fix: Match OPA behavior for split (microsoft#295)

In case of empty delimiter, Rust's split returns leading and trailing
empty strings whereas Golang's doesn't.
Change behavior to match Golang/OPA.

fixes microsoft#291

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

src/builtins/strings.rs

@@ -146,11 +146,18 @@ fn split(span: &Span, params: &[Ref<Expr>], args: &[Value], _strict: bool) -> Re
     let s = ensure_string(name, &params[0], &args[0])?;
     let delimiter = ensure_string(name, &params[1], &args[1])?;
 
-    Ok(Value::from_array(
+    // Handle https://github.com/microsoft/regorus/issues/291
+    let parts: Vec<Value> = if delimiter.as_ref() == "" {
+        // If delimiter is "", str::split returns a leading and trailing "" whereas Golang's split doesn't.
+        // Therefore avoid str::split and instead return each char as a Value::String.
+        s.chars().map(|c| Value::from(c.to_string())).collect()
+    } else {
         s.split(delimiter.as_ref())
             .map(|s| Value::String(s.into()))
-            .collect(),
-    ))
+            .collect()
+    };
+
+    Ok(Value::from(parts))
 }
 
 fn to_string(v: &Value, unescape: bool) -> String {

tests/interpreter/cases/builtins/strings/split.yaml

@@ -0,0 +1,17 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+cases:
+  - note: empty separator
+    data: {}
+    modules: []
+    query: "x := split(\"test\", \"\")"
+    want_result:
+      x: ["t", "e", "s", "t"]
+
+  - note: empty separator, empty string
+    data: {}
+    modules: []
+    query: "x := split(\"\", \"\")"
+    want_result:
+      x: []