From d61178b418f790e69df7263290e8c30c28f71ae1 Mon Sep 17 00:00:00 2001
From: Nate Cook <natecook@apple.com>
Date: Thu, 9 Jun 2022 16:11:36 -0500
Subject: [PATCH 1/2] Allow CustomConsuming types to match w/ zero width

We previously asserted if a custom consuming type matches with zero
width, but that isn't necessary or good. A custom type can implement
a lookaround assertion or act as a tracer.
---
 .../_StringProcessing/Engine/Processor.swift  |  2 +-
 Tests/RegexBuilderTests/RegexDSLTests.swift   | 36 +++++++++++++++++++
 2 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/Sources/_StringProcessing/Engine/Processor.swift b/Sources/_StringProcessing/Engine/Processor.swift
index 8f777ad33..5865e7a09 100644
--- a/Sources/_StringProcessing/Engine/Processor.swift
+++ b/Sources/_StringProcessing/Engine/Processor.swift
@@ -120,7 +120,7 @@ extension Processor {
   mutating func advance(to nextIndex: Input.Index) {
     assert(nextIndex >= bounds.lowerBound)
     assert(nextIndex <= bounds.upperBound)
-    assert(nextIndex > currentPosition)
+    assert(nextIndex >= currentPosition)
     currentPosition = nextIndex
   }
 
diff --git a/Tests/RegexBuilderTests/RegexDSLTests.swift b/Tests/RegexBuilderTests/RegexDSLTests.swift
index f325b579f..c1aa996f6 100644
--- a/Tests/RegexBuilderTests/RegexDSLTests.swift
+++ b/Tests/RegexBuilderTests/RegexDSLTests.swift
@@ -950,6 +950,42 @@ class RegexDSLTests: XCTestCase {
       XCTAssertEqual(str.wholeMatch(of: parser)?.output, version)
     }
   }
+  
+  func testZeroWidthConsumer() throws {
+    struct Trace: CustomConsumingRegexComponent {
+      typealias RegexOutput = Void
+      var label: String
+      init(_ label: String) { self.label = label }
+
+      static var traceOutput = ""
+
+      func consuming(_ input: String, startingAt index: String.Index, in bounds: Range<String.Index>) throws -> (upperBound: String.Index, output: Void)? {
+        print("Matching '\(label)'", to: &Self.traceOutput)
+        print(input, to: &Self.traceOutput)
+        let dist = input.distance(from: input.startIndex, to: index)
+        print(String(repeating: " ", count: dist) + "^", to: &Self.traceOutput)
+        return (index, ())
+      }
+    }
+    
+    let regex = Regex {
+      OneOrMore(.word)
+      Trace("end of key")
+      ":"
+      Trace("start of value")
+      OneOrMore(.word)
+    }
+    XCTAssertNotNil("hello:goodbye".firstMatch(of: regex))
+    XCTAssertEqual(Trace.traceOutput, """
+      Matching 'end of key'
+      hello:goodbye
+           ^
+      Matching 'start of value'
+      hello:goodbye
+            ^
+      
+      """)
+  }
 }
 
 extension Unicode.Scalar {

From 5a981ec541546822e2baa2eaf643db86f8872ae9 Mon Sep 17 00:00:00 2001
From: Nate Cook <natecook@apple.com>
Date: Thu, 16 Jun 2022 20:17:51 -0500
Subject: [PATCH 2/2] Rename Processor.advance(to:) to resume(at:)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Since the given index doesn’t need to advance, this name is less
misleading.
---
 .../_StringProcessing/Engine/Processor.swift   | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/Sources/_StringProcessing/Engine/Processor.swift b/Sources/_StringProcessing/Engine/Processor.swift
index 85cd27206..d6c1306f0 100644
--- a/Sources/_StringProcessing/Engine/Processor.swift
+++ b/Sources/_StringProcessing/Engine/Processor.swift
@@ -117,11 +117,15 @@ extension Processor {
     return true
   }
 
-  mutating func advance(to nextIndex: Input.Index) {
-    assert(nextIndex >= bounds.lowerBound)
-    assert(nextIndex <= bounds.upperBound)
-    assert(nextIndex >= currentPosition)
-    currentPosition = nextIndex
+  /// Continue matching at the specified index.
+  ///
+  /// - Precondition: `bounds.contains(index) || index == bounds.upperBound`
+  /// - Precondition: `index >= currentPosition`
+  mutating func resume(at index: Input.Index) {
+    assert(index >= bounds.lowerBound)
+    assert(index <= bounds.upperBound)
+    assert(index >= currentPosition)
+    currentPosition = index
   }
 
   func doPrint(_ s: String) {
@@ -358,7 +362,7 @@ extension Processor {
         signalFailure()
         return
       }
-      advance(to: nextIndex)
+      resume(at: nextIndex)
       controller.step()
 
     case .assertBy:
@@ -386,7 +390,7 @@ extension Processor {
           return
         }
         registers[valReg] = val
-        advance(to: nextIdx)
+        resume(at: nextIdx)
         controller.step()
       } catch {
         abort(error)