1 files changed, 42 insertions, 0 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
 }