Add arbitrary base math log (#7409)

Adds new command `math log` that takes as a required positional argument
a base.

Specialized for `math log 2` and `math log 10` for better performance
and precision that matches the expectations there. This leads to
discontinuities in numerical error but should make a better trade-off
for common usecases.


Example testing of the happy path

crates/nu-command/src/default_context.rs

@@ -422,6 +422,7 @@ pub fn create_default_context() -> EngineState {
             MathTau,
             MathEuler,
             MathLn,
+            MathLog,
         };
 
         // Network

crates/nu-command/src/math/log.rs

@@ -0,0 +1,134 @@
+use nu_engine::CallExt;
+use nu_protocol::ast::Call;
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::{
+    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Type, Value,
+};
+
+#[derive(Clone)]
+pub struct SubCommand;
+
+impl Command for SubCommand {
+    fn name(&self) -> &str {
+        "math log"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build("math log")
+            .required(
+                "base",
+                SyntaxShape::Number,
+                "Base for which the logarithm should be computed",
+            )
+            .input_output_types(vec![(Type::Number, Type::Float)])
+            .vectorizes_over_list(true)
+            .category(Category::Math)
+    }
+
+    fn usage(&self) -> &str {
+        "Returns the logarithm for an arbitrary base."
+    }
+
+    fn search_terms(&self) -> Vec<&str> {
+        vec!["base", "exponent", "inverse", "euler"]
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        call: &Call,
+        input: PipelineData,
+    ) -> Result<nu_protocol::PipelineData, nu_protocol::ShellError> {
+        let head = call.head;
+        let base: Spanned<f64> = call.req(engine_state, stack, 0)?;
+
+        if base.item <= 0.0f64 {
+            return Err(ShellError::UnsupportedInput(
+                "Base has to be greater 0".into(),
+                base.span,
+            ));
+        }
+
+        let base = base.item;
+        input.map(
+            move |value| operate(value, head, base),
+            engine_state.ctrlc.clone(),
+        )
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![
+            Example {
+                description: "Get the logarithm of 100 to the base 10",
+                example: "100 | math log 10",
+                result: Some(Value::test_float(2.0f64)),
+            },
+            Example {
+                example: "[16 8 4] | math log 2",
+                description: "Get the log2 of a list of values",
+                result: Some(Value::List {
+                    vals: vec![
+                        Value::test_float(4.0),
+                        Value::test_float(3.0),
+                        Value::test_float(2.0),
+                    ],
+                    span: Span::test_data(),
+                }),
+            },
+        ]
+    }
+}
+
+fn operate(value: Value, head: Span, base: f64) -> Value {
+    match value {
+        numeric @ (Value::Int { .. } | Value::Float { .. }) => {
+            let (val, span) = match numeric {
+                Value::Int { val, span } => (val as f64, span),
+                Value::Float { val, span } => (val, span),
+                _ => unreachable!(),
+            };
+
+            if val <= 0.0 {
+                return Value::Error {
+                    error: ShellError::UnsupportedInput(
+                        "'math log' undefined for values outside the open interval (0, Inf)."
+                            .into(),
+                        span,
+                    ),
+                };
+            }
+            // Specialize for better precision/performance
+            let val = if base == 10.0 {
+                val.log10()
+            } else if base == 2.0 {
+                val.log2()
+            } else {
+                val.log(base)
+            };
+
+            Value::Float { val, span }
+        }
+        other => Value::Error {
+            error: ShellError::UnsupportedInput(
+                format!(
+                    "Only numerical values are supported, input type: {:?}",
+                    other.get_type()
+                ),
+                other.span().unwrap_or(head),
+            ),
+        },
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_examples() {
+        use crate::test_examples;
+
+        test_examples(SubCommand {})
+    }
+}

crates/nu-command/src/math/mod.rs

@@ -13,6 +13,7 @@ mod euler;
 mod eval;
 mod floor;
 mod ln;
+mod log;
 pub mod math_;
 mod max;
 mod median;
@@ -68,4 +69,5 @@ pub use euler::SubCommand as MathEuler;
 pub use pi::SubCommand as MathPi;
 pub use tau::SubCommand as MathTau;
 
+pub use self::log::SubCommand as MathLog;
 pub use ln::SubCommand as MathLn;