Replace O(n) function lookup with O(1) hash map and fix memory leaks

[Copilot]

Function lookups used linear search through a linked list (strcmp on every node), degrading performance with program size. Additionally, functions were created twice (parse + execute phases) causing memory leaks.

Changes

Function storage

• Replaced function_table linked list with function_map hash map
• Removed Function.next pointer from struct
• get_function() now O(1) instead of O(n)

Memory management

• Added duplicate check in create_function() to prevent double-creation
• Iterate hash map in free_function_table() to properly free all functions
• Added hm_free_shallow() for hash maps storing pointers to externally-managed memory

Code quality

• Replaced strdup() with safe_strdup() for consistent error handling
• Cache strlen() results in add_variable_to_scope() to avoid redundant calculations

Example

Before:

Function *get_function(const char *name) {
    Function *func = function_table;
    while (func != NULL) {
        if (strcmp(func->name, name) == 0)  // O(n) scan
            return func;
        func = func->next;
    }
    return NULL;
}

After:

Function *get_function(const char *name) {
    if (!function_map || !name) return NULL;
    Function **func_ptr = (Function **)hm_get(function_map, name, strlen(name));  // O(1)
    return func_ptr ? *func_ptr : NULL;
}

Testing

All 63 unit tests and 69 valgrind memory tests pass with zero leaks detected.

Original prompt

Identify and suggest improvements to slow or inefficient code

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[SIGMazer]

@leo-aa88 check this out :"

[leo-aa88]

@leo-aa88 check this out :"

[leo-aa88]

LGTM

[SIGMazer]

@leo-aa88 can u approve this again

[SIGMazer]

@copilot u can merge the pr now

[Copilot]

@copilot u can merge the pr now

I don't have permission to merge PRs directly. The PR can be merged by maintainers with write access through GitHub's interface. The PR is ready - all tests pass and it has been approved.

[SIGMazer]

somehow i can't merge it can u do @leo-aa88