From 82ad57901bb43c6beb349ba9be15b2d464b8cb46 Mon Sep 17 00:00:00 2001
From: Ziyang Hu <hu.ziyang@cantab.net>
Date: Fri, 2 Sep 2022 20:05:15 +0800
Subject: [PATCH] reverse

---
 docs/functions.md           |  2 ++
 src/data/expr.rs            |  1 +
 src/data/functions.rs       | 10 ++++++++++
 src/data/tests/functions.rs | 17 ++++++++++++++++-
 4 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/docs/functions.md b/docs/functions.md
index ad85ea28..af8cc58d 100644
--- a/docs/functions.md
+++ b/docs/functions.md
@@ -124,6 +124,8 @@ The four _basic arithmetic operators_ `+`, `-`, `*`, and `/` do what you expect,
 
 `prepend(l, x)`, `append(l, x)`: prepends / appends the element `x` to the list `l`.
 
+`reverse(l)` reverses the list.
+
 `sorted(l)`: returns the sorted list as defined by the total order detailed in [datatypes](datatypes.md).
 
 `chunks(l, n)`: splits the list `l` into chunks of `n`, e.g. `chunks([1, 2, 3, 4, 5], 2) == [[1, 2], [3, 4], [5]]`.
diff --git a/src/data/expr.rs b/src/data/expr.rs
index e7dc6daf..a3095a6b 100644
--- a/src/data/expr.rs
+++ b/src/data/expr.rs
@@ -431,6 +431,7 @@ pub(crate) fn get_op(name: &str) -> Option<&'static Op> {
         "is_nan" => &OP_IS_NAN,
         "length" => &OP_LENGTH,
         "sorted" => &OP_SORTED,
+        "reverse" => &OP_REVERSE,
         "append" => &OP_APPEND,
         "prepend" => &OP_PREPEND,
         "unicode_normalize" => &OP_UNICODE_NORMALIZE,
diff --git a/src/data/functions.rs b/src/data/functions.rs
index c02849a3..768a64bc 100644
--- a/src/data/functions.rs
+++ b/src/data/functions.rs
@@ -978,6 +978,16 @@ pub(crate) fn op_sorted(args: &[DataValue]) -> Result<DataValue> {
     Ok(DataValue::List(arg))
 }
 
+define_op!(OP_REVERSE, 1, false);
+pub(crate) fn op_reverse(args: &[DataValue]) -> Result<DataValue> {
+    let mut arg = args[0]
+        .get_list()
+        .ok_or_else(|| anyhow!("cannot apply 'reverse' to {:?}", args))?
+        .to_vec();
+    arg.reverse();
+    Ok(DataValue::List(arg))
+}
+
 define_op!(OP_HAVERSINE, 4, false);
 pub(crate) fn op_haversine(args: &[DataValue]) -> Result<DataValue> {
     let gen_err = || anyhow!("cannot computer haversine distance for {:?}", args);
diff --git a/src/data/tests/functions.rs b/src/data/tests/functions.rs
index a60bb153..d75b38fd 100644
--- a/src/data/tests/functions.rs
+++ b/src/data/tests/functions.rs
@@ -932,7 +932,7 @@ fn test_unicode_normalize() {
 }
 
 #[test]
-fn test_sort() {
+fn test_sort_reverse() {
     assert_eq!(
         op_sorted(&[DataValue::List(vec![
             DataValue::from(2.0),
@@ -947,6 +947,21 @@ fn test_sort() {
             DataValue::from(2),
             DataValue::from(2.0),
         ])
+    );
+    assert_eq!(
+        op_reverse(&[DataValue::List(vec![
+            DataValue::from(2.0),
+            DataValue::from(1),
+            DataValue::from(2),
+            DataValue::Null
+        ])])
+        .unwrap(),
+        DataValue::List(vec![
+            DataValue::Null,
+            DataValue::from(2),
+            DataValue::from(1),
+            DataValue::from(2.0),
+        ])
     )
 }