59 lines
2.3 KiB
Swift
59 lines
2.3 KiB
Swift
public extension RandomAccessCollection where Index == Int {
|
|
/// Returns the first index in which an element of the collection satisfies the given predicate.
|
|
/// The collection assumed to be sorted. If collection is not have sorted values the result is undefined.
|
|
///
|
|
/// The idea is to get first index of a function for which the given predicate evaluates to true.
|
|
///
|
|
/// let values = [1,2,3,4,5]
|
|
/// let idx = values.firstIndexAssumingSorted(where: { $0 > 3 })
|
|
///
|
|
/// // false, false, false, true, true
|
|
/// // ^
|
|
/// // therefore idx == 3
|
|
///
|
|
/// - parameter predicate: A closure that takes an element as its argument
|
|
/// and returns a Boolean value that indicates whether the passed element
|
|
/// represents a match.
|
|
///
|
|
/// - returns: The index of the first element for which `predicate` returns `true`.
|
|
/// If no elements in the collection satisfy the given predicate, returns `nil`.
|
|
///
|
|
/// - complexity: O(log(*n*)), where *n* is the length of the collection.
|
|
///
|
|
/// - throws: Rethrows errors thrown by the predicate.
|
|
@inlinable
|
|
func firstIndexAssumingSorted(where predicate: (Self.Element) throws -> Bool) rethrows -> Int? {
|
|
// Predicate should divide a collection to two pairs of values
|
|
// "bad" values for which predicate returns `false``
|
|
// "good" values for which predicate return `true`
|
|
|
|
// false false false false false true true true
|
|
// ^
|
|
// The idea is to get _first_ index which for which the predicate returns `true`
|
|
|
|
let lastIndex = count
|
|
|
|
// The index that represents where bad values start
|
|
var badIndex = -1
|
|
|
|
// The index that represents where good values start
|
|
var goodIndex = lastIndex
|
|
var midIndex = (badIndex + goodIndex) / 2
|
|
|
|
while badIndex + 1 < goodIndex {
|
|
if try predicate(self[midIndex]) {
|
|
goodIndex = midIndex
|
|
} else {
|
|
badIndex = midIndex
|
|
}
|
|
midIndex = (badIndex + goodIndex) / 2
|
|
}
|
|
|
|
// We're out of bounds, no good items in array
|
|
if midIndex == lastIndex {
|
|
return nil
|
|
}
|
|
return goodIndex
|
|
}
|
|
}
|