SegmentQuery

Eonil, 2019.

Dynamically stores additive values and get arbitrary sub-range sums in O(log(n)) time.

This is an implementation of something called "segment-tree" or "interval-tree". You insert/remove lengths of consecutive segments and query positions on it. I'm not sure which one is correct name.

How to Use

Add dependency.

.package(url: "https://github.com/eonil/swift-segment-query", .branch("master")),

And use.

import SegmentQuery

var x = SegmentQuery<Int>()

/// Append segment lengths.
x.append(contentsOf: [111,222,333])

/// Get subrange sum.
let s = x[1..<3].sum
assert(s == 222+333)

/// Get start/end offsets of a segment 1.
let a = x[..<1].sum
let b = x[...1].sum
let c = a..<b

/// Get index of segment and in-segment-offset from an offset.
let (a,b) = x.location(at: 230)
assert(a == 1)
assert(b == 8)

Requirements

• Swift 5.x.
• Swift Package Manager.

Complexity

• O(log(n)) for element query/insert/update/remove and sub-sum query.

Performance

I couldn't find other segment query library written in Swift for comparison. Instead, I performed insert/remove performance comparison with well-known B-Tree library written by Lőrentey. This is one-by-one random Int number insert/remove at random location on 64-bit macOS,

With list of 128K values,

• 4x faster than Swift.Array.
• Very close speed to BTree.List.

With list of 1 million values,

• 4x faster than BTree.List.

Run SegmentQueryBenchmark to get numbers on your machine.

Implementation

• B-Tree based with no key stored.
• Only values are stored.
• Values are stored only on leafs.
• Buffer sizes are optimized for ordered list behavior with dynamic automatic sum.

Missing Features

• Balancing after remove. (Now only insertion is balanced by B-Tree insertion algorithm)
• Bulk insert/remove implementation.
• Sequential element iteration in O(n) time. Now it's O(n log n).
• Sub-sum indexing in each node. We can store sub-sum for each branch/leaf node children to perform binary search to get an item. There is a trade-off, but I didn't try due to lack of time.

Credit & License

Copyright(C) Eonil 2019. All rights reserved. Using of this code is licensed under "MIT License".