# class Prime::EratosthenesSieve

Internal use. An implementation of eratosthenes's sieve

### Constants

BITS_PER_ENTRY
ENTRIES_PER_TABLE
FILLED_ENTRY
NUMS_PER_ENTRY
NUMS_PER_TABLE

### Public Instance Methods

next_to(n) click to toggle source

returns the least odd prime number which is greater than `n`.

```# File lib/prime.rb, line 441
def next_to(n)
n = (n-1).div(2)*2+3 # the next odd number to given n
table_index, integer_index, bit_index = indices(n)
loop do
extend_table until @tables.length > table_index
for j in integer_index...ENTRIES_PER_TABLE
if !@tables[table_index][j].zero?
for k in bit_index...BITS_PER_ENTRY
return NUMS_PER_TABLE*table_index + NUMS_PER_ENTRY*j + 2*k+1 if !@tables[table_index][j][k].zero?
end
end
bit_index = 0
end
table_index += 1; integer_index = 0
end
end```

### Private Instance Methods

extend_table() click to toggle source
```# File lib/prime.rb, line 471
def extend_table
lbound = NUMS_PER_TABLE * @tables.length
ubound = lbound + NUMS_PER_TABLE
new_table = [FILLED_ENTRY] * ENTRIES_PER_TABLE # which represents primarity in lbound...ubound
(3..Integer(Math.sqrt(ubound))).step(2) do |p|
i, j, k = indices(p)
next if @tables[i][j][k].zero?

start = (lbound.div(p)+1)*p  # least multiple of p which is >= lbound
start += p if start.even?
(start...ubound).step(2*p) do |n|
_, j, k = indices(n)
new_table[j] &= FILLED_ENTRY^(1<<k)
end
end
@tables << new_table.freeze
end```
indices(n) click to toggle source

for an odd number `n`, returns (i, j, k) such that @tables[j] represents primarity of the number

```# File lib/prime.rb, line 460
def indices(n)
#   binary digits of n: |0|1|2|3|4|5|6|7|8|9|10|11|....
#   indices:            |-|    k  |  j  |     i
# because of NUMS_PER_ENTRY, NUMS_PER_TABLE

k = (n & 0b00011111) >> 1
j = (n & 0b11100000) >> 5
i = n >> 8
return i, j, k
end```