Implement JPEG-LS Run Mode Encoding (Phase 5.2)

[Copilot]

Implements run mode encoding for JPEG-LS compression, complementing the existing regular mode encoder. Run mode efficiently compresses flat regions by encoding consecutive identical pixel values.

Implementation

• JPEGLSRunMode.swift: Complete run mode encoder

  • Entry detection: Ra == Rb condition
  • Adaptive J[RUNindex] mapping (0, 1, 2, ..., min(i, 32))
  • Run-length encoding with continuation bits and remainder
  • Run interruption handling with prediction error encoding
  • Context adaptation: RUNindex increases for long runs, decreases for short runs
  • Split handling for runs exceeding maxRunLength

• Test suite: 42 tests covering detection, encoding, interruption, adaptation, edge cases

Usage Example

let runMode = try JPEGLSRunMode(parameters: presetParams, near: 0)

// Check if we should enter run mode
if runMode.shouldEnterRunMode(a: leftPixel, b: topPixel) {
    // Detect run length
    let runLength = runMode.detectRunLength(
        pixels: scanLine,
        startIndex: position,
        runValue: leftPixel
    )
    
    // Encode the run
    let encoded = runMode.encodeRunLength(runLength: runLength, runIndex: currentIndex)
    
    // If interrupted, encode the differing pixel
    if let interruption = interruptionPixel {
        let encodedInterruption = runMode.encodeRunInterruption(
            interruptionValue: interruption,
            runValue: leftPixel
        )
    }
    
    // Adapt context for next run
    let newIndex = runMode.adaptRunIndex(
        currentRunIndex: currentIndex,
        completedRunLength: runLength
    )
}

Standards Compliance

ITU-T.87 (ISO/IEC 14495-1:1999) Sections 4.5.1-4.5.4

Next Phase

Phase 5.3: Near-lossless encoding with NEAR parameter support

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[Copilot]

Pull request overview

This PR implements Phase 5.2 of the JPEG-LS encoder: Run Mode Encoding. Run mode is an optimization for encoding flat image regions where consecutive pixels have identical values. The implementation follows the ITU-T.87 standard and complements the existing regular mode encoder from Phase 5.1.

Changes:

• Added complete run mode encoder implementation with adaptive J[RUNindex] mapping, run-length detection, encoding with continuation bits, run interruption handling, and context adaptation
• Added comprehensive test suite with 42 tests covering all run mode functionality including detection, encoding, interruption, adaptation, and edge cases
• Updated README.md and MILESTONES.md to reflect Phase 5.2 completion and 100% test coverage

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
Sources/JPEGLS/Encoder/JPEGLSRunMode.swift	Complete run mode encoder implementation with detection, encoding, interruption handling, and context adaptation
Tests/JPEGLSTests/JPEGLSRunModeTests.swift	Comprehensive test suite with 42 tests achieving 100% code coverage
README.md	Updated to mark Phase 5.2 as completed with test coverage statistics
MILESTONES.md	Updated Phase 5.2 checklist items to completed status

[SureshKViswanathan]

@copilot fix the comments

[Copilot]

@copilot fix the comments

Could you clarify what specific comments you'd like me to fix? The automated code review found no issues, and the CI is passing with 100% test coverage on the run mode implementation. If you're referring to specific code documentation, inline comments, or a different concern, please let me know and I'll address it.