How to Reverse a String in Go, Handling Unicode Combining Diacritical Marks?

Manipulating Strings in Go: A Detailed Guide to Reversing Characters

In Go, strings are treated as byte slices rather than character sequences. This can make it challenging to perform character-level operations like reversing strings.

Understanding the Problem:

Let's consider the following scenario: we have several strings of random characters with varying lengths (100, 200, 300, 400, and 500). We want to invert the characters of each string.

The Challenge:

Attempts to assign characters directly to different indices in the string result in an error, as Go does not allow direct character-level assignment to strings.

Unicode Considerations:

Andrew Sellers provides an innovative approach in his gist that handles Unicode combining diacritical marks (CDMs) and other complex Unicode characters.

He introduces a technique to detect and preserve the order of CDMs, which are part of a complex Unicode block that affects combining characters within a string.

The Solution:

The solution involves iterating through the string in reverse order and identifying CDMs using a range table. CDMs are stored, and then combined with regular characters before adding them to the reversed array.

Handling Complex Unicode Characters:

For characters like emojis and modifiers, the approach requires special consideration to preserve the order of elements and the correct representation of composed glyphs.

Implementing the Solution in Go:

Here's a code snippet based on Andrew Sellers' approach:

<code class="go">package main

import (
    "fmt"
    "os"
    "runtime"
    "unicode"
)

func main() {
    var stringsToReverse = []string{"Hello, World", "??⃠?", "??‍?????‍⚖️", "aͤoͧiͤ  š́ž́ʟ́", "H̙̖ell͔o̙̟͚͎̗̹̬ ̯W̖͝ǫ̬̞̜rḷ̦̣̪d̰̲̗͈"}
    for _, s := range stringsToReverse {
        fmt.Printf("Reverse '%s' => '%s'\n", s, ReverseString(s))
    }

    fmt.Printf("Memory usage: %d bytes\n", runtime.MemStats.Alloc)
    os.Exit(0)
}

// ReverseString reverses the characters in a string, handling Unicode combining diacritical marks
func ReverseString(s string) string {
    sv := []rune(s)
    cv := make([]rune, 0)
    rv := make([]rune, 0)

    for ix := len(sv) - 1; ix >= 0; ix-- {
        r := sv[ix]
        if unicode.In(r, combining) {
            cv = append(cv, r)
            fmt.Printf("Detect combining diacritical mark ' %c'\n", r)
        } else {
            rrv := make([]rune, 0, len(cv)+1)
            rrv = append(rrv, r)
            rrv = append(rrv, cv...)
            fmt.Printf("regular mark '%c' (with '%d' combining diacritical marks '%s') => '%s'\n", r, len(cv), string(cv), string(rrv))
            rv = append(rv, rrv...)
            cv = make([]rune, 0)
        }
    }

    return string(rv)
}</code>

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