begin work on differential fuzzers

fuzz/Cargo.toml

@@ -67,6 +67,10 @@ path = "fuzz_targets/ast_fuzz_regex.rs"
 name = "ast_fuzz_match_bytes"
 path = "fuzz_targets/ast_fuzz_match_bytes.rs"
 
+[[bin]]
+name = "ast_diff_default"
+path = "fuzz_targets/ast_diff_default.rs"
+
 [profile.release]
 opt-level = 3
 debug = true

fuzz/fuzz_targets/ast_diff_default.rs

@@ -0,0 +1,65 @@
+#![no_main]
+
+use {
+    libfuzzer_sys::{fuzz_target, Corpus},
+    regex::RegexBuilder,
+    regex_automata::nfa::thompson::pikevm::PikeVM as NfaRegex,
+    regex_syntax::ast::Ast,
+};
+
+#[derive(Eq, PartialEq, arbitrary::Arbitrary)]
+struct FuzzData {
+    ast: Ast,
+    haystack: String,
+}
+
+impl std::fmt::Debug for FuzzData {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let mut builder = f.debug_struct("FuzzData");
+        builder.field("ast", &format!("{}", self.ast));
+        builder.field("haystack", &self.haystack);
+        builder.finish()
+    }
+}
+
+fuzz_target!(|data: FuzzData| -> Corpus {
+    let _ = env_logger::try_init();
+
+    let pattern = format!("{}", data.ast);
+    let Ok(re) = RegexBuilder::new(&pattern).size_limit(1<<20).build() else {
+        return Corpus::Reject;
+    };
+    let Ok(baseline) = NfaRegex::new(&pattern) else {
+        return Corpus::Reject; // should we error here?
+    };
+    let mut cache = baseline.create_cache();
+
+    assert_eq!(
+        re.is_match(&data.haystack),
+        baseline.is_match(&mut cache, &data.haystack)
+    );
+    let found1 = re.find(&data.haystack);
+    let found2 = baseline.find(&mut cache, &data.haystack);
+    if let Some(found1) = found1 {
+        let found2 = found2.expect("Found in target, but not in baseline!");
+        assert_eq!(found1.start(), found2.start());
+        assert_eq!(found1.end(), found2.end());
+    }
+    if let Some(captures) = re.captures(&data.haystack) {
+        let mut baseline_captures = baseline.create_captures();
+
+        baseline.captures(&mut cache, &data.haystack, &mut baseline_captures);
+        drop(cache);
+        assert_eq!(captures.len(), baseline_captures.group_len());
+        for (c1, c2) in captures.iter().zip(baseline_captures.iter()) {
+            if let Some(c1) = c1 {
+                let c2 = c2.expect("Matched in target, but not baseline!");
+                assert_eq!(c1.start(), c2.start);
+                assert_eq!(c1.end(), c2.end);
+            } else {
+                assert!(!c2.is_some(), "Matched in baseline, but not target!");
+            }
+        }
+    }
+    Corpus::Keep
+});