Don't use input.count in PrefixThrough and PrefixUpTo for speedup.

Calling Substring.count will calculate the length of the string the first time it is called which for large strings can be slow.
pointfreeco · Oct 9, 2024 · d3fa32f · d3fa32f
1 parent 19457ac
commit d3fa32f
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diff --git a/Sources/Parsing/ParserPrinters/PrefixThrough.swift b/Sources/Parsing/ParserPrinters/PrefixThrough.swift
@@ -32,12 +32,11 @@ public struct PrefixThrough<Input: Collection>: Parser where Input.SubSequence =
     let original = input
     while let index = input.firstIndex(where: { self.areEquivalent(first, $0) }) {
       input = input[index...]
-      if input.count >= count,
-        zip(input[index...], self.possibleMatch).allSatisfy(self.areEquivalent)
+      if let matchEndIndex = input.index(index, offsetBy: count, limitedBy: input.endIndex),
+        zip(input[..<matchEndIndex], self.possibleMatch).allSatisfy(self.areEquivalent)
       {
-        let index = input.index(index, offsetBy: count)
-        input = input[index...]
-        return original[..<index]
+        input = input[matchEndIndex...]
+        return original[..<matchEndIndex]
       }
       input.removeFirst()
     }

diff --git a/Sources/Parsing/ParserPrinters/PrefixUpTo.swift b/Sources/Parsing/ParserPrinters/PrefixUpTo.swift
@@ -32,8 +32,8 @@ public struct PrefixUpTo<Input: Collection>: Parser where Input.SubSequence == I
     let original = input
     while let index = input.firstIndex(where: { self.areEquivalent(first, $0) }) {
       input = input[index...]
-      if input.count >= count,
-        zip(input[index...], self.possibleMatch).allSatisfy(self.areEquivalent)
+      if let matchEndIndex = input.index(index, offsetBy: count, limitedBy: input.endIndex),
+        zip(input[..<matchEndIndex], self.possibleMatch).allSatisfy(self.areEquivalent)
       {
         return original[..<index]
       }