Rolling-Checksum Byte Scanner: Delta Sync in 2D

File Editing (local)

Block size 64 B

Mutation kind

Mutation size 15%

Network

Uplink bandwidth 20 Mbps

Transfer result

Bandwidth saved vs. full re-upload
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Rolling-scan literal

0 B

Aligned-only literal

0 B

Weak-hash rejects

Full re-upload

0 B

Rolling sync time

0.0 s

Full-upload time

0.0 s

How this differs from a fixed-block compare

The file is a 4096-byte buffer drawn as two 64×64 grids: baseline (cloud copy) on top, local copy below. This engine implements the actual rsync byte-level algorithm, not just per-index block comparison:

weak(k,S)  = a mod M, b mod M   (M = 65521)
             a = Σ X_i,  b = Σ (S-i)·X_i,  i over window [k, k+S)
rolling:     a' = a - X_k + X_{k+S}
             b' = b - S·X_k + a'      → O(1) per byte, no re-sum
strong(B)  = FNV-1a 32-bit hash over the window (confirms a weak hit)

The local scanner slides the window one byte at a time and looks up the weak value in a table of the baseline's fixed-offset block checksums. This finds a matching block at a shifted offset after an insert/delete — something a per-index comparison (dashed "aligned-only" number) cannot do, because every block after the shift point has a different index even though its bytes are unchanged. That's the whole reason real delta-sync tools use a rolling hash instead of hashing each aligned block once.

Phase: idle — mutate the file, then sync

Unchanged byte Mutated (pending) Copy instruction (matched) Literal (uploaded)