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string json = """
{
  "test": 0,
  "data": "value"
}
""";

JsonStringRunner runner = new();

List<FAMatch> matches = new();
FAMatch current = default;
Stopwatch sw = new();
sw.Start();
do{
    current = runner.GetMatch(json);
    matches.Add(current);
} while(!runner.isDone);
sw.Stop();
matches.Dump();
sw.Dump();

internal record struct FAMatch(int token, string match, int position, int length, int column)
{
    internal static FAMatch Create(int token, string match, int position, int length, int column)
        => new(token, match, position, length, column);
}

internal abstract class FAStringRunner
{
    protected int position = -1, line = 0, column = 0;
    internal bool isDone = false;
}

internal sealed partial class JsonStringRunner : FAStringRunner
{
    private void Advance(string s, ref int ch, ref int len, bool flag)
    {
        // Assuming Advance takes consecutive characters in the string.
        ch = s[position];
        position++;
        len++;
        isDone = !(position < s.Length);
    }
    private FAMatch NextMatchImpl(string s)
    {
        int ch;
        int len;
        int l;
        int c;
        ch = -1;
        len = 0;
        if ((this.position is -1))
        {
            this.position = 0;
        }
        int p = this.position;
        l = this.line;
        c = this.column;
        this.Advance(s, ref ch, ref len, true);
        // q0:
        switch (ch)
        {
            // [\t-\n\r ]
            case 9 or 10 or 13 or 32:
                if(ch is 10 or 13){
                    l = line++;
                }
                return q1();
            // [\"]
            case 34:
                return q2();
            // [,]
            case 44:
                return q9();
            // [\-]
            case 45:
                return q10();
            // [0]
            case 48:
                return q11();
            // [1-9]
            case >= 49 and <= 57:
                return q17();
            // [\:]
            case 58:
                return q18();
            // [\[]
            case 91:
                return q19();
            // [\]]
            case 93:
                return q20();
            // [f]
            case 102:
                return q21();
            // [n]
            case 110:
                return q26();
            // [t]
            case 116:
                return q30();
            // [\{]
            case 123:
                return q32();
            // [\}]
            case 125:
                return q33();
        }
        return errorout();

        [MethodImpl(MethodImplOptions.AggressiveInlining)] FAMatch q1()
        {
            // [\t-\n\r ]
            while (ch is 9 or 10 or 13 or 32)
            {
                this.Advance(s, ref ch, ref len, false);
            }
            return FAMatch.Create(10, s.Substring(p, len), p, l, c);
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)] FAMatch q2()
        {
            // [\0-\t\v-!#-\[\]-\x10ffff]
            while (ch is (((((>= 0)
                        and (<= 9))
                        or ((>= 11)
                        and (<= 33)))
                        or ((>= 35)
                        and (<= 91)))
                        or ((>= 93)
                        and (<= 1114111))))
            {
                this.Advance(s, ref ch, ref len, false);
            }
            // [\"]
            if ((ch is 34))
            {
                this.Advance(s, ref ch, ref len, false);
                return q3();
            }
            // [\\]
            if ((ch is 92))
            {
                this.Advance(s, ref ch, ref len, false);
                return q4();
            }

            return default;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)] FAMatch q3() => FAMatch.Create(9, s.Substring(p, len), p, l, c);

        [MethodImpl(MethodImplOptions.AggressiveInlining)] FAMatch q4()
        {
            // [\"\/\\bfnrt]
            if (ch is 34 or 47
                        or 92
                        or 98
                        or 102
                        or 110
                        or 114
                        or 116)
            {
                this.Advance(s, ref ch, ref len, false);
                return q2();
            }

            // [u]
            if (ch is 117)
            {
                this.Advance(s, ref ch, ref len, false);
                return q5();
            }

            return errorout();
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)] FAMatch q5()
        {
            // [0-9A-Fa-f]
            if (ch is (>= 48 and <= 57) or (>= 65 and <= 70)
                        or (>= 97 and <= 102))
            {
                this.Advance(s, ref ch, ref len, false);
                return q6();
            }
            return errorout();
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)] FAMatch q6()
        {
            // [0-9A-Fa-f]
            if (ch is (>= 48 and <= 57) or (>= 65 and <= 70)
                        or (>= 97 and <= 102))
            {
                this.Advance(s, ref ch, ref len, false);
                return q7();
            }
            return errorout();
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)] FAMatch q7() => FAMatch.Create(7, s.Substring(p, len), p, l, c);
        [MethodImpl(MethodImplOptions.AggressiveInlining)] FAMatch q9() => FAMatch.Create(9, s.Substring(p, len), p, l, c);
        [MethodImpl(MethodImplOptions.AggressiveInlining)] FAMatch q10() => FAMatch.Create(10, s.Substring(p, len), p, l, c);
        [MethodImpl(MethodImplOptions.AggressiveInlining)] FAMatch q11() => FAMatch.Create(11, s.Substring(p, len), p, l, c);
        [MethodImpl(MethodImplOptions.AggressiveInlining)] FAMatch q17() => FAMatch.Create(17, s.Substring(p, len), p, l, c);
        [MethodImpl(MethodImplOptions.AggressiveInlining)] FAMatch q18() => FAMatch.Create(18, s.Substring(p, len), p, l, c);
        [MethodImpl(MethodImplOptions.AggressiveInlining)] FAMatch q19() => FAMatch.Create(19, s.Substring(p, len), p, l, c);
        [MethodImpl(MethodImplOptions.AggressiveInlining)] FAMatch q20() => FAMatch.Create(20, s.Substring(p, len), p, l, c);
        [MethodImpl(MethodImplOptions.AggressiveInlining)] FAMatch q21() => FAMatch.Create(21, s.Substring(p, len), p, l, c);
        [MethodImpl(MethodImplOptions.AggressiveInlining)] FAMatch q26() => FAMatch.Create(26, s.Substring(p, len), p, l, c);
        [MethodImpl(MethodImplOptions.AggressiveInlining)] FAMatch q30() => FAMatch.Create(110, s.Substring(p, len), p, l, c);
        [MethodImpl(MethodImplOptions.AggressiveInlining)] FAMatch q32() => FAMatch.Create(123, s.Substring(p, len), p, l, c);
        [MethodImpl(MethodImplOptions.AggressiveInlining)] FAMatch q33() => FAMatch.Create(125, s.Substring(p, len), p, l, c);

        [MethodImpl(MethodImplOptions.AggressiveInlining)] FAMatch errorout() => FAMatch.Create(0, s.Substring(p, len), p, l, c); // Dunno what you do in this section of code.
    }

    public FAMatch GetMatch(string s) => NextMatchImpl(s);
}

Quote:

Farewell to thee, farewell to thee
Thou charming one who dwells in shaded bowers
One fond embrace ere I depart
Until we meet again.

Chris Maunder wrote:

discussing, in painful detail, the issues that we face using our tools as part of a thoughtful dialogue