From 419f409db8a4195a58354a6e3d0c1aac26d2984f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Te-j=C3=A9=20Rodgers?= <tjd.rodgers@gmail.com>
Date: Thu, 25 Jun 2020 19:06:21 +0200
Subject: [PATCH] [#524] add implementation for `Math.expm1()`

---
 boa/src/builtins/math/mod.rs   | 18 ++++++++++++++++++
 boa/src/builtins/math/tests.rs | 30 ++++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/boa/src/builtins/math/mod.rs b/boa/src/builtins/math/mod.rs
index 9379318bb64..0bb3bd848f0 100644
--- a/boa/src/builtins/math/mod.rs
+++ b/boa/src/builtins/math/mod.rs
@@ -223,6 +223,23 @@ impl Math {
         Ok(args.get(0).map_or(f64::NAN, |x| f64::from(x).exp()).into())
     }
 
+    /// The Math.expm1() function returns e^x - 1, where x is the argument, and e the base of
+    /// the natural logarithms. The result is computed in a way that is accurate even when the
+    /// value of x is close 0
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///  - [MDN documentation][mdn]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-math.expm1
+    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/expm1
+    pub(crate) fn expm1(_: &Value, args: &[Value], _: &mut Interpreter) -> ResultValue {
+        Ok(args
+            .get(0)
+            .map_or(f64::NAN, |x| f64::from(x).exp_m1())
+            .into())
+    }
+
     /// Get the highest integer below a number.
     ///
     /// More information:
@@ -511,6 +528,7 @@ impl Math {
         make_builtin_fn(Self::cos, "cos", &math, 1);
         make_builtin_fn(Self::cosh, "cosh", &math, 1);
         make_builtin_fn(Self::exp, "exp", &math, 1);
+        make_builtin_fn(Self::expm1, "expm1", &math, 1);
         make_builtin_fn(Self::floor, "floor", &math, 1);
         make_builtin_fn(Self::log, "log", &math, 1);
         make_builtin_fn(Self::log10, "log10", &math, 1);
diff --git a/boa/src/builtins/math/tests.rs b/boa/src/builtins/math/tests.rs
index e661148135d..6b68c3046d7 100644
--- a/boa/src/builtins/math/tests.rs
+++ b/boa/src/builtins/math/tests.rs
@@ -279,6 +279,36 @@ fn exp() {
     assert_eq!(c.to_number(), 7.389_056_098_930_65);
 }
 
+#[test]
+fn expm1() {
+    let realm = Realm::create();
+    let mut engine = Interpreter::new(realm);
+    let init = r#"
+        var a = Math.expm1();
+        var b = Math.expm1({});
+        var c = Math.expm1(1);
+        var d = Math.expm1(-1);
+        var e = Math.expm1(0);
+        var f = Math.expm1(2);
+        "#;
+
+    eprintln!("{}", forward(&mut engine, init));
+
+    let a = forward(&mut engine, "a");
+    let b = forward(&mut engine, "b");
+    let c = forward_val(&mut engine, "c").unwrap();
+    let d = forward_val(&mut engine, "d").unwrap();
+    let e = forward_val(&mut engine, "e").unwrap();
+    let f = forward_val(&mut engine, "f").unwrap();
+
+    assert_eq!(a, String::from("NaN"));
+    assert_eq!(b, String::from("NaN"));
+    assert_eq!(c.to_number(), 1.718_281_828_459_045);
+    assert_eq!(d.to_number(), -0.632_120_558_828_557_7);
+    assert_eq!(e.to_number(), 0_f64);
+    assert_eq!(f.to_number(), 6.389_056_098_930_65);
+}
+
 #[test]
 fn floor() {
     let realm = Realm::create();