4 files changed, 50 insertions, 3 deletions

diff --git a/src/error.rs b/src/error.rs
index ec2b555..e05d4e3 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -2,11 +2,16 @@
  * Refer to LICENCE for more information.
  * */
 
+use std::iter::ExactSizeIterator;
+
+use crate::lex;
+
 #[derive(Debug)]
 pub enum CalcError {
     Math(Math),
     Syntax(String),
     Parser(String),
+    Help,
 }
 
 #[derive(Debug)]
@@ -25,5 +30,42 @@ pub fn handler(e: CalcError) -> String {
         },
         CalcError::Syntax(details) => format!("Syntax Error: {}", details),
         CalcError::Parser(details) => format!("Parser Error: {}", details),
+        CalcError::Help => {
+            // calculate max width but ideally this should be calculated once
+            let mut max_width = 79; // capped at 79
+            if let Some((w, _)) = term_size::dimensions() {
+                if w < max_width {
+                    max_width = w;
+                }
+            }
+            let operators: Vec<_> = lex::OPERATORS.keys().map(|c| c.to_string()).collect();
+            format!(
+                "Constants\n{}\nFunctions\n{}\nOperators\n{}\n",
+                blocks(max_width, lex::CONSTANTS.keys().cloned()),
+                blocks(max_width, lex::FUNCTIONS.keys().cloned()),
+                operators.join(" ")
+            )
+        }
+    }
+}
+
+/// Convert iterator into strings of chunks of 8 right padded with space.
+fn blocks(
+    max_width: usize,
+    mut iter: impl Iterator<Item = &'static str> + ExactSizeIterator,
+) -> String {
+    // multiply by eight since we are formatting it into chunks of 8
+    let items_per_line = max_width / 8;
+    let full_bytes = (iter.len() - iter.len() % items_per_line) * 8;
+    let part_bytes = iter.len() % items_per_line * 8; // leftovers
+    let n_newlines = iter.len() / items_per_line + if part_bytes > 0 { 1 } else { 0 };
+    let mut s = String::with_capacity(full_bytes + part_bytes + n_newlines);
+    for _ in 0..n_newlines {
+        for item in iter.by_ref().take(items_per_line) {
+            s.extend(format!("{:>8}", item).chars());
+        }
+        s.push('\n');
     }
+    debug_assert_eq!(s.capacity(), s.len()); // check capacity calculation
+    s
 }
diff --git a/src/lex.rs b/src/lex.rs
index a2de98a..76f61a0 100644
--- a/src/lex.rs
+++ b/src/lex.rs
@@ -69,14 +69,14 @@ pub enum Token {
 }
 
 lazy_static! {
-    static ref CONSTANTS: HashMap<&'static str, Token> = {
+    pub static ref CONSTANTS: HashMap<&'static str, Token> = {
         let mut m = HashMap::new();
         m.insert("e",  Token::Num(std::f64::consts::E));
         m.insert("pi", Token::Num(std::f64::consts::PI));
         m
     };
 
-    static ref FUNCTIONS: HashMap<&'static str, Token> = {
+    pub static ref FUNCTIONS: HashMap<&'static str, Token> = {
         let mut m = HashMap::new();
         m.insert("sin",   Function::token_from_fn("sin".into(),   |x| is_radian_mode(x, CONFIGURATION.radian_mode).sin()));
         m.insert("cos",   Function::token_from_fn("cos".into(),   |x| is_radian_mode(x, CONFIGURATION.radian_mode).cos()));
@@ -105,7 +105,7 @@ lazy_static! {
         m
     };
 
-    static ref OPERATORS: HashMap<char, Token> = {
+    pub static ref OPERATORS: HashMap<char, Token> = {
         let mut m = HashMap::new();
         m.insert('+', Operator::token_from_op('+', |x, y| x + y, 2, true));
         m.insert('-', Operator::token_from_op('-', |x, y| x - y, 2, true));
diff --git a/src/main.rs b/src/main.rs
index 5540da9..94f4cbc 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -193,6 +193,9 @@ fn parse_arguments() -> Configuration {
 
 pub fn eval_math_expression(input: &str, prev_ans: Option<f64>) -> Result<f64, CalcError> {
     let input = input.trim().replace(" ", "");
+    if input == "help" {
+        return Err(CalcError::Help);
+    }
     if input.is_empty() {
         return Ok(0.);
     }
diff --git a/src/readline.rs b/src/readline.rs
index ea195ee..d689f95 100644
--- a/src/readline.rs
+++ b/src/readline.rs
@@ -12,6 +12,7 @@ use directories::ProjectDirs;
 
 use regex::Regex;
 
+use crate::error::CalcError;
 use crate::eval_math_expression;
 
 pub struct RLHelper {
@@ -67,6 +68,7 @@ impl Highlighter for LineHighlighter {
                 }
                 Owned(coloured)
             }
+            Err(CalcError::Help) => Owned(line.replace("help", "\x1b[36mhelp\x1b[0m")),
             Err(_) => Owned(format!("\x1b[31m{}\x1b[0m", line)),
         }
     }