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CombinationsSequence: underestimatedCount == count #220

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Jan 20, 2024
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CombinationsSequence: underestimatedCount == count #220

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d57af2f
CombinationsSequence: underestimatedCount == count
dabrahams Jan 6, 2024
d8c9e58
Remove duplicated test
natecook1000 Jan 17, 2024
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4 changes: 4 additions & 0 deletions Sources/Algorithms/Combinations.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -78,6 +78,10 @@ public struct CombinationsSequence<Base: Collection> {
binomial(n: n, k: $0)
}.reduce(0, +)
}

/// The total number of combinations.
@inlinable
public var underestimatedCount: Int { count }
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Doesn't this conflict with the complexity requirement of Sequence.underestimatedCount?

  /// - Complexity: O(1), except if the sequence also conforms to `Collection`.
  ///   In this case, see the documentation of `Collection.underestimatedCount`.
  var underestimatedCount: Int { get }

The requirement is relaxed for non-random access collections, so not a problem for #219.

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Hum, indeed. What is the complexity of your count? If it's log(N) IMO this is acceptable; complexity requirements are somewhat fungible (e.g. first on a lazy filtered collection is arguably not O(1)). Otherwise, I suppose not.

Oh, but they totally broke the complexity bound of underestimatedCount. It was O(n) before ea49459b7594. What a debacle.

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Actually, that O(1) constraint is meaningless as outlined here. You should ignore it.

}

extension CombinationsSequence: Sequence {
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96 changes: 51 additions & 45 deletions Tests/SwiftAlgorithmsTests/CombinationsTests.swift
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Expand Up @@ -15,57 +15,63 @@ import Algorithms
final class CombinationsTests: XCTestCase {
func testCount() {
let c = "ABCD"

let c0 = c.combinations(ofCount: 0).count
XCTAssertEqual(c0, 1)

let c1 = c.combinations(ofCount: 1).count
XCTAssertEqual(c1, 4)

let c2 = c.combinations(ofCount: 2).count
XCTAssertEqual(c2, 6)

let c3 = c.combinations(ofCount: 3).count
XCTAssertEqual(c3, 4)

let c4 = c.combinations(ofCount: 4).count
XCTAssertEqual(c4, 1)

let c5 = c.combinations(ofCount: 0...0).count
XCTAssertEqual(c5, 1)

let c6 = c.combinations(ofCount: 1...1).count
XCTAssertEqual(c6, 4)

let c7 = c.combinations(ofCount: 1...2).count
XCTAssertEqual(c7, 10)

let c8 = c.combinations(ofCount: 1...3).count
XCTAssertEqual(c8, 14)

let c9 = c.combinations(ofCount: 2...4).count
XCTAssertEqual(c9, 11)


/// XCTAsserts that `x`'s `count` and `underestimatedCount` are both `l` at
/// the given `file` and `line`.
func check(
_ x: CombinationsSequence<String>, countsAre l: Int,
file: StaticString, line: UInt)
{
XCTAssertEqual(x.count, l, "unexpected count", file: file, line: line)
XCTAssertEqual(
x.underestimatedCount, l, "unexpected underestimatedCount",
file: file, line: line)
}

/// XCTAsserts that the `count` and `underestimatedCount` of
/// `c.combinations(ofCount: l)` are both `n` at the given `file` and
/// `line`.
func check(
cHas n: Int,
combinationsOfLength l: Int,
file: StaticString = #filePath, line: UInt = #line)
{
check(c.combinations(ofCount: l), countsAre: n, file: file, line: line)
}

/// XCTAsserts that the `count` and `underestimatedCount` of
/// `c.combinations(ofCount: l)` are both `n` at the given `file` and
/// `line`.
func check<R: RangeExpression>(
cHas n: Int,
combinationsOfLengths l: R,
file: StaticString = #filePath, line: UInt = #line) where R.Bound == Int
{
check(c.combinations(ofCount: l), countsAre: n, file: file, line: line)
}

check(cHas: 1, combinationsOfLength: 0)
check(cHas: 4, combinationsOfLength: 1)
check(cHas: 6, combinationsOfLength: 2)
check(cHas: 1, combinationsOfLength: 4)

check(cHas: 1, combinationsOfLengths: 0...0)
check(cHas: 4, combinationsOfLengths: 1...1)
check(cHas: 10, combinationsOfLengths: 1...2)
check(cHas: 14, combinationsOfLengths: 1...3)
check(cHas: 11, combinationsOfLengths: 2...4)

// `k` greater than element count results in same number of combinations
let c10 = c.combinations(ofCount: 3...10).count
XCTAssertEqual(c10, 5)
check(cHas: 5, combinationsOfLengths: 3...10)

// `k` greater than element count results in same number of combinations
let c11 = c.combinations(ofCount: 4...10).count
XCTAssertEqual(c11, 1)
check(cHas: 1, combinationsOfLengths: 4...10)

// `k` entirely greater than element count results in no combinations
let c12 = c.combinations(ofCount: 5...10).count
XCTAssertEqual(c12, 0)

let c13 = c.combinations(ofCount: 0...).count
XCTAssertEqual(c13, 16)

let c14 = c.combinations(ofCount: ...3).count
XCTAssertEqual(c14, 15)
check(cHas: 0, combinationsOfLengths: 5...10)

let c15 = c.combinations(ofCount: 0...).count
XCTAssertEqual(c15, 16)
check(cHas: 16, combinationsOfLengths: 0...)
check(cHas: 15, combinationsOfLengths: ...3)
}

func testCombinations() {
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