implement builtin function math.log

src/builtin/modules/math.rs

@@ -1,6 +1,6 @@
 use std::cmp::Ordering;
 
-use num_traits::{One, Zero};
+use num_traits::{One, Signed, Zero};
 
 use crate::{
     args::CallArgs,
@@ -107,7 +107,69 @@ fn hypot(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
 
 fn log(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(2)?;
+
+    let number = match args.get_err(0, "number")? {
+        Value::Dimension(Some(n), Unit::None, ..) => n,
+        v @ Value::Dimension(Some(..), ..) => {
+            return Err((
+                format!(
+                    "$number: Expected {} to be unitless.",
+                    v.inspect(args.span())?
+                ),
+                args.span(),
+            )
+                .into())
+        }
+        v @ Value::Dimension(None, ..) => return Ok(v),
+        v => {
+            return Err((
+                format!("$number: {} is not a number.", v.inspect(args.span())?),
+                args.span(),
+            )
+                .into())
+        }
+    };
+
+    let base = match args.default_arg(1, "base", Value::Null)? {
+        Value::Null => None,
+        Value::Dimension(Some(n), Unit::None, ..) => Some(n),
+        v @ Value::Dimension(Some(..), ..) => {
+            return Err((
+                format!(
+                    "$number: Expected {} to be unitless.",
+                    v.inspect(args.span())?
+                ),
+                args.span(),
+            )
+                .into())
+        }
+        v @ Value::Dimension(None, ..) => return Ok(v),
+        v => {
+            return Err((
+                format!("$base: {} is not a number.", v.inspect(args.span())?),
+                args.span(),
+            )
+                .into())
+        }
+    };
+
+    Ok(Value::Dimension(
+        if let Some(base) = base {
+            if base.is_zero() {
+                Some(Number::zero())
+            } else {
+                (|| Some(number.ln()? / base.ln()?))()
+            }
+        } else if number.is_negative() {
+            None
+        } else if number.is_zero() {
             todo!()
+        } else {
+            number.ln()
+        },
+        Unit::None,
+        true,
+    ))
 }
 
 fn pow(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
@@ -310,6 +372,7 @@ pub(crate) fn declare(f: &mut Module) {
     f.insert_builtin("acos", acos);
     f.insert_builtin("asin", asin);
     f.insert_builtin("atan", atan);
+    f.insert_builtin("log", log);
     #[cfg(feature = "random")]
     f.insert_builtin("random", random);
 

src/value/number/mod.rs

@@ -122,6 +122,12 @@ impl Number {
             self.as_float()?.sqrt(),
         )?)))
     }
+
+    pub fn ln(self) -> Option<Self> {
+        Some(Number::Big(Box::new(BigRational::from_float(
+            self.as_float()?.ln(),
+        )?)))
+    }
 }
 
 macro_rules! trig_fn(

tests/math-module.rs

@@ -318,3 +318,75 @@ test!(
     "@use 'sass:math';\na {\n  color: math.atan((0 / 0));\n}\n",
     "a {\n  color: NaNdeg;\n}\n"
 );
+test!(
+    log_above_one,
+    "@use 'sass:math';\na {\n  color: math.log(2);\n}\n",
+    "a {\n  color: 0.6931471806;\n}\n"
+);
+test!(
+    log_below_negative_one,
+    "@use 'sass:math';\na {\n  color: math.log(-2);\n}\n",
+    "a {\n  color: NaN;\n}\n"
+);
+test!(
+    log_one,
+    "@use 'sass:math';\na {\n  color: math.log(1);\n}\n",
+    "a {\n  color: 0;\n}\n"
+);
+test!(
+    log_negative_one,
+    "@use 'sass:math';\na {\n  color: math.log(-1);\n}\n",
+    "a {\n  color: NaN;\n}\n"
+);
+test!(
+    #[ignore = "we do not support Infinity"]
+    log_zero,
+    "@use 'sass:math';\na {\n  color: math.log(0);\n}\n",
+    "a {\n  color: -Infinity;\n}\n"
+);
+test!(
+    log_point_five,
+    "@use 'sass:math';\na {\n  color: math.log(.5);\n}\n",
+    "a {\n  color: -0.6931471806;\n}\n"
+);
+test!(
+    log_nan,
+    "@use 'sass:math';\na {\n  color: math.log((0 / 0));\n}\n",
+    "a {\n  color: NaN;\n}\n"
+);
+test!(
+    log_base_nan,
+    "@use 'sass:math';\na {\n  color: math.log(1, (0 / 0));\n}\n",
+    "a {\n  color: NaN;\n}\n"
+);
+test!(
+    log_base_above_one,
+    "@use 'sass:math';\na {\n  color: math.log(2, 2);\n}\n",
+    "a {\n  color: 1;\n}\n"
+);
+test!(
+    log_base_below_negative_one,
+    "@use 'sass:math';\na {\n  color: math.log(2, -2);\n}\n",
+    "a {\n  color: NaN;\n}\n"
+);
+test!(
+    #[ignore = "we do not support Infinity"]
+    log_base_one,
+    "@use 'sass:math';\na {\n  color: math.log(2, 1);\n}\n",
+    "a {\n  color: Infinity;\n}\n"
+);
+test!(
+    log_base_negative_one,
+    "@use 'sass:math';\na {\n  color: math.log(2, -1);\n}\n",
+    "a {\n  color: NaN;\n}\n"
+);
+test!(
+    log_base_zero,
+    "@use 'sass:math';\na {\n  color: math.log(2, 0);\n}\n",
+    "a {\n  color: 0;\n}\n"
+);
+test!(
+    log_base_point_five,
+    "@use 'sass:math';\na {\n  color: math.log(2, .5);\n}\n",
+    "a {\n  color: -1;\n}\n"
+);