Implement builtin function str-slice()

src/builtin/string.rs

@@ -1,5 +1,9 @@
 use std::collections::BTreeMap;
 
+use num_bigint::BigInt;
+use num_traits::cast::ToPrimitive;
+use num_traits::sign::Signed;
+
 use super::Builtin;
 use crate::common::QuoteKind;
 use crate::units::Unit;
@@ -37,4 +41,42 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
     decl!(f "unquote", |args, _| {
         Some(arg!(args, 0, "string").eval().unquote())
     });
+    decl!(f "str-slice", |args, _| {
+        let (string, quotes) = match arg!(args, 0, "string").eval() {
+            Value::Ident(s, q) => (s, q),
+            _ => todo!("____ is not a string")
+        };
+        let str_len = string.len();
+        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) => (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) => (BigInt::from(str_len + 1) + n.to_integer()).to_usize().unwrap(),
+            Value::Dimension(..) => todo!("$end: Expected ___ to have no units."),
+            Value::Null => str_len,
+            _ => todo!("$end-at: ____ is not a number")
+        };
+
+        if end > str_len {
+            end = str_len;
+        }
+
+        if start > end || start > str_len {
+            match quotes {
+                QuoteKind::Double | QuoteKind::Single => Some(Value::Ident(String::new(), QuoteKind::Double)),
+                QuoteKind::None => Some(Value::Null),
+            }
+        } else {
+            let s = string[start-1..end].to_string();
+            match quotes {
+                QuoteKind::Double | QuoteKind::Single => Some(Value::Ident(s, QuoteKind::Double)),
+                QuoteKind::None => Some(Value::Ident(s, QuoteKind::None)),
+            }
+        }
+
+    });
 }

tests/strings.rs

@@ -33,3 +33,33 @@ test!(
     "a {\n  color: str-length($string: aBc123);\n}\n",
     "a {\n  color: 6;\n}\n"
 );
+test!(
+    str_slice_dbl_quote,
+    "a {\n  color: str-slice(\"abcd\", 2, 3);\n}\n",
+    "a {\n  color: \"bc\";\n}\n"
+);
+test!(
+    str_slice_sgl_quote,
+    "a {\n  color: str-slice('abcd', 2, 3);\n}\n",
+    "a {\n  color: \"bc\";\n}\n"
+);
+test!(
+    str_slice_no_quote,
+    "a {\n  color: str-slice(abcd, 2, 3);\n}\n",
+    "a {\n  color: bc;\n}\n"
+);
+test!(
+    str_slice_no_end,
+    "a {\n  color: str-slice(abcd, 2);\n}\n",
+    "a {\n  color: bcd;\n}\n"
+);
+test!(
+    str_slice_negative_start_negative_end,
+    "a {\n  color: str-slice(abcd, -3, -2);\n}\n",
+    "a {\n  color: bc;\n}\n"
+);
+test!(
+    str_slice_negative_end,
+    "a {\n  color: str-slice(abcd, 2, -2);\n}\n",
+    "a {\n  color: bc;\n}\n"
+);