Unraveling the Mystery: Why Does slices.BinarySearchFunc Call the Comparison Function with the Matched Element Twice?

The Curious Case of Binary Search

Binary search, a fundamental concept in computer science, has been a staple in many programming languages. In Go, the `sort` package provides a binary search function, `BinarySearch`, which takes a slice and a comparison function as arguments. However, a peculiar behavior has been observed: when the comparison function is called with the matched element, it’s called twice. But why?

A Brief Overview of Binary Search

Before diving into the mystery, let’s quickly revisit the basics of binary search. Binary search is a fast search algorithm that finds an element in a sorted array by dividing the search interval in half and repeatedly searching for the element in the appropriate half until it’s found.

+---------------+
|  sorted array  |
+---------------+
       |
       |
       v
+---------------+
|  mid element   |
+---------------+
       |
       |
       v
+---------------+
|  left half     |
|  or right half  |
+---------------+
       |
       |
       v
      ...

The Sort Package and BinarySearchFunc

In Go, the `sort` package provides a `BinarySearch` function, which takes a slice of any type and a comparison function as arguments. The comparison function is called repeatedly to determine the direction of the search. The function signature is as follows:

func BinarySearch(data []Type, f func(i int) int) int

The `BinarySearch` function returns the index of the matched element if found, or the index where the element should be inserted to maintain the sorted order.

The Mysterious Double Call

Now, let’s explore the curious case of the double call. When the comparison function is called with the matched element, it’s called twice. But why?

func main() {
    data := []int{1, 2, 3, 4, 5, 6, 7, 8, 9}
    idx := sort.BinarySearch(data, func(i int) int {
        fmt.Printf("Called with index %d, value %d\n", i, data[i])
        return data[i] - 5
    })
    fmt.Println("Matched index:", idx)
}

The output will show that the comparison function is called with the matched element (index 4, value 5) twice:

Called with index 4, value 5
Called with index 4, value 5
Matched index: 4

The Reason Behind the Double Call

The reason lies in the implementation of the `BinarySearch` function. The function uses a loop to narrow down the search interval. When the matched element is found, the loop continues to execute one more iteration to ensure that the element is indeed the exact match.

func BinarySearch(data []Type, f func(i int) int) int {
    lo, hi := 0, len(data) - 1
    for lo <= hi {
        mid := lo + (hi - lo) / 2
        cmp := f(mid)
        if cmp < 0 {
            lo = mid + 1
        } else if cmp > 0 {
            hi = mid - 1
        } else {
            // matched element found, but loop one more time to be sure
            hi = mid
        }
    }
    if lo == 0 || f(lo - 1) != 0 {
        return lo
    }
    return lo - 1
}

In the implementation, when the matched element is found, the loop sets `hi` to `mid`, which causes the loop to execute one more iteration. In this final iteration, the comparison function is called again with the matched element, resulting in the double call.

Why Not Optimize It Away?

One might wonder why the Go developers didn’t optimize the double call away. The reason is that the double call ensures the correctness of the binary search algorithm. By calling the comparison function one more time, the algorithm verifies that the matched element is indeed the exact match, rather than relying on the previous iteration’s result.

This extra iteration also helps to handle edge cases, such as when the slice contains duplicate elements or when the search element is not present in the slice.

Conclusion

In conclusion, the double call of the comparison function with the matched element in `BinarySearchFunc` is an intentional design decision that ensures the correctness and robustness of the binary search algorithm. While it may seem counterintuitive at first, this behavior is essential for handling edge cases and verifying the exact match.

As developers, it’s essential to understand the underlying implementation of the libraries and frameworks we use, rather than just relying on their surface-level behavior. By doing so, we can write more effective and efficient code that leverages the full potential of the language and its ecosystem.

Taking Away

• Binary search is a fundamental concept in computer science.
• The Go language’s `sort` package provides a `BinarySearch` function.
• The comparison function is called twice with the matched element.
• The double call ensures the correctness and robustness of the binary search algorithm.
• Understanding the underlying implementation is essential for writing effective and efficient code.

Function	Arguments	Returns
BinarySearch	slice of any type, comparison function	index of the matched element, or index where the element should be inserted

Remember, the next time you encounter a seemingly peculiar behavior in a library or framework, take a step back, and explore the underlying implementation. You never know what hidden gems you might discover!

Frequently Asked Question

Get ready to slice through the mystery of slices.BinarySearchFunc!