Review, remove_trait_impls

Related type:

TraitsResolver

TraitsResolver helps to find the implementation for a given trait function and concrete type arguments.

TraitsResolver has a function resolve_trait_function_reference (see below) for a given polynomial reference, it resolves its trait

one of the input to this polynomialReference,

using fibo_no_public example, PolynomialReference looks like:

Understanding the remove_trait_impls Function in Rust

This post explains the remove_trait_impls function, its purpose, and how it works step-by-step.

Function Overview

/// Update the data structure after a certain set of trait impls have been removed.
/// This just updates the `index` fields.
/// Assumes that `to_remove` is sorted.
pub fn remove_trait_impls(&mut self, to_remove: &[usize]) {
    for map in self.impls.values_mut() {
        *map = map
            .drain()
            .filter_map(|(type_args, mut impl_data)| {
                match to_remove.binary_search(&impl_data.index) {
                    Ok(_) => None,
                    Err(index) => {
                        // `index` is the number of removed elements before this one.
                        impl_data.index -= index;
                        Some((type_args, impl_data))
                    }
                }
            })
            .collect();
    }
}

Purpose of the Function

The remove_trait_impls function is designed to update and maintain consistency in a data structure (SolvedTraitImpls) that maps trait implementations. Specifically, it:

• Removes certain trait implementations specified by their indices in to_remove.
• Adjusts the index fields of the remaining implementations to account for the removed ones.

Key Assumption

The to_remove slice is sorted. This is crucial for the correct functioning of binary_search.

Components of the Function

1. Iterating Over Trait Implementations

for map in self.impls.values_mut() {

self.impls is a HashMap containing trait implementations. The .values_mut() method gives mutable access to the inner maps, which are also HashMaps.

2. Using drain to Remove All Elements

.map.drain()

The drain() method removes all elements from the inner HashMap and returns an iterator over the removed elements. This empties the map, allowing you to selectively rebuild it.

3. Filtering and Adjusting Remaining Elements

.filter_map(|(type_args, mut impl_data)| {
    match to_remove.binary_search(&impl_data.index) {
        Ok(_) => None,
        Err(index) => {
            impl_data.index -= index;
            Some((type_args, impl_data))
        }
    }
})

filter_map: Filters and transforms elements in one step.

• Returns None for elements to be removed.
• Returns Some((type_args, impl_data)) for elements to keep, adjusting their index.

binary_search: Searches to_remove for impl_data.index:

• Ok(_): The index exists in to_remove, so the implementation is removed.
• Err(index): The index is adjusted by subtracting the number of removed elements before it (index).

4. Collecting the Updated Map

.collect();

The filter_map iterator produces only the elements to be kept, with updated indices. collect reconstructs the HashMap from the filtered elements.

Detailed Explanation of binary_search

The binary_search method plays a central role in this function. It determines whether an index should be removed or adjusted.

Return Values of binary_search

• Ok(_): The value is in the to_remove list.
• Err(index): The value is not in to_remove. The index indicates the position where it would fit, which is equal to the number of elements in to_remove that are less than this value.

Example

let sorted_array = [1, 3, 5, 7];
let value = 4;

match sorted_array.binary_search(&value) {
    Ok(_) => println!("Value is in the array."),
    Err(index) => println!("Value not found. It would fit at index: {}", index),
}

Output:

Value not found. It would fit at index: 2

Example Workflow of remove_trait_impls

Input Data

self.impls = {
    "trait1": {
        [Type::Int]: ImplData { index: 0, function: ... },
        [Type::Float]: ImplData { index: 1, function: ... },
        [Type::String]: ImplData { index: 2, function: ... },
    },
    "trait2": {
        [Type::Int]: ImplData { index: 0, function: ... },
        [Type::Bool]: ImplData { index: 1, function: ... },
    }
};

to_remove = [1]; // Remove trait impls with index 1

Execution Steps

1. For trait1, [Type::Float] is removed, and [Type::String] has its index adjusted to 1.
2. For trait2, [Type::Bool] is removed.

Final Output

self.impls = {
    "trait1": {
        [Type::Int]: ImplData { index: 0, function: ... },
        [Type::String]: ImplData { index: 1, function: ... },
    },
    "trait2": {
        [Type::Int]: ImplData { index: 0, function: ... },
    }
};

Summary

The remove_trait_impls function efficiently:

• Removes trait implementations based on the to_remove list.
• Adjusts the indices of the remaining implementations to maintain consistency.
• Relies on the sorted to_remove list and the behavior of binary_search for performance.