fix(clob book): show best bid/ask at the top of the orderbook

src/output/clob/books.rs

@@ -1,12 +1,28 @@
 use polymarket_client_sdk_v2::clob::types::response::{
-    LastTradePriceResponse, LastTradesPricesResponse, OrderBookSummaryResponse,
+    LastTradePriceResponse, LastTradesPricesResponse, OrderBookSummaryResponse, OrderSummary,
 };
 use serde_json::json;
 use tabled::settings::Style;
 use tabled::{Table, Tabled};
 
 use crate::output::{DASH, OutputFormat, truncate};
 
+/// Returns bids in display order (best bid first).
+///
+/// The CLOB API returns bids ascending by price, so the best (highest-priced)
+/// bid is reversed to the top for human-facing display.
+fn bids_in_display_order(bids: &[OrderSummary]) -> Vec<&OrderSummary> {
+    bids.iter().rev().collect()
+}
+
+/// Returns asks in display order (best ask first).
+///
+/// The CLOB API returns asks descending by price, so the best (lowest-priced)
+/// ask is reversed to the top for human-facing display.
+fn asks_in_display_order(asks: &[OrderSummary]) -> Vec<&OrderSummary> {
+    asks.iter().rev().collect()
+}
+
 pub fn print_order_book(
     result: &OrderBookSummaryResponse,
     output: &OutputFormat,
@@ -35,9 +51,8 @@ pub fn print_order_book(
                 println!("No bids.");
             } else {
                 println!("Bids:");
-                let rows: Vec<Row> = result
-                    .bids
-                    .iter()
+                let rows: Vec<Row> = bids_in_display_order(&result.bids)
+                    .into_iter()
                     .map(|o| Row {
                         price: o.price.to_string(),
                         size: o.size.to_string(),
@@ -53,9 +68,8 @@ pub fn print_order_book(
                 println!("No asks.");
             } else {
                 println!("Asks:");
-                let rows: Vec<Row> = result
-                    .asks
-                    .iter()
+                let rows: Vec<Row> = asks_in_display_order(&result.asks)
+                    .into_iter()
                     .map(|o| Row {
                         price: o.price.to_string(),
                         size: o.size.to_string(),
@@ -158,3 +172,57 @@ pub fn print_last_trades_prices(
     }
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use super::{OrderSummary, asks_in_display_order, bids_in_display_order};
+    use rust_decimal_macros::dec;
+
+    fn order(price: rust_decimal::Decimal) -> OrderSummary {
+        OrderSummary::builder().price(price).size(dec!(100)).build()
+    }
+
+    // The CLOB API returns bids ascending (worst price first).
+    // The display order helper must reverse that so the best (highest) bid is first.
+    #[test]
+    fn bids_in_display_order_puts_best_bid_first() {
+        let api_order = vec![order(dec!(0.30)), order(dec!(0.40)), order(dec!(0.50))];
+
+        let displayed: Vec<rust_decimal::Decimal> = bids_in_display_order(&api_order)
+            .into_iter()
+            .map(|o| o.price)
+            .collect();
+
+        assert_eq!(
+            displayed,
+            vec![dec!(0.50), dec!(0.40), dec!(0.30)],
+            "bids must be reversed for display so the highest-priced bid is on top"
+        );
+    }
+
+    // The CLOB API returns asks descending (worst price first).
+    // The display order helper must reverse that so the best (lowest) ask is first.
+    #[test]
+    fn asks_in_display_order_puts_best_ask_first() {
+        let api_order = vec![order(dec!(0.70)), order(dec!(0.60)), order(dec!(0.50))];
+
+        let displayed: Vec<rust_decimal::Decimal> = asks_in_display_order(&api_order)
+            .into_iter()
+            .map(|o| o.price)
+            .collect();
+
+        assert_eq!(
+            displayed,
+            vec![dec!(0.50), dec!(0.60), dec!(0.70)],
+            "asks must be reversed for display so the lowest-priced ask is on top"
+        );
+    }
+
+    // Empty input must not panic and must produce an empty display order.
+    #[test]
+    fn empty_inputs_produce_empty_output() {
+        let empty: Vec<OrderSummary> = vec![];
+        assert!(bids_in_display_order(&empty).is_empty());
+        assert!(asks_in_display_order(&empty).is_empty());
+    }
+}