create a bitset using managedbuffer in swift

Here's an example of how to create a bitset using ManagedBuffer in Swift:


main.swift
final class BitSet {
    private let buffer: ManagedBuffer<Int, UnsafeMutablePointer<Int>>

    private var words: UnsafeMutablePointer<Int> {
        return buffer.withUnsafeMutablePointerToElements({ $0 })
    }

    var capacity: Int {
        return buffer.capacity * Int.bitWidth
    }

    init(capacity: Int, initialValue: Bool = false) {
        let wordCount = (capacity + Int.bitWidth - 1) / Int.bitWidth
        self.buffer = ManagedBuffer<Int, UnsafeMutablePointer<Int>>.create(
            minimumCapacity: wordCount) { buffer in
                let words = buffer.withUnsafeMutablePointerToElements { $0 }
                words.initialize(repeating: initialValue ? -1 : 0, count: wordCount)
                return words
        }
    }

    deinit {
        words.deinitialize(count: buffer.capacity)
        buffer.deallocate()
    }

    subscript(index: Int) -> Bool {
        get {
            assert(index < capacity)
            let wordIndex = index / Int.bitWidth
            let bitIndex  = index % Int.bitWidth
            return (words[wordIndex] & (1 << bitIndex)) != 0
        }
        set(newValue) {
            assert(index < capacity)
            let wordIndex = index / Int.bitWidth
            let bitIndex  = index % Int.bitWidth
            if newValue {
                words[wordIndex] |= (1 << bitIndex)
            } else {
                words[wordIndex] &= ~(1 << bitIndex)
            }
        }
    }
}
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This implementation creates a fixed-size bitset with a specified capacity in bits. It uses ManagedBuffer to allocate a contiguous block of memory to store the words containing the bit values. The words are initialized to either all 0s or all 1s depending on the initial value parameter.

The subscript operator is used to read and write individual bits by calculating the word and bit indexes and using bitwise operators to access and modify the bit values.

Note that this implementation assumes that the capacity is a multiple of the word size (i.e. 64 bits on most modern platforms). If the capacity is not a multiple of the word size, the implementation could be adjusted by padding the last word with unused bits or using a dynamic word size.

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