EFCore.NamingConventions/EFCore.NamingConventions/NamingConventions/Internal/SnakeCaseNameRewriter.cs

using System;
using System.Globalization;
using System.Text;

namespace EFCore.NamingConventions.Internal
{
    public class SnakeCaseNameRewriter : INameRewriter
    {
        private readonly CultureInfo _culture;

        public SnakeCaseNameRewriter(CultureInfo culture) => _culture = culture;

        public virtual string RewriteName(string name)
        {
            if (string.IsNullOrEmpty(name))
            {
                return name;
            }

            var builder = new StringBuilder(name.Length + Math.Min(2, name.Length / 5));
            var previousCategory = default(UnicodeCategory?);

            for (var currentIndex = 0; currentIndex < name.Length; currentIndex++)
            {
                var currentChar = name[currentIndex];
                if (currentChar == '_')
                {
                    builder.Append('_');
                    previousCategory = null;
                    continue;
                }

                var currentCategory = char.GetUnicodeCategory(currentChar);
                switch (currentCategory)
                {
                case UnicodeCategory.UppercaseLetter:
                case UnicodeCategory.TitlecaseLetter:
                    if (previousCategory == UnicodeCategory.SpaceSeparator ||
                        previousCategory == UnicodeCategory.LowercaseLetter ||
                        previousCategory != UnicodeCategory.DecimalDigitNumber &&
                        previousCategory != null &&
                        currentIndex > 0 &&
                        currentIndex + 1 < name.Length &&
                        char.IsLower(name[currentIndex + 1]))
                    {
                        builder.Append('_');
                    }

                    currentChar = char.ToLower(currentChar, _culture);
                    break;

                case UnicodeCategory.LowercaseLetter:
                case UnicodeCategory.DecimalDigitNumber:
                    if (previousCategory == UnicodeCategory.SpaceSeparator)
                    {
                        builder.Append('_');
                    }
                    break;

                default:
                    if (previousCategory != null)
                    {
                        previousCategory = UnicodeCategory.SpaceSeparator;
                    }
                    continue;
                }

                builder.Append(currentChar);
                previousCategory = currentCategory;
            }

            return builder.ToString();
        }
    }
}