using System;

//abc
static void Match_ABC(ReadOnlySpan<char> input)
{
    int ptr = 0;
    int region_start = ptr;
    int region_end = region_start;

    const string pattern = @"abc";
    if(input[0..3].SequenceEqual(pattern))
    {
        ptr += 3;
        region_end = ptr;
    }
}

//a[abc]d
static void Match_A_charset_ABC_C(ReadOnlySpan<char> input)
{
    int ptr = 0;
    int region_start = ptr;
    int region_end = region_start;

    if(input[0] == 'a' && (input[1] == 'a' || input[1] == 'b' || input[1] == 'c') && input[2] == 'd')
    {
        ptr += 3;
        region_end = ptr;
    }
}

//[abc]
static void Match_charset_ABC(ReadOnlySpan<char> input)
{
    int ptr = 0;
    int region_start = ptr;
    int region_end = region_start;

    if(input[0] == 'a' || input[0] == 'b' || input[0] == 'c')
    {
        ptr += 1;
        region_end = ptr;
    }
}

//.
static void Match_Dot(ReadOnlySpan<char> input)
{
    int ptr = 0;
    int region_start = ptr;
    int region_end = region_start;

    ptr += 1;
    region_end = ptr;
}

//a{3,5}

static void Match_A_3_5(ReadOnlySpan<char> input)
{
    int ptr = 0;
    int region_start = ptr;
    int region_end = region_start;

    do
    {
        if(input[ptr..].StartsWith("aaa"))
        {
            ptr += 3;
            region_end = ptr;
        }
        else
        {
            break;
        }
        for(int i = 0; i < 2; i++)
        {
            if(input[ptr] == 'a')
            {
                ptr += 1;
                region_end = ptr;
            }
            else
            {
                break;
            }
        }

    } while(false);
}

//(ab){2,3}
static void Match_AB_2_3(ReadOnlySpan<char> input)
{
    int ptr = 0;
    int region_start = ptr;
    int region_end = region_start;

    do
    {
        if(input[ptr..].StartsWith("abab"))
        {
            ptr += 4;
            region_end = ptr;
        }
        else
        {
            break;
        }
        for(int i = 0; i < 1; i++)
        {
            if(input[ptr..].StartsWith("ab"))
            {
                ptr += 2;
                region_end = ptr;
            }
            else
            {
                break;
            }
        }

    } while(false);
}

// (ab|[cd]|.)x
static void MatchDiffCase(ReadOnlySpan<char> seq)
{
    /*
     * [000] 0001, DEF , Sccs, NONE, 0000, MATCH_SEQ "ab"
     * [001] 0001, Fail, Sccs, LINE, 0000, MATCH_SET "[cd]"
     * [002] 0001, Fail, Sccs, LINE, 0001, MATCH_SET "."
     * [003] 0000, Fail, DEF , LINE, 0002, POP FAIL
     * [004] 0000, DEF , DEF , NONE, 0000, MATCH_SEQ "x"
    */
    
    int ptr = 0;
    if(seq.StartsWith("ab"))
    {
        ptr+=2;
    }
    else if((seq[0] == 'c' || seq[0] == 'd'))
    {
        ptr+=1;
    }
    else if(true)
    {
        ptr+=1;
    }
    else
    {
        return;
    }
}

// (?<name>.*)x
static void MatchCountAnyGroup(ReadOnlySpan<char> seq)
{
    /*
     * [000] 0001, DEF , DEF , NONE, 0000, FIND_COMPLEX FURTHEST COUNT: -1
     * [001] 0001, Sccs, DEF , LINE, 0000, PACK SUCCESS GROUP: "name"
     * [002] 0000, DEF , DEF , NONE, 0000, MATCH_SEQ "234"
    */

    int ptr = 0;
    
    int group_start = ptr;
    int furthest = -1;
    do
    {
        if((furthest = seq.LastIndexOf("123"))>0)
        {
            ptr += furthest + 3;
            break;
        }
    } while(true);
    // grouping
    if(furthest >= 0)
    {
        ptr = furthest;
        int group_end = ptr;
        // group "name" matched from group_start to group_end
    }
}