Microoptimizations

using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

class Program
{
    static void Main()
    {
        string sentence = "Hello how are you?";
        string targetWord = "how";
        string replaceWith = "what is";
        string indent = "  ";

        // Calculate lengths
        int originalLength = sentence.Length;
        int targetLength = targetWord.Length;
        int replacementLength = replaceWith.Length;
        int indentLength = indent.Length;

        // Calculate the length for the new sentence
        int newLength = originalLength - targetLength + replacementLength + (indentLength * 6); // 6 indentations

        Span<char> originalSpan = stackalloc char[originalLength];
        Span<char> newSpan = stackalloc char[newLength];

        // Copy original sentence and target word to spans
        sentence.AsSpan().CopyTo(originalSpan);
        targetWord.AsSpan().CopyTo(stackalloc char[targetLength]);

        int newSpanIndex = 0;

        // Add initial indentation
        CopyAndAdvance(ref newSpan, indent.AsSpan(), ref newSpanIndex);

        // Find and replace target word, add indentations
        for (int i = 0; i < originalSpan.Length; i++)
        {
            if (IsWordMatch(originalSpan.Slice(i, originalSpan.Length - i), targetWord.AsSpan()))
            {
                CopyAndAdvance(ref newSpan, replaceWith.AsSpan(), ref newSpanIndex);
                i += targetLength - 1; // Skip the target word in original sentence
            }
            else
            {
                newSpan[newSpanIndex++] = originalSpan[i];
            }

            // Add indentation after each word or at the end
            if (i == originalSpan.Length - 1 || originalSpan[i] == ' ')
            {
                CopyAndAdvance(ref newSpan, indent.AsSpan(), ref newSpanIndex);
            }
        }

        // Convert result back to string
        string result = new string(newSpan);
        Console.WriteLine($"Original: {sentence}");
        Console.WriteLine($"Modified: {result}");
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    static void CopyAndAdvance(ref Span<char> destination, ReadOnlySpan<char> source, ref int index)
    {
        source.CopyTo(destination.Slice(index));
        index += source.Length;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    static bool IsWordMatch(ReadOnlySpan<char> sentence, ReadOnlySpan<char> word)
    {
        return sentence.Length >= word.Length && sentence.Slice(0, word.Length).SequenceEqual(word);
    }
}