LibJS: Implement BigInt.asIntN

Author: https://github.com/trflynn89
Commit: https://github.com/SerenityOS/serenity/commit/460c2caaf73
Pull-request: https://github.com/SerenityOS/serenity/pull/12307
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibJS/Runtime/BigIntConstructor.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <AK/String.h>
+#include <LibJS/Runtime/AbstractOperations.h>
 #include <LibJS/Runtime/BigInt.h>
 #include <LibJS/Runtime/BigIntConstructor.h>
 #include <LibJS/Runtime/BigIntObject.h>
@@ -14,6 +15,8 @@
 
 namespace JS {
 
+static const Crypto::SignedBigInteger BIGINT_ONE { 1 };
+
 BigIntConstructor::BigIntConstructor(GlobalObject& global_object)
     : NativeFunction(vm().names.BigInt.as_string(), *global_object.function_prototype())
 {
@@ -27,9 +30,8 @@ void BigIntConstructor::initialize(GlobalObject& global_object)
     // 21.2.2.3 BigInt.prototype, https://tc39.es/ecma262/#sec-bigint.prototype
     define_direct_property(vm.names.prototype, global_object.bigint_prototype(), 0);
 
-    // TODO: Implement these functions below and uncomment this.
-    // u8 attr = Attribute::Writable | Attribute::Configurable;
-    // define_native_function(vm.names.asIntN, as_int_n, 2, attr);
+    u8 attr = Attribute::Writable | Attribute::Configurable;
+    define_native_function(vm.names.asIntN, as_int_n, 2, attr);
     // define_native_function(vm.names.asUintN, as_uint_n, 2, attr);
 
     define_direct_property(vm.names.length, Value(1), Attribute::Configurable);
@@ -67,7 +69,27 @@ ThrowCompletionOr<Object*> BigIntConstructor::construct(FunctionObject&)
 // 21.2.2.1 BigInt.asIntN ( bits, bigint ), https://tc39.es/ecma262/#sec-bigint.asintn
 JS_DEFINE_NATIVE_FUNCTION(BigIntConstructor::as_int_n)
 {
-    TODO();
+    // 1. Set bits to ? ToIndex(bits).
+    auto bits = TRY(vm.argument(0).to_index(global_object));
+
+    // 2. Set bigint to ? ToBigInt(bigint).
+    auto* bigint = TRY(vm.argument(1).to_bigint(global_object));
+
+    // 3. Let mod be ℝ(bigint) modulo 2^bits.
+    // FIXME: For large values of `bits`, this can likely be improved with a SignedBigInteger API to
+    //        drop the most significant bits.
+    auto bits_shift_left = BIGINT_ONE.shift_left(bits);
+    auto mod = modulo(bigint->big_integer(), bits_shift_left);
+
+    // 4. If mod ≥ 2^(bits-1), return ℤ(mod - 2^bits); otherwise, return ℤ(mod).
+    // NOTE: Some of the below conditionals are non-standard, but are to protect SignedBigInteger from
+    //       allocating an absurd amount of memory if `bits - 1` overflows to NumericLimits<size_t>::max.
+    if ((bits == 0) && (mod >= BIGINT_ONE))
+        return js_bigint(vm, mod.minus(bits_shift_left));
+    if ((bits > 0) && (mod >= BIGINT_ONE.shift_left(bits - 1)))
+        return js_bigint(vm, mod.minus(bits_shift_left));
+
+    return js_bigint(vm, mod);
 }
 
 // 21.2.2.2 BigInt.asUintN ( bits, bigint ), https://tc39.es/ecma262/#sec-bigint.asuintn

Userland/Libraries/LibJS/Tests/builtins/BigInt/BigInt.asIntN.js

@@ -0,0 +1,79 @@
+describe("errors", () => {
+    test("invalid index", () => {
+        expect(() => {
+            BigInt.asIntN(-1, 0n);
+        }).toThrowWithMessage(RangeError, "Index must be a positive integer");
+
+        expect(() => {
+            BigInt.asIntN(Symbol(), 0n);
+        }).toThrowWithMessage(TypeError, "Cannot convert symbol to number");
+    });
+
+    test("invalid BigInt", () => {
+        expect(() => {
+            BigInt.asIntN(1, 1);
+        }).toThrowWithMessage(TypeError, "Cannot convert number to BigInt");
+
+        expect(() => {
+            BigInt.asIntN(1, Symbol());
+        }).toThrowWithMessage(TypeError, "Cannot convert symbol to BigInt");
+
+        expect(() => {
+            BigInt.asIntN(1, "foo");
+        }).toThrowWithMessage(SyntaxError, "Invalid value for BigInt: foo");
+    });
+});
+
+describe("correct behavior", () => {
+    test("length is 2", () => {
+        expect(BigInt.asIntN).toHaveLength(2);
+    });
+
+    test("basic functionality", () => {
+        expect(BigInt.asIntN(0, -2n)).toBe(0n);
+        expect(BigInt.asIntN(0, -1n)).toBe(0n);
+        expect(BigInt.asIntN(0, 0n)).toBe(0n);
+        expect(BigInt.asIntN(0, 1n)).toBe(0n);
+        expect(BigInt.asIntN(0, 2n)).toBe(0n);
+
+        expect(BigInt.asIntN(1, -3n)).toBe(-1n);
+        expect(BigInt.asIntN(1, -2n)).toBe(0n);
+        expect(BigInt.asIntN(1, -1n)).toBe(-1n);
+        expect(BigInt.asIntN(1, 0n)).toBe(0n);
+        expect(BigInt.asIntN(1, 1n)).toBe(-1n);
+        expect(BigInt.asIntN(1, 2n)).toBe(0n);
+        expect(BigInt.asIntN(1, 3n)).toBe(-1n);
+
+        expect(BigInt.asIntN(2, -3n)).toBe(1n);
+        expect(BigInt.asIntN(2, -2n)).toBe(-2n);
+        expect(BigInt.asIntN(2, -1n)).toBe(-1n);
+        expect(BigInt.asIntN(2, 0n)).toBe(0n);
+        expect(BigInt.asIntN(2, 1n)).toBe(1n);
+        expect(BigInt.asIntN(2, 2n)).toBe(-2n);
+        expect(BigInt.asIntN(2, 3n)).toBe(-1n);
+
+        expect(BigInt.asIntN(4, -3n)).toBe(-3n);
+        expect(BigInt.asIntN(4, -2n)).toBe(-2n);
+        expect(BigInt.asIntN(4, -1n)).toBe(-1n);
+        expect(BigInt.asIntN(4, 0n)).toBe(0n);
+        expect(BigInt.asIntN(4, 1n)).toBe(1n);
+        expect(BigInt.asIntN(4, 2n)).toBe(2n);
+        expect(BigInt.asIntN(4, 3n)).toBe(3n);
+
+        const extremelyBigInt = 123456789123456789123456789123456789123456789123456789n;
+
+        expect(BigInt.asIntN(0, extremelyBigInt)).toBe(0n);
+        expect(BigInt.asIntN(1, extremelyBigInt)).toBe(-1n);
+        expect(BigInt.asIntN(2, extremelyBigInt)).toBe(1n);
+        expect(BigInt.asIntN(4, extremelyBigInt)).toBe(5n);
+        expect(BigInt.asIntN(128, extremelyBigInt)).toBe(-99061374399389259395070030194384019691n);
+        expect(BigInt.asIntN(256, extremelyBigInt)).toBe(extremelyBigInt);
+
+        expect(BigInt.asIntN(0, -extremelyBigInt)).toBe(0n);
+        expect(BigInt.asIntN(1, -extremelyBigInt)).toBe(-1n);
+        expect(BigInt.asIntN(2, -extremelyBigInt)).toBe(-1n);
+        expect(BigInt.asIntN(4, -extremelyBigInt)).toBe(-5n);
+        expect(BigInt.asIntN(128, -extremelyBigInt)).toBe(99061374399389259395070030194384019691n);
+        expect(BigInt.asIntN(256, -extremelyBigInt)).toBe(-extremelyBigInt);
+    });
+});