parsing: improve function handling

parsing/src/splitting.rs

@@ -1,17 +1,61 @@
 use crate::parsing::is_variable;
+use crate::SUPPORTED_FUNCTIONS;
+
+/// Protect function names that start with variable characters (like 'e' in 'exp')
+/// by replacing them with a placeholder during parsing, then restoring them after.
+/// This prevents incorrect splitting like "exp" -> "e" * "xp".
+fn protect_function_names(input: &str) -> (String, Vec<(&'static str, String)>) {
+    let mut result = input.to_string();
+    let mut replacements = Vec::new();
+
+    // Only protect functions that start with a variable character
+    // Sort by length descending to replace longer matches first (e.g., "exp" before "e")
+    let mut funcs: Vec<&'static str> = SUPPORTED_FUNCTIONS
+        .iter()
+        .filter(|&&func| {
+            func.chars()
+                .next()
+                .map(|c| is_variable(&c))
+                .unwrap_or(false)
+        })
+        .copied()
+        .collect();
+    funcs.sort_by(|a, b| b.len().cmp(&a.len()));
+
+    for func in funcs {
+        // Use a placeholder made of letters that will be treated as a function name
+        // The placeholder won't be split because it's all letters
+        let placeholder = format!("zzzfn{}", replacements.len());
+        result = result.replace(func, &placeholder);
+        replacements.push((func, placeholder));
+    }
+
+    (result, replacements)
+}
+
+/// Restore protected function names from their placeholders
+fn restore_function_names(input: &str, replacements: &[(&'static str, String)]) -> String {
+    let mut result = input.to_string();
+    for (func, placeholder) in replacements {
+        result = result.replace(placeholder, func);
+    }
+    result
+}
 
 pub fn split_function(input: &str, split: SplitType) -> Vec<String> {
+    // Protect function names that could be incorrectly split
+    let (protected, replacements) = protect_function_names(input);
+
     split_function_chars(
-        &input
+        &protected
             .replace("pi", "π") // replace "pi" text with pi symbol
             .replace("**", "^") // support alternate manner of expressing exponents
-            .replace("exp", "\u{1fc93}") // stop-gap solution to fix the `exp` function
             .chars()
             .collect::<Vec<char>>(),
         split,
     )
     .iter()
-    .map(|x| x.replace('\u{1fc93}', "exp")) // Convert back to `exp` text
+    .map(|x| restore_function_names(x, &replacements))
     .collect::<Vec<String>>()
 }
 
@@ -232,4 +276,13 @@ fn split_function_test() {
         &["2", "sin(π) + 2", "cos(tau)"],
         SplitType::Multiplication,
     );
+
+    // Test that exp() function is properly handled (not split into e*xp)
+    assert_test("exp(x)", &["exp(x)"], SplitType::Multiplication);
+    assert_test("2exp(x)", &["2", "exp(x)"], SplitType::Multiplication);
+    assert_test(
+        "exp(x)sin(x)",
+        &["exp(x)", "sin(x)"],
+        SplitType::Multiplication,
+    );
 }