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LibJS: Optimize & Bring String.prototype.repeat closer to the spec

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Specifically, we immediately return an empty string when `this` is an
empty string, instead of wasting time in a loop doing nothing N times.
This commit is contained in:
Idan Horowitz 2021-06-30 15:19:38 +03:00 • committed by Linus Groh
parent 5f09d78b9d
commit 7d055dd039
1 changed files with 15 additions and 6 deletions
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21
Userland/Libraries/LibJS/Runtime/StringPrototype.cpp
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@ -167,21 +167,30 @@ JS_DEFINE_NATIVE_FUNCTION(StringPrototype::repeat)
auto string = ak_string_from(vm, global_object); auto string = ak_string_from(vm, global_object);
if (!string.has_value()) if (!string.has_value())
return {}; return {};
if (!vm.argument_count())
return js_string(vm, String::empty()); auto n = vm.argument(0).to_integer_or_infinity(global_object);
auto count = vm.argument(0).to_integer_or_infinity(global_object);
if (vm.exception()) if (vm.exception())
return {}; return {};
if (count < 0) {
if (n < 0) {
vm.throw_exception<RangeError>(global_object, ErrorType::StringRepeatCountMustBe, "positive"); vm.throw_exception<RangeError>(global_object, ErrorType::StringRepeatCountMustBe, "positive");
return {}; return {};
} }
if (Value(count).is_infinity()) {
if (Value(n).is_positive_infinity()) {
vm.throw_exception<RangeError>(global_object, ErrorType::StringRepeatCountMustBe, "finite"); vm.throw_exception<RangeError>(global_object, ErrorType::StringRepeatCountMustBe, "finite");
return {}; return {};
} }
if (n == 0)
return js_string(vm, String::empty());
// NOTE: This is an optimization, it is not required by the specification but it produces equivalent behaviour
if (string->is_empty())
return js_string(vm, String::empty());
StringBuilder builder; StringBuilder builder;
for (size_t i = 0; i < count; ++i) for (size_t i = 0; i < n; ++i)
builder.append(*string); builder.append(*string);
return js_string(vm, builder.to_string()); return js_string(vm, builder.to_string());
} }