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div_sum computes proper divisor sum via trial division up to sqrt(n):
let div_sum n =
let s = ref 1 in
let i = ref 2 in
while !i * !i <= n do
if n mod !i = 0 then begin
s := !s + !i;
let q = n / !i in
if q <> !i then s := !s + q
end;
i := !i + 1
done;
if n = 1 then 0 else !s
Outer loop finds amicable pairs (a, b) with d(a) = b, d(b) = a,
a != b. Only pair under 300 is (220, 284); 220 + 284 = 504.
Real PE21 uses 10000 (answer 31626). 300 keeps the run under
budget while exercising the same divisor-sum trick.
119 baseline programs total.
26 lines
462 B
OCaml
26 lines
462 B
OCaml
let div_sum n =
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let s = ref 1 in
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let i = ref 2 in
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while !i * !i <= n do
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if n mod !i = 0 then begin
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s := !s + !i;
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let q = n / !i in
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if q <> !i then s := !s + q
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end;
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i := !i + 1
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done;
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if n = 1 then 0 else !s
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let euler21 limit =
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let total = ref 0 in
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for a = 2 to limit do
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let b = div_sum a in
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if b <> a && b > a && b <= limit && div_sum b = a then
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total := !total + a + b
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done;
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!total
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;;
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euler21 300
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