bit.(i) <- bit.(i) + delta;
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update (i + lowbit i) delta
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end
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let rec prefix_sum i =
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if i <= 0 then 0
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else bit.(i) + prefix_sum (i - lowbit i)
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let range_sum l r = prefix_sum r - prefix_sum (l - 1)
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;;
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let a = [| 1; 3; 5; 7; 9; 11; 13; 15 |] in
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for i = 0 to n - 1 do
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update (i + 1) a.(i)
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done;
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let total = prefix_sum n in
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update 1 100;
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let after = prefix_sum n in
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total + after
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