first commit

util/julia/files/_erastothenes_sieve.jl

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+mutable struct sieve
+    sieve::BitVector
+    _length::Int
+    _target::Int
+    _prerun_size::Int
+end
+
+function get_sieve()
+    return sieve([false], 1, 1, 1000)
+end
+
+function _elongate!(s::sieve)
+    length = s._length
+    target = s._target
+    diff = target - length
+    if diff > 0
+        s.sieve = [s.sieve; trues(diff)]
+        for i = 1:floor(Int, sqrt(target))
+            if s.sieve[i]
+                _floor = length÷i + 1
+                _ceil = target÷i
+                for j = _floor:_ceil
+                    actual = i*j
+                    if actual > i
+                        s.sieve[actual] = false
+                    end
+                end
+            end
+        end
+        s._length = target
+    end
+    return nothing
+end
+
+# runs the sieve until index n
+function run!(s::sieve, n::Int)
+    s._target = n
+    _elongate!(s)
+    return nothing
+end
+
+# fetches data about a specific integer
+function get(s::sieve, n::Int, prerun=true::Bool)
+    if n > s._length
+        run(s, prerun ? n + s._prerun_size : n)
+    end
+    return s.sieve[n]
+end
+
+# returns all primes in the sieve
+function get_primes(s::sieve, n_max=(-1)::Int)
+    if n_max == -1
+        trimmed_sieve = s.sieve
+    else
+        trimmed_sieve = s.sieve[1:n_max]
+    end
+    n = length(trimmed_sieve)
+    c = count(trimmed_sieve)
+    primes = Vector(undef, c)
+    j = 1
+    for i = 1:n
+        if s.sieve[i]
+            primes[j] = i
+            j += 1
+        end
+    end
+    return primes
+end
+
+function get_composites(s::sieve)
+    n = length(s.sieve)
+    c = count(s.sieve)
+    composites = Vector(undef, n-c)
+    j = 1
+    for i = 1:n
+        if !s.sieve[i]
+            composites[j] = i
+            j += 1
+        end
+    end
+    return composites
+end
+
+function is_prime(s::sieve, i::Int)
+    if i < 0 return false end
+    run(s, i)
+    return s.sieve[i]
+end