Handle negative numbers in str-slice()

src/builtin/string.rs

@@ -50,12 +50,14 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
         let start = match arg!(args, 1, "start-at").eval() {
             Value::Dimension(n, Unit::None) if n.to_integer().is_positive() => n.to_integer().to_usize().unwrap(),
             Value::Dimension(n, Unit::None) if n == Number::from(0) => 1_usize,
+            Value::Dimension(n, Unit::None) if n < -Number::from(str_len) => 1_usize,
             Value::Dimension(n, Unit::None) => (BigInt::from(str_len + 1) + n.to_integer()).to_usize().unwrap(),
             Value::Dimension(..) => todo!("$start: Expected ___ to have no units."),
             _ => todo!("$start-at: ____ is not a number")
         };
         let mut end = match arg!(args, 2, "end-at"=Value::Null).eval() {
             Value::Dimension(n, Unit::None) if n.to_integer().is_positive() => n.to_integer().to_usize().unwrap(),
+            Value::Dimension(n, Unit::None) if n < -Number::from(str_len) => 0_usize,
             Value::Dimension(n, Unit::None) => (BigInt::from(str_len + 1) + n.to_integer()).to_usize().unwrap(),
             Value::Dimension(..) => todo!("$end: Expected ___ to have no units."),
             Value::Null => str_len,
@@ -78,6 +80,5 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
                 QuoteKind::None => Some(Value::Ident(s, QuoteKind::None)),
             }
         }
-
     });
 }

src/value/number.rs

@@ -1,6 +1,6 @@
 use std::convert::From;
 use std::fmt::{self, Display, Write};
-use std::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Rem, RemAssign, Sub, SubAssign};
+use std::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Rem, RemAssign, Sub, SubAssign, Neg};
 
 use num_bigint::BigInt;
 use num_rational::BigRational;
@@ -196,3 +196,13 @@ impl RemAssign for Number {
         self.val %= other.val
     }
 }
+
+impl Neg for Number {
+    type Output = Self;
+
+    fn neg(self) -> Self {
+        Number {
+            val: -self.val,
+        }
+    }
+}

tests/strings.rs

@@ -68,3 +68,13 @@ test!(
     "a {\n  color: str-slice(cde, 0);\n}\n",
     "a {\n  color: cde;\n}\n"
 );
+test!(
+    str_slice_start_below_negative_str_len,
+    "a {\n  color: str-slice(cde, -100);\n}\n",
+    "a {\n  color: cde;\n}\n"
+);
+test!(
+    str_slice_end_below_negative_str_len,
+    "a {\n  color: str-slice(\"cde\", 0, -100);\n}\n",
+    "a {\n  color: \"\";\n}\n"
+);