[GGUF] buildGgufHeader function

[mishig25]

buildGgufHeader: GGUF header reconstruction with proper alignment

🎯 Why serializeGgufMetadata #1740 isn't enough

serializeGgufMetadata only handles metadata serialization but lacks critical functionality:

• ❌ No tensor info preservation - loses original tensor definitions
• ❌ No alignment calculation - creates invalid memory layout
• ❌ Incomplete headers - generates metadata-only files
• ❌ Framework incompatible - resulting files can't be loaded

🚀 buildGgufHeader: Complete solution

buildGgufHeader is a comprehensive function that uses serializeGgufMetadata internally but adds essential missing pieces:

export async function buildGgufHeader(originalFileBlob, updatedMetadata, options) {
  // 1. ✅ Use serializeGgufMetadata for metadata
  const newHeaderBytes = serializeGgufMetadata(updatedMetadata, {
    littleEndian: options.littleEndian,
    alignment,
  });

  // 2. ✅ Preserve original tensor info (missing in serializeGgufMetadata)
  const originalTensorInfoBlob = originalFileBlob.slice(tensorInfoStart, tensorInfoEnd);

  // 3. ✅ Calculate proper alignment (missing in serializeGgufMetadata)  
  const prePadLenNew = kvEndOffset + tensorInfoSize;
  const GGML_PAD = (x, n) => (x + n - 1) & ~(n - 1);
  const targetTensorDataOffset = GGML_PAD(prePadLenNew, alignment);
  const padLen = targetTensorDataOffset - prePadLenNew;

  // 4. ✅ Reconstruct complete, valid header
  return new Blob([
    newHeaderBytes.slice(0, kvEndOffset),  // Metadata from serializeGgufMetadata
    originalTensorInfoBlob,                // Preserved tensor info
    new Uint8Array(padLen)                 // Correct alignment padding
  ]);
}

📊 Comparison

Function	Metadata	Tensor Info	Alignment	Complete Header
serializeGgufMetadata	✅	❌	❌	❌
buildGgufHeader	✅	✅	✅	✅

🎯 Key Innovation

buildGgufHeader = serializeGgufMetadata + tensor preservation + alignment calculation

This creates production-ready GGUF files that maintain framework compatibility while enabling metadata updates.

[mishig25]

Merging it to get the moon PR going. Happy to address comments in follow up PR

[julien-c]

ok 🤷