Bitsets

Bitsets are simple sets of integral-indexed bits originally introduced to support attribute testing of Unicode char-sets so both integers and Unicode code points can be used as indices. You can use them as a set of flags much like bitmasks in C.

The first bit is index 0 (zero).

simple-bitsets.idio
;; create a five element bitset -- all bits are initially unset
bs1 := make-bitset 5

printf "bs1 is %s\n" bs1

;; set the first two bits
bitset-set! bs1 0
bitset-set! bs1 1

printf "bs1 is %s\n" bs1

;; if we know the starting bits we can create it immediately
bs2 := #B{ 5 11 }
printf "bs2 is %s\n" bs2

;; more than 8 bits
bs3 := make-bitset 40

;; set the 9th and tenth bits
bitset-set! bs3 8
bitset-set! bs3 9

printf "bs3 is %s\n" bs3

;; an inverted bs3
bs4 := not-bitset bs3

;; clear the last bit
bitset-clear! bs4 ((bitset-size bs4) - 1)

printf "bs4 is %s\n" bs4
$ idio simple-bitsets
bs1 is #B{ 5 }
bs1 is #B{ 5 11000 }
bs2 is #B{ 5 11000 }
bs3 is #B{ 40 8:11000000 }
bs4 is #B{ 40 0-0 00111111 10-18 11111110 }

The printed (and reader) form is a condensed set with listed ranges all set (otherwise the bits are unset). You can use the #B{ ... } form as bitset constructors.

In the previous example’s output:

  • bs3 has the first two bits set in the block starting at index 8 (a hex index)

  • bs4 has

    • all bits in the blocks starting at (hex) 0 through to the block starting at (hex) 0 inclusive are set

    • the next block has the first two bits unset then the rest set

    • all bits in the blocks starting at (hex) 10 through to the block starting at (hex) 18 inclusive are set

    • the next block has all the bits set except the last

SRFI-14

SRFI-14 defines several sets of char-sets which use arrays of bitsets (representing Unicode planes) to cover the 221 bits of Unicode code points.

simple-char-sets.idio
require SRFI-14

;; the ASCII-only variant of char-set:lower-case
printf "%s\n" %char-set:lower-case
$ idio simple-char-sets
#<SI sparse-char-set size=1114112 planes=#[
     #B{ 65536 60:01111111 68-70 11100000 }
     #f #f #f #f #f #f #f #f #f #f #f #f #f #f #f #f ]>

NB. The output has been edited for readability.

For the ASCII-only variant you can see the bitset for the first plane is #B{ 65536 60:01111111 68-70 11100000 } which reads as bits from index 61 (hex) through to 7a (hex). Which looks reasonable.

Try char-set:lower-case (without the %) for the complete Unicode SRFI 14 definition of lower case code points.

Last built at 2024-06-17T06:11:37Z+0000 from 77077af (dev) for Idio 0.3