mirror of
https://github.com/featurebasedb/featurebase.git
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120 lines
3.6 KiB
Go
120 lines
3.6 KiB
Go
// Copyright 2020 Pilosa Corp.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package pilosa
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import "math/bits"
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// bsiData contains BSI-structured data.
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type bsiData []*Row
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// insert a value for a column in the BSI data.
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func (bsi *bsiData) insert(column uint64, value uint64) {
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data := *bsi
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for value != 0 {
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bit := bits.TrailingZeros64(value)
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value &^= 1 << bit
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for len(data) <= bit {
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data = append(data, NewRow())
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}
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data[bit].SetBit(column)
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}
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*bsi = data
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}
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// pivotDescending loops over nonzero BSI values in descending order.
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// For each value, the provided function is called with the value and a slice of the associated columns.
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func (bsi bsiData) pivotDescending(filter *Row, branch uint64, limit, offset *uint64, fn func(uint64, ...uint64)) {
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// This "pivot" algorithm works by treating the BSI data as a tree.
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// Each branch of this tree corresponds to a power-of-2-sized range of BSI values.
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// Each range is subdivided into 2 ranges of half size, which form lower branches.
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// Eventually, a range of width 1 cannot be subdivided and forms a leaf.
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// At each branch and leaf, there is a bitmap of all columns within the corresponding range.
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// The lower branches are formed as a difference or intersect of the upper branch's bitmap with the BSI bit that subdivides the range.
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// This function uses a depth-first search over this virtual tree.
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switch {
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case !filter.Any():
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// There are no remaining data.
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case offset != nil && *offset >= filter.Count():
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// Skip this entire branch.
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*offset -= filter.Count()
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case limit != nil && *limit == 0:
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// The limit has been reached.
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// No more data is necessary.
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case len(bsi) == 0:
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// This is a leaf node.
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cols := filter.Columns()
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if offset != nil {
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cols = cols[*offset:]
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*offset = 0
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}
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if limit != nil {
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if *limit < uint64(len(cols)) {
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cols = cols[:*limit]
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}
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*limit -= uint64(len(cols))
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}
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fn(branch, cols...)
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default:
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// Pivot over the highest bit.
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upperBranch, lowerBranch := branch|(1<<uint(len(bsi)-1)), branch
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splitBit := bsi[len(bsi)-1]
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lowerBits := bsi[:len(bsi)-1]
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lowerBits.pivotDescending(filter.Intersect(splitBit), upperBranch, limit, offset, fn)
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lowerBits.pivotDescending(filter.Difference(splitBit), lowerBranch, limit, offset, fn)
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}
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}
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/*
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// distribution generates a BSI histogram for the input.
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// TODO: I forgot what I was going to use this for.
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// Could probbably use this for:
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// - quartile queries
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// - TopN on int
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func (bsi bsiData) distribution(filter *Row) bsiData {
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var dist bsiData
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bsi.pivotDescending(filter, 0, nil, nil, func(count uint64, values ...uint64) {
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dist.insert(count, uint64(len(values)))
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})
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return dist
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}
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*/
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// addBSI adds BSI values together.
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func addBSI(x, y bsiData) bsiData {
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if len(x) > len(y) {
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x, y = y, x
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}
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carry := NewRow()
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out := make(bsiData, 0, len(y))
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for i, v := range x {
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out = append(out, v.Xor(y[i]).Xor(carry))
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carry = v.Intersect(y[i]).Union(v.Intersect(carry), y[i].Intersect(carry))
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}
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for _, v := range y[len(x):] {
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out = append(out, v.Xor(carry))
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carry = v.Intersect(carry)
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}
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if carry.Any() {
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out = append(out, carry)
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}
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return out
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}
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