avm2: Implement String.charAt

core/src/avm2/globals/string.rs

@@ -4,9 +4,11 @@ use crate::avm2::class::{Class, ClassAttributes};
 use crate::avm2::method::Method;
 use crate::avm2::names::{Namespace, QName};
 use crate::avm2::object::{Object, TObject};
+use crate::avm2::string::AvmString;
 use crate::avm2::value::Value;
 use crate::avm2::Error;
 use crate::avm2::{activation::Activation, traits::Trait};
+use crate::string_utils;
 use gc_arena::{GcCell, MutationContext};
 
 /// Implements `String`'s instance initializer.
@@ -54,6 +56,36 @@ fn length<'gc>(
     Ok(Value::Undefined)
 }
 
+/// Implements `String.charAt`
+fn char_at<'gc>(
+    activation: &mut Activation<'_, 'gc, '_>,
+    this: Option<Object<'gc>>,
+    args: &[Value<'gc>],
+) -> Result<Value<'gc>, Error> {
+    if let Some(this) = this {
+        if let Value::String(s) = this.value_of(activation.context.gc_context)? {
+            // This function takes Number, so if we use coerce_to_i32 instead of coerce_to_number, the value may overflow.
+            let n = args
+                .get(0)
+                .unwrap_or(&Value::Number(0.0))
+                .coerce_to_number(activation)?;
+            if n < 0.0 {
+                return Ok("".into());
+            }
+
+            let index = if !n.is_nan() { n as usize } else { 0 };
+            let ret = s
+                .encode_utf16()
+                .nth(index)
+                .map(|c| string_utils::utf16_code_unit_to_char(c).to_string())
+                .unwrap_or_default();
+            return Ok(AvmString::new(activation.context.gc_context, ret).into());
+        }
+    }
+
+    Ok(Value::Undefined)
+}
+
 /// Construct `String`'s class.
 pub fn create_class<'gc>(mc: MutationContext<'gc, '_>) -> GcCell<'gc, Class<'gc>> {
     let class = Class::new(
@@ -72,5 +104,10 @@ pub fn create_class<'gc>(mc: MutationContext<'gc, '_>) -> GcCell<'gc, Class<'gc>
         Method::from_builtin(length),
     ));
 
+    write.define_instance_trait(Trait::from_method(
+        QName::new(Namespace::as3_namespace(), "charAt"),
+        Method::from_builtin(char_at),
+    ));
+
     class
 }

core/tests/regression_tests.rs

@@ -506,6 +506,7 @@ swf_tests! {
     (as3_movieclip_dispatchevent_selfadd, "avm2/movieclip_dispatchevent_selfadd", 1),
     (as3_string_constr, "avm2/string_constr", 1),
     (as3_string_length, "avm2/string_length", 1),
+    (as3_string_char_at, "avm2/string_char_at", 1),
 }
 
 // TODO: These tests have some inaccuracies currently, so we use approx_eq to test that numeric values are close enough.

core/tests/swfs/avm2/string_char_at/Test.as

@@ -0,0 +1,32 @@
+package {
+	public class Test {}
+}
+
+trace("//\"abcdefg\".charAt();")
+trace("abcdefg".charAt());
+trace("//\"abcdefg\".charAt(1);")
+trace("abcdefg".charAt(1));
+trace("//\"abcdefg\".charAt(1.1);")
+trace("abcdefg".charAt(1.1));
+trace("//\"abcdefg\".charAt(1.5);")
+trace("abcdefg".charAt(1.5));
+trace("//\"abcdefg\".charAt(7);")
+trace("abcdefg".charAt(7));
+trace("//\"abcdefg\".charAt(-1);")
+trace("abcdefg".charAt(-1));
+trace("//\"abcdefg\".charAt(NaN);")
+trace("abcdefg".charAt(NaN));
+trace("//\"abcdefg\".charAt(1.79e+308);")
+trace("abcdefg".charAt(1.79e+308));
+trace("//\"abcdefg\".charAt(Infinity);")
+trace("abcdefg".charAt(Infinity));
+trace("//\"abcdefg\".charAt(-Infinity);")
+trace("abcdefg".charAt(-Infinity));
+trace("//\"あいうえお\".charAt(1);")
+trace("あいうえお".charAt(1));
+trace("//\"مَرحَبًا\".charAt(1);")
+trace("مَرحَبًا".charAt(1));
+trace("//\"👨‍👨‍👧‍👦\".charAt(0);")
+trace("👨‍👨‍👧‍👦".charAt(0));
+trace("//\"\".charAt(0);")
+trace("".charAt(0));

core/tests/swfs/avm2/string_char_at/output.txt

@@ -0,0 +1,28 @@
+//"abcdefg".charAt();
+a
+//"abcdefg".charAt(1);
+b
+//"abcdefg".charAt(1.1);
+b
+//"abcdefg".charAt(1.5);
+b
+//"abcdefg".charAt(7);
+
+//"abcdefg".charAt(-1);
+
+//"abcdefg".charAt(NaN);
+a
+//"abcdefg".charAt(1.79e+308);
+
+//"abcdefg".charAt(Infinity);
+
+//"abcdefg".charAt(-Infinity);
+
+//"あいうえお".charAt(1);
+い
+//"مَرحَبًا".charAt(1);
+َ
+//"👨‍👨‍👧‍👦".charAt(0);
+<EFBFBD>
+//"".charAt(0);
+