From 53980774c327675e886179c0a2c140744dcf9b95 Mon Sep 17 00:00:00 2001
From: Jonas Maier <jonas@x77.dev>
Date: Sat, 6 Jun 2026 12:15:52 +0200
Subject: special cased regex for performance

---
 src/regex/simple.rs | 125 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 125 insertions(+)
 create mode 100644 src/regex/simple.rs

(limited to 'src/regex/simple.rs')

diff --git a/src/regex/simple.rs b/src/regex/simple.rs
new file mode 100644
index 0000000..00bc9b4
--- /dev/null
+++ b/src/regex/simple.rs
@@ -0,0 +1,125 @@
+use crate::regex::CharacterClass;
+
+use super::{Match, Pattern, RegexEngine};
+
+fn empty_match() -> Option<Match> {
+    Some(Match {
+        submatches: [].into(),
+    })
+}
+
+pub struct Anything;
+
+#[derive(Debug, Clone)]
+pub struct NotASimpleWildcard;
+
+impl RegexEngine for Anything {
+    type CompileError = NotASimpleWildcard;
+
+    fn compile(pat: Pattern) -> Result<Self, Self::CompileError> {
+        if let Pattern::Rep(pat, 0, None, _) = pat
+            && let Pattern::CharacterClass(CharacterClass::Everything) = *pat
+        {
+            Ok(Anything)
+        } else {
+            Err(NotASimpleWildcard)
+        }
+    }
+
+    fn run(&self, _input: &[u8]) -> Option<Match> {
+        empty_match()
+    }
+}
+
+pub struct Nothing;
+#[derive(Debug, Clone)]
+pub struct NotASimpleNothing;
+
+impl RegexEngine for Nothing {
+    type CompileError = NotASimpleNothing;
+
+    fn compile(pat: Pattern) -> Result<Self, Self::CompileError> {
+        match pat {
+            Pattern::Range(a, b) if a > b => Ok(Nothing),
+            Pattern::CharacterClass(CharacterClass::Nothing) => Ok(Nothing),
+            Pattern::Alt(pats) => {
+                let all_impossible = pats.into_iter().map(Self::compile).all(|p| p.is_ok());
+                if all_impossible {
+                    Ok(Nothing)
+                } else {
+                    Err(NotASimpleNothing)
+                }
+            }
+            Pattern::Concat(pats) => {
+                if let Some(pat) = pats.into_iter().next() {
+                    Self::compile(pat)
+                } else {
+                    Err(NotASimpleNothing)
+                }
+            }
+            Pattern::Rep(_, x, Some(y), _) if y < x => Ok(Nothing),
+            Pattern::Rep(_, 0, None, _) => Err(NotASimpleNothing),
+            Pattern::Rep(pat, _gt_0, _, _) => Self::compile(*pat),
+            Pattern::Submatch(pat) => Self::compile(*pat),
+            _ => Err(NotASimpleNothing),
+        }
+    }
+
+    fn run(&self, _input: &[u8]) -> Option<Match> {
+        None
+    }
+}
+
+pub struct Exact {
+    bytes: Vec<u8>,
+}
+
+const MEM_LIMIT: usize = 25_000;
+
+#[derive(Debug, Clone)]
+pub struct NotSimplyAString;
+
+fn ce(pat: Pattern) -> Option<Vec<u8>> {
+    match pat {
+        Pattern::Byte(b) => Some(vec![b]),
+        Pattern::Concat(patterns) => {
+            let mut pats = patterns.into_iter().map(ce).collect::<Option<Vec<_>>>()?;
+            let mut out = Vec::new();
+            for p in pats.iter_mut() {
+                out.append(p);
+            }
+            Some(out)
+        }
+        Pattern::Rep(pat, min, Some(max), _) if min == max => {
+            if let Some(bytes) = ce(*pat)
+                && bytes.len() * (min as usize) < MEM_LIMIT
+            {
+                Some(bytes.repeat(min as usize))
+            } else {
+                None
+            }
+        }
+        Pattern::Submatch(_) => None, // TODO: submatches could be stored as constant offsets
+        Pattern::Nothing => Some(Vec::new()),
+        _ => None,
+    }
+}
+
+impl RegexEngine for Exact {
+    type CompileError = NotSimplyAString;
+
+    fn compile(pat: Pattern) -> Result<Self, Self::CompileError> {
+        match ce(pat) {
+            Some(bytes) => Ok(Self { bytes }),
+            None => Err(NotSimplyAString),
+        }
+    }
+
+    fn run(&self, input: &[u8]) -> Option<Match> {
+        if input == self.bytes {
+            empty_match()
+        } else {
+            None
+        }
+    }
+}
-- 
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