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// This source file is part of the Swift.org open source project
//
// Copyright (c) 2020 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
/// Combine two sequences which are already ordered with respect to
/// `compare` into a single ordered sequence with the items from each
/// sequence, in order.
///
/// - Parameters:
/// - lhs: The left-hand side sequence.
/// - rhs: The right-hand side sequence.
/// - areInIncreasingOrder: A predicate that returns true if its first
/// argument should be ordered before its second argument; otherwise,
/// false. Each sequence *MUST* already be in order with respect to this
/// predicate (this is checked, in debug builds).
/// - Returns: A list of pairs, ordered with respect to `compare`. The first
/// element in the pair will come from the LHS sequence, and the second will
/// come from the RHS sequence, and equal elements appear in both lists will be
/// returned together.
@inlinable
public func orderedZip<S: Sequence>(
_ lhs: S,
_ rhs: S,
by areInIncreasingOrder: (S.Element, S.Element) -> Bool
) -> [(S.Element?, S.Element?)] {
var result: [(S.Element?, S.Element?)] = []
result.reserveCapacity(max(lhs.underestimatedCount, rhs.underestimatedCount))
// Initialize.
var lhsIt = lhs.makeIterator()
var rhsIt = rhs.makeIterator()
var lhsItem = lhsIt.next()
var rhsItem = rhsIt.next()
// While each list has items...
while let a = lhsItem, let b = rhsItem {
// If a < b, take a.
if areInIncreasingOrder(a, b) {
result.append((a, nil))
lhsItem = lhsIt.next()
assert(lhsItem == nil || !areInIncreasingOrder(lhsItem!, a))
continue
}
// If b < a, take b.
if areInIncreasingOrder(b, a) {
result.append((nil, b))
rhsItem = rhsIt.next()
assert(rhsItem == nil || !areInIncreasingOrder(rhsItem!, b))
continue
}
// Otherwise, a == b, take them both.
result.append((a, b))
lhsItem = lhsIt.next()
assert(lhsItem == nil || !areInIncreasingOrder(lhsItem!, a))
rhsItem = rhsIt.next()
assert(rhsItem == nil || !areInIncreasingOrder(rhsItem!, b))
}
// Add an remaining items from either list (only one of these can actually be true).
while let a = lhsItem {
result.append((a, nil))
lhsItem = lhsIt.next()
assert(lhsItem == nil || !areInIncreasingOrder(lhsItem!, a))
}
while let b = rhsItem {
result.append((nil, b))
rhsItem = rhsIt.next()
assert(rhsItem == nil || !areInIncreasingOrder(rhsItem!, b))
}
return result
}
/// Combine a list of sequences which are already ordered with respect to
/// `compare` into a single ordered sequence with the items from each sequence,
/// in order.
///
/// - Parameters:
/// - sequences: The list of sequences.
/// - areInIncreasingOrder: A predicate that returns true if its first
/// argument should be ordered before its second argument; otherwise,
/// false. Each sequence *MUST* already be in order with respect to this
/// predicate (this is checked, in debug builds).
/// - Returns: A sequence of arrays, ordered with respect to `compare`. Each row
/// in the result will have exactly `sequences.count` entries, and each Nth item
/// either be nil or the equivalently ordered item from the Nth sequence.
@inlinable
public func orderedZip<S: Sequence>(
sequences: [S],
by areInIncreasingOrder: (S.Element, S.Element) -> Bool
) -> [[S.Element?]] {
var result: [[S.Element?]] = []
result.reserveCapacity(sequences.map{ $0.underestimatedCount }.max() ?? 0)
// Initialize iterators.
var iterators = sequences.map{ $0.makeIterator() }
// We keep a "current" item for each iterator.
//
// This strategy is not particularly efficient if we have many many
// sequences with highly varied lengths, but is simple.
var items: [S.Element?] = []
for i in 0 ..< iterators.count {
items.append(iterators[i].next())
}
// Iterate...
while true {
// Find the smallest item.
var maybeSmallest: S.Element?
var smallestIndex: Int = -1
for (i, itemOpt) in items.enumerated() {
if let item = itemOpt, maybeSmallest == nil || areInIncreasingOrder(item, maybeSmallest!) {
maybeSmallest = item
smallestIndex = i
}
}
// If there was no smallest, we have reached the end of all lists.
guard let smallest = maybeSmallest else {
// We are done.
break
}
// Now, we have to take all items equivalent to the smallest. Since we
// have already taken the smallest from each list, we can find this by
// reversing the equivalence (assuming a proper predicate). We do this
// in lieu of tracking equivalence while we iterate through the items.
var row: [S.Element?] = []
for (i, itemOpt) in items.enumerated() {
// If this is the smallest item, or is not greater than the smallest
// (and thus is equal), it should go in the list.
if let item = itemOpt, i == smallestIndex || !areInIncreasingOrder(smallest, item) {
// Walk the item (and validate individual sequence ordering, in debug mode).
let next = iterators[i].next()
assert(next == nil || !areInIncreasingOrder(next!, item))
items[i] = next
row.append(item)
} else {
// Otherwise, no entry for this sequence.
row.append(nil)
}
}
result.append(row)
}
return result
}
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