Is Go's `append` Function Truly Constant Time, or Does Its Complexity Depend on Implementation?

Understanding append Complexity

The append function in Go is a fundamental operation used to extend slices or arrays. However, its time complexity can vary depending on the specific implementation. This article delves into the computational complexity of the append operation in the Go programming language.

Linear vs. Constant Time

The question arises whether append operates in linear time, where reallocation and copying occur on each append, or in amortized constant time, as seen in vector implementations in other languages.

Implementation-Dependent Complexity

According to the Go Programming Language Specification, append reallocates if necessary. The precise algorithm to grow the slice depends on the implementation. For the current gc compiler, the algorithm is amortized constant time.

Amortized Constant Time Algorithm

The Go gc compiler uses a dynamic array amortized constant time algorithm to grow the target slice when necessary. This algorithm ensures that the average time complexity of successive append operations remains constant, although individual operations may take longer at times.

Implementation Variations

It's important to note that the Go Programming Language Specification allows for different implementations of the append function. Implementors can choose to be parsimonious or generous in allocating memory. The Go gc compiler uses a generous algorithm, while other implementations may opt for a more parsimonious approach.

Examples of Different Implementations

The following code snippet illustrates two legal implementations of append. The first implementation uses a generous constant algorithm, while the second uses a parsimonious variable algorithm. Both algorithms are compared to the regular append function and the Go gccgo compiler.

Conclusion

The computational complexity of the append operation in Go depends on the implementation. The Go gc compiler uses an amortized constant time algorithm, providing efficient slice extension operations. However, implementations may vary, potentially affecting the time complexity of append. It's crucial to consider this variation when using append in performance-sensitive applications.

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