Tests for arrays builtins

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

src/builtins/arrays.rs

@@ -3,7 +3,7 @@
 
 use crate::ast::Expr;
 use crate::builtins;
-use crate::builtins::utils::{ensure_args_count, ensure_array};
+use crate::builtins::utils::{ensure_args_count, ensure_array, ensure_numeric};
 use crate::lexer::Span;
 use crate::value::Value;
 
@@ -15,6 +15,7 @@ use std::rc::Rc;
 pub fn register(m: &mut HashMap<&'static str, builtins::BuiltinFcn>) {
     m.insert("array.concat", concat);
     m.insert("array.reverse", reverse);
+    m.insert("array.slice", slice);
 }
 
 fn concat(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
@@ -29,13 +30,36 @@ fn concat(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
 
 fn reverse(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
     let name = "array.reverse";
-    ensure_args_count(span, name, params, args, 2)?;
+    ensure_args_count(span, name, params, args, 1)?;
 
     let mut v1 = ensure_array(name, &params[0], args[0].clone())?;
     Rc::make_mut(&mut v1).reverse();
     Ok(Value::Array(v1))
 }
 
-/*
 fn slice(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
-*/
+    let name = "array.slice";
+    ensure_args_count(span, name, params, args, 3)?;
+
+    let array = ensure_array(name, &params[0], args[0].clone())?;
+    let start = ensure_numeric(name, &params[1], &args[1].clone())?;
+    let stop = ensure_numeric(name, &params[2], &args[2].clone())?;
+
+    if start != start.floor() || stop != stop.floor() {
+        return Ok(Value::Undefined);
+    }
+
+    // TODO: usize conversion checks.
+    let start = start as usize;
+    let stop = match stop as usize {
+        s if s > array.len() => array.len(),
+        s => s,
+    };
+
+    if start >= stop {
+        return Ok(Value::new_array());
+    }
+
+    let slice = &array[start..stop];
+    Ok(Value::from_array(slice.to_vec()))
+}