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bit: Optimized int_bitfield_extract.

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This commit is contained in:
Jeroen van Rijn
2021-08-11 20:59:53 +02:00
parent 463003e86a
commit 35d8976de4
3 changed files with 70 additions and 68 deletions
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core/math/big/helpers.odin
+62 -40
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@@ -191,20 +191,8 @@ neg :: proc(dest, src: ^Int, allocator := context.allocator) -> (err: Error) {
/*
Helpers to extract values from the `Int`.
*/
extract_bit :: proc(a: ^Int, bit_offset: int) -> (bit: DIGIT, err: Error) {
/*
Check that `a`is usable.
*/
if err = clear_if_uninitialized(a); err != nil { return 0, err; }
limb := bit_offset / _DIGIT_BITS;
if limb < 0 || limb >= a.used {
return 0, .Invalid_Argument;
}
i := DIGIT(1 << DIGIT((bit_offset % _DIGIT_BITS)));
return 1 if ((a.digit[limb] & i) != 0) else 0, nil;
int_bitfield_extract_single :: proc(a: ^Int, offset: int) -> (bit: _WORD, err: Error) {
return int_bitfield_extract(a, offset, 1);
}
int_bitfield_extract :: proc(a: ^Int, offset, count: int) -> (res: _WORD, err: Error) {
@@ -212,38 +200,72 @@ int_bitfield_extract :: proc(a: ^Int, offset, count: int) -> (res: _WORD, err: E
Check that `a` is usable.
*/
if err = clear_if_uninitialized(a); err != nil { return 0, err; }
if count > _WORD_BITS || count < 1 { return 0, .Invalid_Argument; }
for shift := 0; shift < count; shift += 1 {
bit_offset := offset + shift;
limb := bit_offset / _DIGIT_BITS;
mask := DIGIT(1 << DIGIT((bit_offset % _DIGIT_BITS)));
if (a.digit[limb] & mask) != 0 {
res += _WORD(1) << uint(shift);
}
}
return res, nil;
}
int_bitfield_extract_fast :: proc(a: ^Int, offset, count: int) -> (res: _WORD, err: Error) {
/*
Check that `a` is usable.
Early out for single bit.
*/
if err = clear_if_uninitialized(a); err != nil { return 0, err; }
if count == 1 {
limb := offset / _DIGIT_BITS;
if limb < 0 || limb >= a.used { return 0, .Invalid_Argument; }
i := _WORD(1 << _WORD((offset % _DIGIT_BITS)));
return 1 if ((_WORD(a.digit[limb]) & i) != 0) else 0, nil;
}
if count > _WORD_BITS || count < 1 { return 0, .Invalid_Argument; }
for shift := 0; shift < count; shift += 1 {
bit_offset := offset + shift;
/*
There are 3 possible cases.
- [offset:][:count] covers 1 DIGIT,
e.g. offset: 0, count: 60 = bits 0..59
- [offset:][:count] covers 2 DIGITS,
e.g. offset: 5, count: 60 = bits 5..59, 0..4
e.g. offset: 0, count: 120 = bits 0..59, 60..119
- [offset:][:count] covers 3 DIGITS,
e.g. offset: 40, count: 100 = bits 40..59, 0..59, 0..19
e.g. offset: 40, count: 120 = bits 40..59, 0..59, 0..39
*/
limb := offset / _DIGIT_BITS;
bits_left := count;
bits_offset := offset % _DIGIT_BITS;
num_bits := min(bits_left, _DIGIT_BITS - bits_offset);
// fmt.printf("offset: %v | count: %v\n\n", offset, count);
// fmt.printf("left: %v | bits_offset: %v | limb: %v | num: %v\n\n", bits_left, bits_offset, limb, num_bits);
shift := offset % _DIGIT_BITS;
mask := (_WORD(1) << uint(num_bits)) - 1;
// fmt.printf("shift: %v | mask: %v\n", shift, mask);
// fmt.printf("d: %v\n", a.digit[limb]);
res = (_WORD(a.digit[limb]) >> uint(shift)) & mask;
// fmt.printf("res: %v\n", res);
bits_left -= num_bits;
if bits_left == 0 { return res, nil; }
res_shift := num_bits;
num_bits = min(bits_left, _DIGIT_BITS);
mask = (1 << uint(num_bits)) - 1;
v := (_WORD(a.digit[limb + 1]) & mask) << uint(res_shift);
res |= v;
bits_left -= num_bits;
if bits_left == 0 { return res, nil; }
// fmt.printf("bits_left: %v | offset: %v | num: %v\n", bits_left, offset, num_bits);
mask = (1 << uint(bits_left)) - 1;
res_shift += _DIGIT_BITS;
v = (_WORD(a.digit[limb + 2]) & mask) << uint(res_shift);
res |= v;
limb := bit_offset / _DIGIT_BITS;
mask := DIGIT(1 << DIGIT((bit_offset % _DIGIT_BITS)));
if (a.digit[limb] & mask) != 0 {
res += _WORD(1) << uint(shift);
}
}
return res, nil;
}