asset slice + testing initiative: fixed assets, property/mutation/fuzz testing, docs

fuzz/fuzz_targets/fuzz_entry_validate.rs

@@ -0,0 +1,51 @@
+// Fuzz target: arbitrary JSON as a journal entry. If validation accepts
+// it, the entry MUST balance exactly — validation is the enforcement
+// point, and this pins it. Staged here; `cargo fuzz init` wires it up.
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+use ledger_core::{AccountType, Currency, JournalEntry, Ledger, Side};
+use rust_decimal::Decimal;
+
+fn test_chart() -> Ledger {
+    let mut ledger = Ledger::new(Currency::USD);
+    for (code, name, typ) in [
+        ("1000", "Cash", AccountType::Asset),
+        ("2000", "Payable", AccountType::Liability),
+        ("3000", "Equity", AccountType::Equity),
+        ("4000", "Revenue", AccountType::Revenue),
+        ("6000", "Rent", AccountType::Expense),
+    ] {
+        ledger.add_account(code, name, typ, None, false).unwrap();
+    }
+    ledger
+}
+
+fuzz_target!(|data: &[u8]| {
+    if let Ok(entry) = serde_json::from_slice::<JournalEntry>(data) {
+        let ledger = test_chart();
+        // Validation must never panic on hostile input.
+        if entry.validate(ledger.chart()).is_ok() {
+            let debits: Decimal = entry
+                .lines()
+                .iter()
+                .filter(|l| l.side == Side::Debit)
+                .map(|l| l.amount.amount)
+                .sum();
+            let credits: Decimal = entry
+                .lines()
+                .iter()
+                .filter(|l| l.side == Side::Credit)
+                .map(|l| l.amount.amount)
+                .sum();
+            assert_eq!(
+                debits, credits,
+                "validate() accepted an unbalanced entry: {entry:?}"
+            );
+            assert!(
+                debits > Decimal::ZERO,
+                "validate() accepted a zero-total entry"
+            );
+        }
+    }
+});