Don't use input.count in PrefixThrough and PrefixUpTo for speedup. (#354

) Calling Substring.count will calculate the length of the string the first time it is called which for large strings can be slow. Co-authored-by: Stephen Celis <[email protected]>
pointfreeco · Oct 9, 2024 · b23c636 · b23c636
1 parent e1e155c
commit b23c636
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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]
       }