51 lines
1.4 KiB
Nim
51 lines
1.4 KiB
Nim
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import math, strutils
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const N = 1_000_000
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var comp: array[2..(N - 1), bool] # True is composite, so default is prime.
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for n in 2..<N:
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if not comp[n]:
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for k in countup(n * n, N - 1, n):
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comp[k] = true
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var primes = @[2]
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for n in countup(3, N - 1, 2):
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if not comp[n]:
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primes.add n
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iterator groups(primes: seq[int]; diffs: varargs[int]): seq[int] =
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## Yield groups of successive primes with given differences.
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var cumdiffs = cumsummed(diffs) # Compute differences from first prime of group.
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let groupSize = diffs.len + 1
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for i in 0..(primes.len - groupSize):
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let p = primes[i]
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var group = @[p]
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for k, diff in cumdiffs:
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if primes[i + k + 1] != p + diff: break
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group.add p + diff
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if group.len == groupSize:
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yield group
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proc findGroups(primes: seq[int]; diffs: varargs[int]) =
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## In the given list of primes and for the given differences,
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## find the first group, the last group and the count of groups.
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var
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first, last: seq[int]
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count = 0
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for group in primes.groups(diffs):
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if first.len == 0: first = group
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last = group
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inc count
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echo "Differences: ", diffs.join(", ")
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echo "– first: ($#)" % first.join(", ")
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echo "– last: ($#)" % last.join(", ")
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echo "– count: ", count
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echo()
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primes.findGroups(2)
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primes.findGroups(1)
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primes.findGroups(2, 2)
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primes.findGroups(2, 4)
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primes.findGroups(4, 2)
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primes.findGroups(6, 4, 2)
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