From a614600a87f5f2b38af13a72f435f655d6eae7b0 Mon Sep 17 00:00:00 2001
From: Stephen Waits <1+swaits@noreply.git.swaits.com>
Date: Sun, 11 Oct 2026 00:46:13 +0000
Subject: [PATCH] Initial commit: wanderbooks accounting semantics engine

---
 crates/ledger-core/src/error.rs | 78 +++++++++++++++++++++++++++++++++
 1 file changed, 78 insertions(+)
 create mode 100644 crates/ledger-core/src/error.rs

diff --git a/crates/ledger-core/src/error.rs b/crates/ledger-core/src/error.rs
new file mode 100644
index 0000000..cd7ef9d
--- /dev/null
+++ b/crates/ledger-core/src/error.rs
@@ -0,0 +1,78 @@
+//! Error type for the ledger engine.
+
+use rust_decimal::Decimal;
+use thiserror::Error;
+
+/// Every way a ledger command can fail.
+///
+/// Variants are part of the engine's contract: agents adding features add
+/// variants here rather than inventing ad-hoc error reporting.
+#[derive(Debug, Error, PartialEq, Eq)]
+pub enum LedgerError {
+    /// A journal entry must have at least two lines (one debit, one credit).
+    #[error("journal entry has fewer than two lines")]
+    TooFewLines,
+
+    /// Debits and credits must be exactly equal. No epsilon, ever.
+    #[error("journal entry does not balance: debits {debits} != credits {credits}")]
+    UnbalancedEntry {
+        /// Sum of debit lines.
+        debits: Decimal,
+        /// Sum of credit lines.
+        credits: Decimal,
+    },
+
+    /// Zero-amount lines are noise; they are rejected, not ignored.
+    #[error("journal line has a zero amount")]
+    ZeroAmountLine,
+
+    /// The referenced account does not exist in the chart of accounts.
+    #[error("unknown account id {0}")]
+    UnknownAccount(u64),
+
+    /// Placeholder (header) accounts organize the chart; they take no postings.
+    #[error("account '{0}' is a placeholder and cannot take postings")]
+    PlaceholderAccount(String),
+
+    /// All lines of an entry (and their accounts) must share one currency.
+    #[error("currency mismatch: expected {expected}, found {found}")]
+    CurrencyMismatch {
+        /// The currency that was expected.
+        expected: String,
+        /// The currency that was found.
+        found: String,
+    },
+
+    /// Currency codes are three ASCII letters (ISO 4217 alpha-3).
+    #[error("invalid currency code '{0}': must be three ASCII letters")]
+    InvalidCurrencyCode(String),
+
+    /// Posted entries are immutable. This is load-bearing: it is what makes
+    /// the ledger an audit trail instead of a spreadsheet.
+    #[error("entry {0} is already posted and immutable")]
+    EntryAlreadyPosted(u64),
+
+    /// Only posted entries can be reversed.
+    #[error("entry {0} is not posted yet")]
+    EntryNotPosted(u64),
+
+    /// The referenced journal entry does not exist.
+    #[error("entry {0} does not exist")]
+    UnknownEntry(u64),
+
+    /// Account codes must be unique within a chart of accounts.
+    #[error("account code '{0}' is already in use")]
+    DuplicateAccountCode(String),
+
+    /// Account codes must not be empty.
+    #[error("account code must not be empty")]
+    EmptyAccountCode,
+
+    /// The referenced parent account does not exist.
+    #[error("unknown parent account id {0}")]
+    UnknownParentAccount(u64),
+
+    /// Any attempt to mutate a posted entry: add lines, delete it, edit it.
+    #[error("cannot modify a posted entry; post a reversing entry instead")]
+    PostedEntryImmutable,
+}