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//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift open source project
//
// Copyright (c) 2019 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See http://swift.org/LICENSE.txt for license information
// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
/// An assignment that is either decided upon during decision making or derived
/// from previously known incompatibilities during unit propagation.
///
/// All assignments store a term (a package identifier and a version
/// requirement) and a decision level, which represents the number of decisions
/// at or before it in the partial solution that caused it to be derived. This
/// is later used during conflict resolution to figure out how far back to jump
/// when a conflict is found.
public struct Assignment: Equatable {
public let term: Term
public let decisionLevel: Int
public let cause: Incompatibility?
public let isDecision: Bool
private init(
term: Term,
decisionLevel: Int,
cause: Incompatibility?,
isDecision: Bool
) {
self.term = term
self.decisionLevel = decisionLevel
self.cause = cause
self.isDecision = isDecision
}
/// An assignment made during decision making.
public static func decision(_ term: Term, decisionLevel: Int) -> Assignment {
assert(term.requirement.isExact, "Cannot create a decision assignment with a non-exact version selection: \(term.requirement)")
return self.init(
term: term,
decisionLevel: decisionLevel,
cause: nil,
isDecision: true
)
}
/// An assignment derived from previously known incompatibilities during
/// unit propagation.
public static func derivation(
_ term: Term,
cause: Incompatibility,
decisionLevel: Int
) -> Assignment {
self.init(
term: term,
decisionLevel: decisionLevel,
cause: cause,
isDecision: false
)
}
}
extension Assignment: CustomStringConvertible {
public var description: String {
switch self.isDecision {
case true:
return "[Decision \(self.decisionLevel): \(self.term)]"
case false:
return "[Derivation: \(self.term) ← \(self.cause?.description ?? "-")]"
}
}
}
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