From b23c636818b579943a7c27b294866b75b36dc0ed Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B8rn=20Olav=20Ruud?=
 <BjornRuud@users.noreply.github.com>
Date: Wed, 9 Oct 2024 20:19:45 +0200
Subject: [PATCH] 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 <stephen@stephencelis.com>
---
 Sources/Parsing/ParserPrinters/PrefixThrough.swift | 9 ++++-----
 Sources/Parsing/ParserPrinters/PrefixUpTo.swift    | 4 ++--
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/Sources/Parsing/ParserPrinters/PrefixThrough.swift b/Sources/Parsing/ParserPrinters/PrefixThrough.swift
index 5c08341b5a..d7db24f823 100644
--- 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
index 6f032417ce..899b459013 100644
--- 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]
       }