semantics: Use a BitSet #11819
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| Original file line number | Diff line number | Diff line change |
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| /* | ||
| Copyright 2022 The Vitess Authors. | ||
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| Licensed under the Apache License, Version 2.0 (the "License"); | ||
| you may not use this file except in compliance with the License. | ||
| You may obtain a copy of the License at | ||
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| http://www.apache.org/licenses/LICENSE-2.0 | ||
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| Unless required by applicable law or agreed to in writing, software | ||
| distributed under the License is distributed on an "AS IS" BASIS, | ||
| WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | ||
| See the License for the specific language governing permissions and | ||
| limitations under the License. | ||
| */ | ||
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| package bitset | ||
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| import ( | ||
| "math/bits" | ||
| "unsafe" | ||
| ) | ||
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| // A Bitset is an immutable collection of bits. You can perform logical operations | ||
| // on it, but all mutable operations return a new Bitset. | ||
| // It is safe to compare directly using the comparison operator and to use as a map key. | ||
| type Bitset string | ||
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| const bitsetWidth = 8 | ||
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| func bitsetWordSize(max int) int { | ||
| return max/bitsetWidth + 1 | ||
| } | ||
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| func toBitset(words []byte) Bitset { | ||
| if len(words) == 0 { | ||
| return "" | ||
| } | ||
| return *(*Bitset)(unsafe.Pointer(&words)) | ||
| } | ||
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| func minlen(a, b Bitset) int { | ||
| if len(a) < len(b) { | ||
| return len(a) | ||
| } | ||
| return len(b) | ||
| } | ||
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| // Overlaps returns whether this Bitset and the input have any bits in common | ||
| func (bs Bitset) Overlaps(b2 Bitset) bool { | ||
| min := minlen(bs, b2) | ||
| for i := 0; i < min; i++ { | ||
| if bs[i]&b2[i] != 0 { | ||
| return true | ||
| } | ||
| } | ||
| return false | ||
| } | ||
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| // Or returns the logical OR of the two Bitsets as a new Bitset | ||
| func (bs Bitset) Or(b2 Bitset) Bitset { | ||
| if len(bs) == 0 { | ||
| return b2 | ||
| } | ||
| if len(b2) == 0 { | ||
| return bs | ||
| } | ||
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| small, large := bs, b2 | ||
| if len(small) > len(large) { | ||
| small, large = large, small | ||
| } | ||
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| merged := make([]byte, len(large)) | ||
| m := 0 | ||
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| for m < len(small) { | ||
| merged[m] = small[m] | large[m] | ||
| m++ | ||
| } | ||
| for m < len(large) { | ||
| merged[m] = large[m] | ||
| m++ | ||
| } | ||
| return toBitset(merged) | ||
| } | ||
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| // AndNot returns the logical AND NOT of the two Bitsets as a new Bitset | ||
| func (bs Bitset) AndNot(b2 Bitset) Bitset { | ||
| if len(b2) == 0 { | ||
| return bs | ||
| } | ||
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| merged := make([]byte, len(bs)) | ||
| m := 0 | ||
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| for m = 0; m < len(bs); m++ { | ||
| if m < len(b2) { | ||
| merged[m] = bs[m] & ^b2[m] | ||
| } else { | ||
| merged[m] = bs[m] | ||
| } | ||
| } | ||
| for ; m > 0; m-- { | ||
| if merged[m-1] != 0 { | ||
| break | ||
| } | ||
| } | ||
| return toBitset(merged[:m]) | ||
| } | ||
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| // And returns the logical AND of the two bitsets as a new Bitset | ||
| func (bs Bitset) And(b2 Bitset) Bitset { | ||
| if len(bs) == 0 || len(b2) == 0 { | ||
| return "" | ||
| } | ||
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| merged := make([]byte, minlen(bs, b2)) | ||
| m := 0 | ||
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| for m = 0; m < len(merged); m++ { | ||
| merged[m] = bs[m] & b2[m] | ||
| } | ||
| for ; m > 0; m-- { | ||
| if merged[m-1] != 0 { | ||
| break | ||
| } | ||
| } | ||
| return toBitset(merged[:m]) | ||
| } | ||
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| // Set returns a copy of this Bitset where the bit at `offset` is set | ||
| func (bs Bitset) Set(offset int) Bitset { | ||
| alloc := len(bs) | ||
| if max := bitsetWordSize(offset); max > alloc { | ||
| alloc = max | ||
| } | ||
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| words := make([]byte, alloc) | ||
| copy(words, bs) | ||
| words[offset/bitsetWidth] |= 1 << (offset % bitsetWidth) | ||
| return toBitset(words) | ||
| } | ||
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| // SingleBit returns the position of the single bit that is set in this Bitset | ||
| // If the Bitset is empty, or contains more than one set bit, it returns -1 | ||
| func (bs Bitset) SingleBit() int { | ||
| offset := -1 | ||
| for i := 0; i < len(bs); i++ { | ||
| t := bs[i] | ||
| if t == 0 { | ||
| continue | ||
| } | ||
| if offset >= 0 || bits.OnesCount8(t) != 1 { | ||
| return -1 | ||
| } | ||
| offset = i*bitsetWidth + bits.TrailingZeros8(t) | ||
| } | ||
| return offset | ||
| } | ||
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| // IsContainedBy returns whether this Bitset is contained by the given Bitset | ||
| func (bs Bitset) IsContainedBy(b2 Bitset) bool { | ||
| if len(bs) > len(b2) { | ||
| return false | ||
| } | ||
| for i := 0; i < len(bs); i++ { | ||
| left := bs[i] | ||
| rigt := b2[i] | ||
| if left&rigt != left { | ||
| return false | ||
| } | ||
| } | ||
| return true | ||
| } | ||
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| // Popcount returns the number of bits that are set in this Bitset | ||
| func (bs Bitset) Popcount() (count int) { | ||
| for i := 0; i < len(bs); i++ { | ||
| count += bits.OnesCount8(bs[i]) | ||
| } | ||
| return | ||
| } | ||
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| // ForEach calls the given callback with the position of each bit set in this Bitset | ||
| func (bs Bitset) ForEach(yield func(int)) { | ||
| for i := 0; i < len(bs); i++ { | ||
| bitset := bs[i] | ||
| for bitset != 0 { | ||
| t := bitset & -bitset | ||
| r := bits.TrailingZeros8(bitset) | ||
| yield(i*bitsetWidth + r) | ||
| bitset ^= t | ||
| } | ||
| } | ||
| } | ||
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There was a problem hiding this comment. an example of how this works will help. A general example for overall implementation will also work. |
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| // Build creates a new immutable Bitset where all the given bits are set | ||
| func Build(bits ...int) Bitset { | ||
| if len(bits) == 0 { | ||
| return "" | ||
| } | ||
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| max := bits[0] | ||
| for _, b := range bits[1:] { | ||
| if b > max { | ||
| max = b | ||
| } | ||
| } | ||
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| words := make([]byte, bitsetWordSize(max)) | ||
| for _, b := range bits { | ||
| words[b/bitsetWidth] |= 1 << (b % bitsetWidth) | ||
| } | ||
| return toBitset(words) | ||
| } | ||
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| const singleton = "\x00\x00\x00\x01\x00\x00\x00\x02\x00\x00\x00\x04\x00\x00\x00\x08\x00\x00\x00\x10\x00\x00\x00\x20\x00\x00\x00\x40\x00\x00\x00\x80" | ||
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| // Single returns a new Bitset where only the given bit is set. | ||
| // If the given bit is less than 32, Single does not allocate to create a new Bitset. | ||
| func Single(bit int) Bitset { | ||
|
Comment on lines
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There was a problem hiding this comment. what is the limit to the number of There was a problem hiding this comment. There is no limit! Any bitset larger than 32 will allocate, but you can make it as large as you want. |
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| switch { | ||
| case bit < 8: | ||
| bit = (bit + 1) << 2 | ||
| return Bitset(singleton[bit-1 : bit]) | ||
| case bit < 16: | ||
| bit = (bit + 1 - 8) << 2 | ||
| return Bitset(singleton[bit-2 : bit]) | ||
| case bit < 24: | ||
| bit = (bit + 1 - 16) << 2 | ||
| return Bitset(singleton[bit-3 : bit]) | ||
| case bit < 32: | ||
| bit = (bit + 1 - 24) << 2 | ||
| return Bitset(singleton[bit-4 : bit]) | ||
| default: | ||
| words := make([]byte, bitsetWordSize(bit)) | ||
| words[bit/bitsetWidth] |= 1 << (bit % bitsetWidth) | ||
| return toBitset(words) | ||
| } | ||
| } | ||
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Does this get the memory address from go runtime?