Fix slicing last n elements using count (#6838)

crates/typst-library/src/foundations/array.rs

@@ -286,11 +286,8 @@ impl Array {
         #[named]
         count: Option<i64>,
     ) -> StrResult<Array> {
-        let mut end = end;
-        if end.is_none() {
-            end = count.map(|c: i64| start + c);
-        }
         let start = self.locate(start, true)?;
+        let end = end.or(count.map(|c| start as i64 + c));
         let end = self.locate(end.unwrap_or(self.len() as i64), true)?.max(start);
         Ok(self.0[start..end].into())
     }

crates/typst-library/src/foundations/bytes.rs

@@ -197,12 +197,8 @@ impl Bytes {
         #[named]
         count: Option<i64>,
     ) -> StrResult<Bytes> {
-        let mut end = end;
-        if end.is_none() {
-            end = count.map(|c: i64| start + c);
-        }
-
         let start = self.locate(start)?;
+        let end = end.or(count.map(|c| start as i64 + c));
         let end = self.locate(end.unwrap_or(self.len() as i64))?.max(start);
         let slice = &self.as_slice()[start..end];
 

crates/typst-library/src/foundations/str.rs

@@ -254,9 +254,9 @@ impl Str {
         #[named]
         count: Option<i64>,
     ) -> StrResult<Str> {
-        let end = end.or(count.map(|c| start + c)).unwrap_or(self.len() as i64);
         let start = self.locate(start)?;
-        let end = self.locate(end)?.max(start);
+        let end = end.or(count.map(|c| start as i64 + c));
+        let end = self.locate(end.unwrap_or(self.len() as i64))?.max(start);
         Ok(self.0[start..end].into())
     }
 

tests/suite/foundations/array.typ

@@ -223,6 +223,8 @@
 #test(range(10).slice(2, 6), (2, 3, 4, 5))
 #test(range(10).slice(4, count: 3), (4, 5, 6))
 #test(range(10).slice(-5, count: 2), (5, 6))
+#test((1, 2, 3).slice(-3, count: 3), (1, 2, 3))
+#test((1, 2, 3).slice(-1, count: 1), (3,))
 #test((1, 2, 3).slice(2, -2), ())
 #test((1, 2, 3).slice(-2, 2), (2,))
 #test((1, 2, 3).slice(-3, 2), (1, 2))
@@ -232,6 +234,10 @@
 // Error: 2-30 array index out of bounds (index: 12, len: 10)
 #range(10).slice(9, count: 3)
 
+--- array-slice-out-of-bounds-from-back ---
+// Error: 2-31 array index out of bounds (index: 12, len: 10)
+#range(10).slice(-2, count: 4)
+
 --- array-slice-out-of-bounds-negative ---
 // Error: 2-24 array index out of bounds (index: -4, len: 3)
 #(1, 2, 3).slice(0, -4)

tests/suite/foundations/bytes.typ

@@ -29,3 +29,18 @@
 --- bytes-bad-conversion-from-dict ---
 // Error: 8-14 expected string, array, or bytes, found dictionary
 #bytes((a: 1))
+
+--- bytes-slice ---
+// Test the `slice` method.
+#test(bytes("abcd").slice(2), bytes("cd"))
+#test(bytes("abcd").slice(0, 3), bytes("abc"))
+#test(bytes("abcd").slice(1, -1), bytes("bc"))
+#test(bytes("abcd").slice(3, 3), bytes(""))
+#test(bytes("abcd").slice(3, 0), bytes(""))
+#test(bytes("abcd").slice(-2), bytes("cd"))
+#test(bytes("abcd").slice(-3, 2), bytes("b"))
+#test(bytes("abcd").slice(-3, -1), bytes("bc"))
+#test(bytes("abcd").slice(-2, -2), bytes(""))
+#test(bytes("abcd").slice(1, count: 3), bytes("bcd"))
+#test(bytes("abcd").slice(-3, count: 3), bytes("bcd"))
+#test(bytes("abcd").slice(2, count: 0), bytes(""))

tests/suite/foundations/str.typ

@@ -145,6 +145,8 @@
 #test("abc🏡def".slice(2, 7), "c🏡")
 #test("abc🏡def".slice(2, -2), "c🏡d")
 #test("abc🏡def".slice(-3, -1), "de")
+#test("x🏡yz".slice(-2, count: 2), "yz")
+#test("x🏡yz".slice(-7, count: 7), "x🏡yz")
 
 --- string-slice-not-a-char-boundary ---
 // Error: 2-21 string index -1 is not a character boundary