PowerToys/doc/devdocs/modules/powerdisplay/MCCS_PARSER_DESIGN.md

MCCS Capabilities String Parser - Recursive Descent Design

Overview

This document describes the recursive descent parser implementation for DDC/CI MCCS (Monitor Control Command Set) capabilities strings.

Grammar Definition (BNF)

capabilities      ::= ['('] segment* [')']
segment           ::= identifier '(' segment_content ')'
segment_content   ::= text | vcp_entries | hex_list
vcp_entries       ::= vcp_entry*
vcp_entry         ::= hex_byte [ '(' hex_list ')' ]
hex_list          ::= hex_byte*
hex_byte          ::= [0-9A-Fa-f]{2}
identifier        ::= [a-z_A-Z]+
text              ::= [^()]+

Example Input

(prot(monitor)type(lcd)model(PD3220U)cmds(01 02 03 07)vcp(10 12 14(04 05 06) 16 60(11 12 0F) DC DF)mccs_ver(2.2)vcpname(F0(Custom Setting)))

Parser Architecture

Component Hierarchy

MccsCapabilitiesParser (main parser)
├── ParseCapabilities()      → MccsParseResult
├── ParseSegment()           → ParsedSegment?
├── ParseBalancedContent()   → string
├── ParseIdentifier()        → ReadOnlySpan<char>
├── ApplySegment()           → void
│   ├── ParseHexList()       → List<byte>
│   ├── ParseVcpEntries()    → Dictionary<byte, VcpCodeInfo>
│   └── ParseVcpNames()      → void
│
├── VcpEntryParser (sub-parser for vcp() content)
│   └── TryParseEntry()      → VcpEntry
│
├── VcpNameParser (sub-parser for vcpname() content)
│   └── TryParseEntry()      → (byte code, string name)
│
└── WindowParser (sub-parser for windowN() content)
    ├── Parse()              → WindowCapability
    └── ParseSubSegment()    → (name, content)?

Design Principles

1. ref struct for Zero Allocation

   • Main parser uses ref struct to avoid heap allocation
   • Works with ReadOnlySpan<char> for efficient string slicing
   • No intermediate string allocations during parsing

2. Recursive Descent Pattern

   • Each grammar rule has a corresponding parse method
   • Methods call each other recursively for nested structures
   • Single-character lookahead via Peek()

3. Error Recovery

   • Errors are accumulated, not thrown
   • Parser attempts to continue after errors
   • Returns partial results when possible

4. Sub-parsers for Specialized Content

   • VcpEntryParser for VCP code entries
   • VcpNameParser for custom VCP names
   • Each sub-parser handles its own grammar subset

Parse Methods Detail

ParseCapabilities()

Entry point. Handles optional outer parentheses and iterates through segments.

private MccsParseResult ParseCapabilities()
{
    // Handle optional outer parens
    // while (!IsAtEnd()) { ParseSegment() }
    // Return result with accumulated errors
}

ParseSegment()

Parses a single identifier(content) segment.

private ParsedSegment? ParseSegment()
{
    // 1. ParseIdentifier()
    // 2. Expect '('
    // 3. ParseBalancedContent()
    // 4. Expect ')'
}

ParseBalancedContent()

Extracts content between balanced parentheses, handling nested parens.

private string ParseBalancedContent()
{
    int depth = 1;
    while (depth > 0) {
        if (char == '(') depth++;
        if (char == ')') depth--;
    }
}

ParseVcpEntries()

Delegates to VcpEntryParser for the specialized VCP entry grammar.

vcp_entry ::= hex_byte [ '(' hex_list ')' ]

Examples:
- "10"           → code=0x10, values=[]
- "14(04 05 06)" → code=0x14, values=[4, 5, 6]
- "60(11 12 0F)" → code=0x60, values=[0x11, 0x12, 0x0F]

Comparison with Other Approaches

Approach	Pros	Cons
Recursive Descent (this)	Clear structure, handles nesting, extensible	More code
Regex (DDCSharp)	Concise	Hard to debug, limited nesting
Mixed (original)	Pragmatic	Inconsistent, hard to maintain

Performance Characteristics

• Time Complexity: O(n) where n = input length
• Space Complexity: O(1) for parsing + O(m) for output where m = number of VCP codes
• Allocations: Minimal - only for output structures

Supported Segments

Segment	Description	Parser
prot(...)	Protocol type	Direct assignment
type(...)	Display type (lcd/crt)	Direct assignment
model(...)	Model name	Direct assignment
cmds(...)	Supported commands	ParseHexList
vcp(...)	VCP code entries	VcpEntryParser
mccs_ver(...)	MCCS version	Direct assignment
vcpname(...)	Custom VCP names	VcpNameParser
windowN(...)	PIP/PBP window capabilities	WindowParser

Window Segment Format

The windowN segment (where N is 1, 2, 3, etc.) describes PIP/PBP window capabilities:

window1(type(PIP) area(25 25 1895 1175) max(640 480) min(10 10) window(10))

Sub-field	Format	Description
type	type(PIP) or type(PBP)	Window type (Picture-in-Picture or Picture-by-Picture)
area	area(x1 y1 x2 y2)	Window area coordinates in pixels
max	max(width height)	Maximum window dimensions
min	min(width height)	Minimum window dimensions
window	window(id)	Window identifier

All sub-fields are optional; missing fields default to zero values.

Error Handling

public readonly struct ParseError
{
    public int Position { get; }    // Character position
    public string Message { get; }  // Human-readable error
}

public sealed class MccsParseResult
{
    public VcpCapabilities Capabilities { get; }
    public IReadOnlyList<ParseError> Errors { get; }
    public bool HasErrors => Errors.Count > 0;
    public bool IsValid => !HasErrors && Capabilities.SupportedVcpCodes.Count > 0;
}

Usage Example

// Parse capabilities string
var result = MccsCapabilitiesParser.Parse(capabilitiesString);

if (result.IsValid)
{
    var caps = result.Capabilities;
    Console.WriteLine($"Model: {caps.Model}");
    Console.WriteLine($"MCCS Version: {caps.MccsVersion}");
    Console.WriteLine($"VCP Codes: {caps.SupportedVcpCodes.Count}");
}

if (result.HasErrors)
{
    foreach (var error in result.Errors)
    {
        Console.WriteLine($"Parse error at {error.Position}: {error.Message}");
    }
}

Edge Cases Handled

1. Missing outer parentheses (Apple Cinema Display)
2. No spaces between hex bytes (010203 vs 01 02 03)
3. Nested parentheses in VCP values
4. Unknown segments (logged but not fatal)
5. Malformed input (partial results returned)