Internally add Collection.endOfPrefix(while:) and BidirectionalCollection.startOfSuffix(while:)

Sources/Algorithms/Chunked.swift

@@ -76,8 +76,7 @@ extension LazyChunked: LazyCollectionProtocol {
   internal func endOfChunk(startingAt start: Base.Index) -> Base.Index {
     let subject = projection(base[start])
     return base[base.index(after: start)...]
-      .firstIndex(where: { !belongInSameGroup(subject, projection($0)) })
-      ?? base.endIndex
+      .endOfPrefix(while: { belongInSameGroup(subject, projection($0)) })
   }
 
   @inlinable
@@ -119,20 +118,8 @@ extension LazyChunked: BidirectionalCollection
 
     // Get the projected value of the last element in the range ending at `end`.
     let subject = projection(base[indexBeforeEnd])
-
-    // Search backward from `end` for the first element whose projection isn't
-    // equal to `subject`.
-    if let firstMismatch = base[..<indexBeforeEnd]
-      .lastIndex(where: { !belongInSameGroup(projection($0), subject) })
-    {
-      // If we found one, that's the last element of the _next_ previous chunk,
-      // and therefore one position _before_ the start of this chunk.
-      return base.index(after: firstMismatch)
-    } else {
-      // If we didn't find such an element, this chunk extends back to the start
-      // of the collection.
-      return base.startIndex
-    }
+    return base[..<indexBeforeEnd]
+      .startOfSuffix(while: { belongInSameGroup(projection($0), subject) })
   }
 
   @inlinable

Sources/Algorithms/Suffix.swift

@@ -27,13 +27,62 @@ extension BidirectionalCollection {
   public func suffix(
     while predicate: (Element) throws -> Bool
   ) rethrows -> SubSequence {
-    let start = startIndex
-    var result = endIndex
-    while result != start {
-      let previous = index(before: result)
-      guard try predicate(self[previous]) else { break }
-      result = previous
+    try self[startOfSuffix(while: predicate)...]
+  }
+}
+
+//===----------------------------------------------------------------------===//
+// endOfPrefix(while:)
+//===----------------------------------------------------------------------===//
+
+extension Collection {
+  /// Returns the exclusive upper bound of the prefix of elements that satisfy
+  /// the predicate.
+  ///
+  /// - Parameter predicate: A closure that takes an element of the collection
+  ///   as its argument and returns `true` if the element is part of the prefix
+  ///   or `false` if it is not. Once the predicate returns `false` it will not
+  ///   be called again.
+  ///
+  /// - Complexity: O(*n*), where *n* is the length of the collection.
+  @usableFromInline
+  internal func endOfPrefix(
+    while predicate: (Element) throws -> Bool
+  ) rethrows -> Index {
+    var index = startIndex
+    while try index != endIndex && predicate(self[index]) {
+      formIndex(after: &index)
+    }
+    return index
+  }
+}
+
+//===----------------------------------------------------------------------===//
+// startOfSuffix(while:)
+//===----------------------------------------------------------------------===//
+
+extension BidirectionalCollection {
+  /// Returns the inclusive lower bound of the suffix of elements that satisfy
+  /// the predicate.
+  ///
+  /// - Parameter predicate: A closure that takes an element of the collection
+  ///   as its argument and returns `true` if the element is part of the suffix
+  ///   or `false` if it is not. Once the predicate returns `false` it will not
+  ///   be called again.
+  ///
+  /// - Complexity: O(*n*), where *n* is the length of the collection.
+  @usableFromInline
+  internal func startOfSuffix(
+    while predicate: (Element) throws -> Bool
+  ) rethrows -> Index {
+    var index = endIndex
+    while index != startIndex {
+      let after = index
+      formIndex(before: &index)
+      if try !predicate(self[index]) {
+        return after
+      }
     }
-    return self[result...]
+    return index
   }
 }

Sources/Algorithms/Trim.swift

@@ -29,20 +29,8 @@ extension BidirectionalCollection {
   public func trimming(
     while predicate: (Element) throws -> Bool
   ) rethrows -> SubSequence {
-    // Consume elements from the front.
-    let sliceStart = try firstIndex { try predicate($0) == false } ?? endIndex
-    // sliceEnd is the index _after_ the last index to match the predicate.
-    var sliceEnd = endIndex
-
-    while sliceStart != sliceEnd {
-      let idxBeforeSliceEnd = index(before: sliceEnd)
-      guard try predicate(self[idxBeforeSliceEnd]) else {
-        return self[sliceStart..<sliceEnd]
-      }
-      sliceEnd = idxBeforeSliceEnd
-    }
-
-    // Trimmed everything.
-    return self[Range(uncheckedBounds: (sliceStart, sliceStart))]
+    let start = try endOfPrefix(while: predicate)
+    let end = try self[start...].startOfSuffix(while: predicate)
+    return self[start..<end]
   }
 }

Tests/SwiftAlgorithmsTests/SuffixTests.swift

@@ -10,7 +10,7 @@
 //===----------------------------------------------------------------------===//
 
 import XCTest
-import Algorithms
+@testable import Algorithms
 
 final class SuffixTests: XCTestCase {
   func testSuffix() {
@@ -23,4 +23,28 @@ final class SuffixTests: XCTestCase {
     let empty: [Int] = []
     XCTAssertEqualSequences(empty.suffix(while: { $0 > 10 }), [])
   }
+
+  func testEndOfPrefix() {
+    let array = Array(0..<10)
+    XCTAssertEqual(array.endOfPrefix(while: { $0 < 3 }), 3)
+    XCTAssertEqual(array.endOfPrefix(while: { _ in false }), array.startIndex)
+    XCTAssertEqual(array.endOfPrefix(while: { _ in true }), array.endIndex)
+
+    let empty = [Int]()
+    XCTAssertEqual(empty.endOfPrefix(while: { $0 < 3 }), 0)
+    XCTAssertEqual(empty.endOfPrefix(while: { _ in false }), empty.startIndex)
+    XCTAssertEqual(empty.endOfPrefix(while: { _ in true }), empty.endIndex)
+  }
+
+  func testStartOfSuffix() {
+    let array = Array(0..<10)
+    XCTAssertEqual(array.startOfSuffix(while: { $0 >= 3 }), 3)
+    XCTAssertEqual(array.startOfSuffix(while: { _ in false }), array.endIndex)
+    XCTAssertEqual(array.startOfSuffix(while: { _ in true }), array.startIndex)
+
+    let empty = [Int]()
+    XCTAssertEqual(empty.startOfSuffix(while: { $0 < 3 }), 0)
+    XCTAssertEqual(empty.startOfSuffix(while: { _ in false }), empty.endIndex)
+    XCTAssertEqual(empty.startOfSuffix(while: { _ in true }), empty.startIndex)
+  }
 }