3 files changed, 43 insertions, 8 deletions

diff --git a/src/input.rs b/src/input.rs
index 2f9d820..462e43c 100644
--- a/src/input.rs
+++ b/src/input.rs
@@ -1,6 +1,6 @@
 use crossterm::event::{KeyCode, KeyEvent, KeyModifiers};
 
-use crate::{Calculator, Function, Operator, Signal};
+use crate::{Calculator, Constant, Function, Operator, Signal};
 
 impl Calculator {
     pub fn handle_input(&mut self, key: KeyEvent) -> Signal {
@@ -32,6 +32,8 @@ impl Calculator {
                         if let Some(st) = self.stack.pop() {
                             self.stack.push(func.call_on(st));
                         }
+                    } else if let Ok(c) = Constant::parse(&self.input_buffer) {
+                        self.stack.push(c.value());
                     } else if let Ok(bu) = self.input_buffer.parse::<f64>() {
                         self.stack.push(bu);
                     }
diff --git a/src/lib.rs b/src/lib.rs
index d8509d5..00163b1 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,3 +1,5 @@
+use std::f64::consts::{E, PI};
+
 mod input;
 pub mod ui;
 
@@ -61,8 +63,8 @@ enum Function {
     RSin, // Sine (radians)
     RCos, // Cosing (radians)
     RTan, // Tangent (radians)
-    Deg,  // Convert from radians to degrees.
-    Rad,  // Convert from degrees to radians.
+    Deg,  // Convert from radians to degrees
+    Rad,  // Convert from degrees to radians
 }
 
 impl Function {
@@ -96,6 +98,30 @@ impl Function {
 
 struct ParseFunctionError(String);
 
+enum Constant {
+    Pi, // Archimedes’ constant (π)
+    E,  // Euler's number (e)
+}
+
+impl Constant {
+    fn parse(s: &str) -> Result<Self, ParseConstantError> {
+        match s {
+            "pi" => Ok(Self::Pi),
+            "e" => Ok(Self::E),
+            _ => Err(ParseConstantError(s.to_string())),
+        }
+    }
+
+    fn value(&self) -> f64 {
+        match self {
+            Self::Pi => PI,
+            Self::E => E,
+        }
+    }
+}
+
+struct ParseConstantError(String);
+
 pub enum Signal {
     None,
     Exit,
diff --git a/src/ui.rs b/src/ui.rs
index 1b50fb5..3d09221 100644
--- a/src/ui.rs
+++ b/src/ui.rs
@@ -12,7 +12,7 @@ use tui::{
     Frame, Terminal,
 };
 
-use crate::{Calculator, Function};
+use crate::{Calculator, Constant, Function};
 
 const WIDTH: u16 = 32;
 
@@ -78,10 +78,7 @@ fn input_buffer_widget(input_buffer: &str) -> impl Widget {
         Span::raw(">"),
         Span::styled(
             format!(" {}", input_buffer),
-            match Function::parse(&input_buffer) {
-                Ok(_) => Style::default().add_modifier(Modifier::BOLD),
-                Err(_) => Style::default(),
-            },
+            input_buffer_style(input_buffer),
         ),
     ]))
     .block(Block::default().borders(Borders::ALL))
@@ -91,3 +88,13 @@ fn version_number_widget() -> impl Widget {
     Paragraph::new(format!("pfc-{}", option_env!("CARGO_PKG_VERSION").unwrap()))
         .alignment(Alignment::Center)
 }
+
+fn input_buffer_style(input_buffer: &str) -> Style {
+    if let Ok(_) = Function::parse(&input_buffer) {
+        Style::default().add_modifier(Modifier::BOLD)
+    } else if let Ok(_) = Constant::parse(&input_buffer) {
+        Style::default().add_modifier(Modifier::BOLD)
+    } else {
+        Style::default()
+    }
+}