Add round-trip fuzz test

Makefile

@@ -18,5 +18,8 @@ clippy: ## run clippy
 journeytests: ## run journey tests
 	./tests/cat.sh
 
+fuzz: ## run fuzz tests for 30 seconds
+	cargo install cargo-fuzz
+	cargo fuzz run round-trip -- -only_ascii=1 -max_total_time=30
 
-tests: docs clippy unittests journeytests  ## run all tests
+tests: docs clippy unittests journeytests fuzz  ## run all tests

fuzz/.gitignore

@@ -0,0 +1,4 @@
+target
+corpus
+artifacts
+coverage

fuzz/Cargo.toml

@@ -0,0 +1,28 @@
+[package]
+name = "pulldown-cmark-to-cmark-fuzz"
+version = "0.0.0"
+publish = false
+edition = "2021"
+
+[package.metadata]
+cargo-fuzz = true
+
+[dependencies]
+libfuzzer-sys = "0.4"
+pulldown-cmark = { version = "0.9.0", default-features = false }
+
+[dependencies.pulldown-cmark-to-cmark]
+path = ".."
+
+# Prevent this from interfering with workspaces
+[workspace]
+members = ["."]
+
+[profile.release]
+debug = 1
+
+[[bin]]
+name = "round-trip"
+path = "fuzz_targets/round-trip.rs"
+test = false
+doc = false

fuzz/fuzz_targets/round-trip.rs

@@ -0,0 +1,42 @@
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+use pulldown_cmark::Parser;
+use pulldown_cmark_to_cmark::cmark;
+
+fn round_trip(text: &str) -> String {
+    let mut result = String::with_capacity(text.len());
+    cmark(Parser::new(&text), &mut result).unwrap();
+    result
+}
+
+fn print_events(text: &str) {
+    eprintln!("{text:?} -> [");
+    for event in Parser::new(&text) {
+        eprintln!("  {event:?}");
+    }
+    eprintln!("]");
+}
+
+fuzz_target!(|text: String| {
+    let round_trip_1 = round_trip(&text);
+    let round_trip_2 = round_trip(&round_trip_1);
+    let round_trip_3 = round_trip(&round_trip_2);
+    let round_trip_4 = round_trip(&round_trip_3);
+    if round_trip_3 != round_trip_4 {
+        print_events(&text);
+        print_events(&round_trip_1);
+        print_events(&round_trip_2);
+        print_events(&round_trip_3);
+        print_events(&round_trip_4);
+
+        panic!(
+            "round-trip failed:\n\
+             -- {text:?}\n\
+             -> {round_trip_1:?}\n\
+             -> {round_trip_2:?}\n\
+             -> {round_trip_3:?}\n\
+             -> {round_trip_4:?}"
+        );
+    }
+});