How Can I Efficiently Access Random Runes in Go Strings Without Using For Loops?

Accessing Random Rune Elements in Strings

Problem

How can random rune elements within a string be accessed efficiently without relying on "for ... range" loops?

Background

Go strings store UTF-8 encoded byte sequences. While "for ... range" loops allow efficient decoding and retrieval of runes, accessing runes via a direct function like "str.At(i)" is unavailable. This leaves developers seeking alternative methods for frequent rune access.

Solution

As mentioned in the provided response, the following limitations apply:

• String values in Go contain UTF-8 byte sequences, necessitating decoding for rune retrieval.
• The string representation does not provide direct rune access at arbitrary indices.

Recommendation

For performance optimization, consider converting input strings to a slice of runes ([]rune) if rune access is required frequently. Unlike strings, which are effectively read-only byte slices, slices of runes allow efficient indexing.

Alternative

If input conversion is not feasible, a cache can be implemented to map strings to their corresponding rune slices. This improves performance for cases involving a small set of recurring strings. However, for unique or infrequent strings, caching can become inefficient and consume excessive memory.

Example

The following code snippet demonstrates a simple cache implementation for rune retrieval:

import (
    "fmt"
    "sync"
)

var cache = sync.Map{}

func RuneAt(s string, idx int) rune {
    rs, ok := cache.Load(s)
    if !ok {
        rs = []rune(s)
        cache.Store(s, rs)
    }
    if idx >= len(rs) {
        return 0
    }
    return rs[idx]
}

func main() {
    str := "你好，世界！"
    for i := 0; i < len(str); i++ {
        fmt.Printf("%c ", RuneAt(str, i))
    }
}

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