initial commit

Signed-off-by: Kamal Tufekcic <kamal@lo.sh>

fuzz/fuzz_targets/roundtrip_arbitrary.rs

@@ -0,0 +1,56 @@
+//! Fuzzer for encoder/decoder self-consistency.
+//!
+//! Interprets the fuzzer's byte input as a sequence of 24-bit signed PCM
+//! samples, clamped to the LAC input range, encodes the frame, and
+//! verifies the decoded output matches the original samples byte-for-byte.
+//!
+//! Catches any encoder bug that produces a bitstream the decoder can't
+//! reconstruct exactly — which is the whole point of a lossless codec.
+//!
+//! This complements `decode_arbitrary`:
+//! - `decode_arbitrary` asserts the decoder is robust to arbitrary input.
+//! - `roundtrip_arbitrary` asserts encoder and decoder agree on meaning.
+
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+
+/// LAC's documented input range (spec §1): `|sample| ≤ 2^23 − 1`.
+/// Fuzzer bytes are interpreted as i32 then clamped to this symmetric
+/// range — note the lower bound is `-(2^23 − 1)`, NOT the two's-complement
+/// `-2^23`; the encoder's debug-asserted input contract excludes the
+/// extra negative value.
+const SAMPLE_MIN: i32 = -((1 << 23) - 1);
+const SAMPLE_MAX: i32 = (1 << 23) - 1;
+
+fuzz_target!(|data: &[u8]| {
+    // Need at least one sample (4 bytes interpreted as i32) and a
+    // reasonable upper bound to keep each fuzz iteration fast. 16k samples
+    // at 48 kHz is ~340 ms of audio, more than enough to exercise every
+    // encoder path including the full MAX_PARTITION_ORDER search.
+    if data.len() < 4 || data.len() > 65_536 {
+        return;
+    }
+
+    // Read i32 chunks little-endian, clamp into LAC's 24-bit range.
+    let samples: Vec<i32> = data
+        .chunks_exact(4)
+        .map(|c| {
+            let raw = i32::from_le_bytes([c[0], c[1], c[2], c[3]]);
+            raw.clamp(SAMPLE_MIN, SAMPLE_MAX)
+        })
+        .collect();
+    if samples.is_empty() {
+        return;
+    }
+
+    let encoded = lac::encode_frame(&samples);
+    let decoded = lac::decode_frame(&encoded)
+        .expect("decoder rejected its own encoder's output");
+    assert_eq!(
+        decoded, samples,
+        "round-trip mismatch: {} samples, encoded {} bytes",
+        samples.len(),
+        encoded.len()
+    );
+});