[ilovejedd]

Originally Posted by NiLuJe

There's possibly a bit of hashing involved, but take that with a grain of salt, as I'm extremely unfamiliar with that bit of code.

Are the actual files identical on your end? How do you transfer them? (Might be a case of Calibre using a different output template?).

Files are bit for bit identical. I was able to trace the digest function used.

Spoiler:

-- calculate partial digest of the document and store in its docsettings to avoid document saving
-- feature to change its checksum.
--
-- To the calculating mechanism itself.
-- since only PDF documents could be modified by KOReader by appending data
-- at the end of the files when highlighting, we use a non-even sampling
-- algorithm which samples with larger weight at file head and much smaller
-- weight at file tail, thus reduces the probability that appended data may change
-- the digest value.
-- Note that if PDF file size is around 1024, 4096, 16384, 65536, 262144
-- 1048576, 4194304, 16777216, 67108864, 268435456 or 1073741824, appending data
-- by highlighting in KOReader may change the digest value.
function Document:fastDigest(docsettings)
    if not self.file then return end
    local file = io.open(self.file, 'rb')
    if file then
        local tmp_docsettings = false
        if not docsettings then -- if not provided, open/create it
            docsettings = require("docsettings"):open(self.file)
            tmp_docsettings = true
        end
        local result = docsettings:readSetting("partial_md5_checksum")
        if not result then
            logger.dbg("computing and storing partial_md5_checksum")
            local bit = require("bit")
            local md5 = require("ffi/MD5")
            local lshift = bit.lshift
            local step, size = 1024, 1024
            local m = md5.new()
            for i = -1, 10 do
                file:seek("set", lshift(step, 2*i))
                local sample = file:read(size)
                if sample then
                    m:update(sample)
                else
                    break
                end
            end
            result = m:sum()
            docsettings:saveSetting("partial_md5_checksum", result)
        end
        if tmp_docsettings then
            docsettings:close()
        end
        file:close()
        return result
    end
end

Looks like KOSync uses the partial_md5_checksum as identifier for syncing. My problem was I just overwrote fanfics with updated versions leading to wrong partial_md5_checksum in metadata.epub.lua.

I deleted the sidecar folder forcing KOReader to recalculate the md5 sum and sync worked.