Kavita/API/Comparators/StringLogicalComparer.cs

//(c) Vasian Cepa 2005
// Version 2
// Taken from: https://www.codeproject.com/Articles/11016/Numeric-String-Sort-in-C

using static System.Char;

namespace API.Comparators;

public static class StringLogicalComparer
{
    public static int Compare(string s1, string s2)
    {
        //get rid of special cases
        if((s1 == null) && (s2 == null)) return 0;
        if(s1 == null) return -1;
        if(s2 == null) return 1;

        if (string.IsNullOrEmpty(s1) && string.IsNullOrEmpty(s2)) return 0;
        if (string.IsNullOrEmpty(s1)) return -1;
        if (string.IsNullOrEmpty(s2)) return -1;

        //WE style, special case
        var sp1 = IsLetterOrDigit(s1, 0);
        var sp2 = IsLetterOrDigit(s2, 0);
        if(sp1 && !sp2) return 1;
        if(!sp1 && sp2) return -1;

        int i1 = 0, i2 = 0; //current index
        while(true)
        {
            var c1 = IsDigit(s1, i1);
            var c2 = IsDigit(s2, i2);
            int r; // temp result
            if(!c1 && !c2)
            {
                bool letter1 = IsLetter(s1, i1);
                bool letter2 = IsLetter(s2, i2);
                if((letter1 && letter2) || (!letter1 && !letter2))
                {
                    if(letter1 && letter2)
                    {
                        r = ToLower(s1[i1]).CompareTo(ToLower(s2[i2]));
                    }
                    else
                    {
                        r = s1[i1].CompareTo(s2[i2]);
                    }
                    if(r != 0) return r;
                }
                else if(!letter1 && letter2) return -1;
                else if(letter1 && !letter2) return 1;
            }
            else if(c1 && c2)
            {
                r = CompareNum(s1, ref i1, s2, ref i2);
                if(r != 0) return r;
            }
            else if(c1)
            {
                return -1;
            }
            else if(c2)
            {
                return 1;
            }
            i1++;
            i2++;
            if((i1 >= s1.Length) && (i2 >= s2.Length))
            {
                return 0;
            }
            if(i1 >= s1.Length)
            {
                return -1;
            }
            if(i2 >= s2.Length)
            {
                return -1;
            }
        }
    }

    private static int CompareNum(string s1, ref int i1, string s2, ref int i2)
    {
        int nzStart1 = i1, nzStart2 = i2; // nz = non zero
        int end1 = i1, end2 = i2;

        ScanNumEnd(s1, i1, ref end1, ref nzStart1);
        ScanNumEnd(s2, i2, ref end2, ref nzStart2);
        var start1 = i1; i1 = end1 - 1;
        var start2 = i2; i2 = end2 - 1;

        var nzLength1 = end1 - nzStart1;
        var nzLength2 = end2 - nzStart2;

        if(nzLength1 < nzLength2) return -1;
        if(nzLength1 > nzLength2) return 1;

        for(int j1 = nzStart1,j2 = nzStart2; j1 <= i1; j1++,j2++)
        {
            var r = s1[j1].CompareTo(s2[j2]);
            if(r != 0) return r;
        }
        // the nz parts are equal
        var length1 = end1 - start1;
        var length2 = end2 - start2;
        if(length1 == length2) return 0;
        if(length1 > length2) return -1;
        return 1;
    }

    //lookahead
    private static void ScanNumEnd(string s, int start, ref int end, ref int nzStart)
    {
        nzStart = start;
        end = start;
        var countZeros = true;
        while(IsDigit(s, end))
        {
            if(countZeros && s[end].Equals('0'))
            {
                nzStart++;
            }
            else countZeros = false;
            end++;
            if(end >= s.Length) break;
        }
    }
}