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HeaderDiff Codec


The specification for the HeaderDiff codec is at:

Delta-encoding modifications

The implementation support three modes for delta-encoding header values. In addition, delta-encoding can be disabled.

The full mode searches for the largest shared prefix between the reference value and the value to encode.

The bounded mode limits the shared prefix, forcing it to end with a character contained in a list of limit characters. For example: /?= ,.

The limit mode limits the number of times an indexed value can be used as a reference for delta-encoding another value.


The implementation now optionally supports a static Huffman encoding for string values.


The HeaderDiff codec supports the following options:

  • buffer for defining the maximum buffer size (default is 32768 bytes).
  • deflate for specifying the windowSize for Deflate. It is an integer between 8 and 15. By default, Deflate is not enabled.
  • delta for enabling or disabling delta-encoding (shared prefix). Enabled by default.
  • delta_type for specifying which type of delta-encoding to use.
    • Full mode: an empty value means that full prefix search is enabled.
    • Bounded mode: a string (possibly quoted) containing the characters defining the possible boundaries for the shared prefix.
    • Limit mode: an integer defining the maximum number of usage of an indexed value as a reference for delta-encoding another value.
  • huffman for enabling Huffman encoding of string values. Disabled by default.


Using the default HeaderDiff codec:

./ -c headerdiff file.har

Using HeaderDiff with a small buffer:

./ -c "headerdiff=buffer=4096" file.har

Using HeaderDiff with Deflate:

./ -c "headerdiff=deflate=12" file.har

Using both a small buffer and Deflate:

./ -c "headerdiff=buffer=4096,deflate=12" file.har

Using HeaderDiff without delta-encoding:

./ -c "headerdiff=delta=false" file.har

Using HeaderDiff with bounded delta-encoding:

./ -c "headerdiff=delta_type='/?= \coma'" file.har

Using HeaderDiff with limited delta-encoding:

./ -c "headerdiff=delta_type=2" file.har

Using HeaderDiff with a static Huffman encoding of strings:

./ -c "headerdiff=huffman" file.har
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