Coalesce character class members

In grapheme semantic mode, coalesce adjacent
character and scalar members of a custom character
class, over which we can perform grapheme breaking.
This involves potentially re-writing ranges such
that they contain a complete grapheme of adjacent
scalars.

Author: hamishknight

Sources/_StringProcessing/ByteCodeGen.swift

@@ -775,9 +775,127 @@ fileprivate extension Compiler.ByteCodeGen {
     builder.label(exit)
   }
 
+  /// Coalesce any adjacent scalar members in a custom character class together.
+  /// This is required in order to produce correct grapheme matching behavior.
+  func coalescingCustomCharacterClassMembers(
+    _ members: [DSLTree.CustomCharacterClass.Member]
+  ) -> [DSLTree.CustomCharacterClass.Member] {
+    struct Accumulator {
+      /// A series of range operands. For example, in `[ab-cde-fg]`, this will
+      /// contain the strings `["ab", "cde", "fg"]`. From there, the resulting
+      /// ranges will be created.
+      private var rangeOperands: [String] = [""]
+
+      /// The current range operand.
+      private var current: String {
+        _read { yield rangeOperands[rangeOperands.count - 1] }
+        _modify { yield &rangeOperands[rangeOperands.count - 1] }
+      }
+
+      /// Try to accumulate a character class member, returning `true` if
+      /// successful, `false` otherwise.
+      mutating func tryAccumulate(
+        _ member: DSLTree.CustomCharacterClass.Member
+      ) -> Bool {
+        switch member {
+        case .atom(let a):
+          guard let c = a.literalCharacterValue else { return false }
+          current.append(c)
+          return true
+        case .quotedLiteral(let str):
+          current += str
+          return true
+        case let .range(lhs, rhs):
+          guard let lhs = lhs.literalCharacterValue,
+                let rhs = rhs.literalCharacterValue
+          else { return false }
+          current.append(lhs)
+          rangeOperands.append(String(rhs))
+          return true
+        default:
+          return false
+        }
+      }
+
+      func finish() -> [DSLTree.CustomCharacterClass.Member] {
+        if rangeOperands.count == 1 {
+          // If we didn't have any additional range operands, this isn't a
+          // range, we can just form a standard quoted literal.
+          return [.quotedLiteral(current)]
+        }
+        var members = [DSLTree.CustomCharacterClass.Member]()
+
+        // We have other range operands, splice them together. For N operands
+        // we have N - 1 ranges.
+        for (i, lhs) in rangeOperands.dropLast().enumerated() {
+          let rhs = rangeOperands[i + 1]
+
+          // If this is the first operand we only need to drop the last
+          // character for its quoted members, otherwise this is both an LHS
+          // and RHS of a range, and as such needs both sides trimmed.
+          let leading = i == 0 ? lhs.dropLast() : lhs.dropFirst().dropLast()
+          if !leading.isEmpty {
+            members.append(.quotedLiteral(String(leading)))
+          }
+          members.append(.range(.char(lhs.last!), .char(rhs.first!)))
+        }
+        // We've handled everything except the quoted portion of the last
+        // operand, add it now.
+        let trailing = rangeOperands.last!.dropFirst()
+        if !trailing.isEmpty {
+          members.append(.quotedLiteral(String(trailing)))
+        }
+        return members
+      }
+    }
+    return members
+      .map { m -> DSLTree.CustomCharacterClass.Member in
+        // First we need to recursively coalsce any child character classes.
+        switch m {
+        case .custom(let ccc):
+          return .custom(coalescingCustomCharacterClass(ccc))
+        case .intersection(let lhs, let rhs):
+          return .intersection(
+            coalescingCustomCharacterClass(lhs),
+            coalescingCustomCharacterClass(rhs))
+        case .subtraction(let lhs, let rhs):
+          return .subtraction(
+            coalescingCustomCharacterClass(lhs),
+            coalescingCustomCharacterClass(rhs))
+        case .symmetricDifference(let lhs, let rhs):
+          return .symmetricDifference(
+            coalescingCustomCharacterClass(lhs),
+            coalescingCustomCharacterClass(rhs))
+        case .atom, .range, .quotedLiteral, .trivia:
+          return m
+        }
+      }
+      .coalescing(with: Accumulator(), into: { $0.finish() }) { accum, member in
+        accum.tryAccumulate(member)
+      }
+  }
+
+  func coalescingCustomCharacterClass(
+    _ ccc: DSLTree.CustomCharacterClass
+  ) -> DSLTree.CustomCharacterClass {
+    // This only needs to be done in grapheme semantic mode. In scalar semantic
+    // mode, we don't want to coalesce any scalars into a grapheme. This
+    // means that e.g `[e\u{301}-\u{302}]` remains a range between U+301 and
+    // U+302.
+    guard options.semanticLevel == .graphemeCluster else { return ccc }
+
+    let members = coalescingCustomCharacterClassMembers(ccc.members)
+    return .init(members: members, isInverted: ccc.isInverted)
+  }
+
   mutating func emitCustomCharacterClass(
     _ ccc: DSLTree.CustomCharacterClass
   ) throws {
+    // Before emitting a custom character class in grapheme semantic mode, we
+    // need to coalesce together any adjacent characters and scalars, over which
+    // we can perform grapheme breaking. This includes e.g range bounds for
+    // `[e\u{301}-\u{302}]`.
+    let ccc = coalescingCustomCharacterClass(ccc)
     if let asciiBitset = ccc.asAsciiBitset(options),
         optimizationsEnabled {
       if options.semanticLevel == .unicodeScalar {

Sources/_StringProcessing/Utility/Misc.swift

@@ -11,11 +11,11 @@
 
 extension Array {
   /// Coalesce adjacent elements using a given accumulator. The accumulator is
-  /// transformed into an element of the array by `finish`. The `accumulate`
+  /// transformed into elements of the array by `finish`. The `accumulate`
   /// function should return `true` if the accumulator has coalesced the
   /// element, `false` otherwise.
   func coalescing<T>(
-    with initialAccumulator: T, into finish: (T) -> Element,
+    with initialAccumulator: T, into finish: (T) -> Self,
     accumulate: (inout T, Element) -> Bool
   ) -> Self {
     var didAccumulate = false
@@ -32,16 +32,28 @@ extension Array {
       if didAccumulate {
         // We have a leftover accumulator, which needs to be finished before we
         // can append the next element.
-        result.append(finish(accumulator))
+        result += finish(accumulator)
         accumulator = initialAccumulator
         didAccumulate = false
       }
       result.append(elt)
     }
     // Handle a leftover accumulation.
     if didAccumulate {
-      result.append(finish(accumulator))
+      result += finish(accumulator)
     }
     return result
   }
+
+  /// Coalesce adjacent elements using a given accumulator. The accumulator is
+  /// transformed into an element of the array by `finish`. The `accumulate`
+  /// function should return `true` if the accumulator has coalesced the
+  /// element, `false` otherwise.
+  func coalescing<T>(
+    with initialAccumulator: T, into finish: (T) -> Element,
+    accumulate: (inout T, Element) -> Bool
+  ) -> Self {
+    coalescing(
+      with: initialAccumulator, into: { [finish($0) ]}, accumulate: accumulate)
+  }
 }