42 lines
1.5 KiB
Swift
42 lines
1.5 KiB
Swift
//
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// Copyright 2023, 2024 New Vector Ltd.
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//
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// SPDX-License-Identifier: AGPL-3.0-only
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// Please see LICENSE in the repository root for full details.
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//
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import Foundation
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import CryptoKit
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/// Object encapsulating an experiment with an arbitrary number of variants, and a method to deterministically
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/// and uniformly assign a variant to user. Variants do not carry any implicit semantics, they are plain numbers
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/// to be interpreted by the caller of the experiment.
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struct Experiment {
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let name: String
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let variants: UInt
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/// Get the assigned variant from the total number of variants and for a given `userId`
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///
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/// This variant is chosen deterministically (the same `userId` and experiment `name` will yield the same variant)
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/// and uniformly (multiple users are distributed roughly evenly among the variants).
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func variant(userId: String) -> UInt {
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// Combine user id with experiment name to avoid identical variant
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// for the same user in different experiments
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let data = (userId + name).data(using: .utf8) ?? Data()
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// Get the first 8 bytes and map to decimal number (UInt64 = 8 bytes)
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let decimal = digest(for: data)
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.prefix(8)
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.reduce(0) { $0 << 8 | UInt64($1) }
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// Compress the decimal into a set number of variants using modulo
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return UInt(decimal % UInt64(variants))
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}
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private func digest(for data: Data) -> SHA256.Digest {
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var sha = SHA256()
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sha.update(data: data)
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return sha.finalize()
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}
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}
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