implement builtin function nth

src/builtin/list.rs

@@ -1,5 +1,7 @@
 use std::collections::HashMap;
 
+use num_traits::cast::ToPrimitive;
+
 use super::Builtin;
 use crate::unit::Unit;
 use crate::value::{Number, Value};
@@ -16,4 +18,41 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
             Ok(Value::Dimension(len, Unit::None))
         }),
     );
+    f.insert(
+        "nth".to_owned(),
+        Box::new(|args, _| {
+            max_args!(args, 2);
+            let list = match arg!(args, 0, "list") {
+                Value::List(v, _) => v,
+                _ => return Err("Missing argument $list.".into()),
+            };
+            let n = match arg!(args, 1, "n") {
+                Value::Dimension(num, _) => num,
+                v => return Err(format!("$n: {} is not a number.", v).into()),
+            };
+
+            if n == Number::from(0) {
+                return Err("$n: List index may not be 0.".into());
+            }
+
+            if n.abs() > Number::from(list.len()) {
+                return Err(format!(
+                    "$n: Invalid index {} for a list with {} elements.",
+                    n,
+                    list.len()
+                )
+                .into());
+            }
+
+            if n.is_decimal() {
+                return Err(format!("$n: {} is not an int.", n).into());
+            }
+
+            if n > Number::from(0) {
+                Ok(list[n.to_integer().to_usize().unwrap() - 1].clone())
+            } else {
+                Ok(list[list.len() - n.abs().to_integer().to_usize().unwrap()].clone())
+            }
+        }),
+    );
 }

src/value/number.rs

@@ -28,25 +28,25 @@ impl Number {
         Number::new(BigRational::new(a.into(), b.into()))
     }
 
-    pub fn round(self) -> Self {
+    pub fn round(&self) -> Self {
         Number {
             val: self.val.round(),
         }
     }
 
-    pub fn ceil(self) -> Self {
+    pub fn ceil(&self) -> Self {
         Number {
             val: self.val.ceil(),
         }
     }
 
-    pub fn floor(self) -> Self {
+    pub fn floor(&self) -> Self {
         Number {
             val: self.val.floor(),
         }
     }
 
-    pub fn abs(self) -> Self {
+    pub fn abs(&self) -> Self {
         Number {
             val: self.val.abs(),
         }

tests/list.rs

@@ -18,3 +18,18 @@ test!(
     "a {\n  color: length((1, 2, 3, 4, 5));\n}\n",
     "a {\n  color: 5;\n}\n"
 );
+test!(
+    nth_space_separated,
+    "a {\n  color: nth(a b c, 1);\n}\n",
+    "a {\n  color: a;\n}\n"
+);
+test!(
+    nth_negative_index,
+    "a {\n  color: nth(a b c, -2);\n}\n",
+    "a {\n  color: b;\n}\n"
+);
+test!(
+    nth_comma_separated,
+    "a {\n  color: nth((a, b, c), 3);\n}\n",
+    "a {\n  color: c;\n}\n"
+);