65 lines
2.7 KiB
Go
65 lines
2.7 KiB
Go
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package credits
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import (
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"crypto/sha256"
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"encoding/hex"
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"strconv"
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"strings"
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"time"
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)
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// Identifying a media *version*, not an item.
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//
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// An Emby item id survives a file being replaced, which is exactly the case this subsystem
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// must not get wrong: Sonarr swapping a 720p rip for a 1080p one leaves the id alone, and a
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// marker measured against the old file would put Skip Credits somewhere arbitrary in the new
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// one. Keying the marker on item *and* fingerprint means a replaced file simply stops
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// matching — the episode becomes a candidate again, and nothing had to notice the swap or
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// run an invalidation pass to make that happen.
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// MediaVersion is what a fingerprint is computed from. Every field is optional except the
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// item id: a source that can only offer runtime still produces a usable fingerprint, it is
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// just a coarser one.
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type MediaVersion struct {
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ItemID string
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RuntimeMs int64
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SizeBytes int64
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ModifiedAt time.Time
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// ETag is Emby's own version token where it offers one. It is the strongest field here
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// because it changes for reasons the other three can miss — a remux to the same length
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// and near-enough the same size.
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ETag string
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}
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// Fingerprint is a short, stable digest of a media version.
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//
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// Truncated to sixteen bytes because it is an equality key rather than a security claim: it
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// is only ever compared against another fingerprint of the same item, and a full digest
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// would double the width of the busiest index in the schema for no gain.
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func Fingerprint(version MediaVersion) string {
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parts := []string{
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strings.TrimSpace(version.ItemID),
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strconv.FormatInt(version.RuntimeMs, 10),
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strconv.FormatInt(version.SizeBytes, 10),
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strings.TrimSpace(version.ETag),
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}
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if !version.ModifiedAt.IsZero() {
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// Second resolution. Filesystems and Emby disagree about sub-second timestamps often
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// enough that finer granularity would invalidate markers on files nothing had
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// touched, which is the expensive direction to be wrong in.
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parts = append(parts, strconv.FormatInt(version.ModifiedAt.UTC().Unix(), 10))
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}
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sum := sha256.Sum256([]byte(strings.Join(parts, "|")))
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return hex.EncodeToString(sum[:16])
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}
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// Weak reports a fingerprint built from too little to detect a file swap.
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//
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// Runtime alone does not distinguish a re-encode of the same episode, so a marker stored
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// against a weak fingerprint is one that could survive a replacement it should not have. The
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// service refuses to store those rather than storing a marker it cannot invalidate — the
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// alternative is a wrong Skip Credits position that nothing will ever correct.
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func (v MediaVersion) Weak() bool {
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return v.SizeBytes <= 0 && strings.TrimSpace(v.ETag) == "" && v.ModifiedAt.IsZero()
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}
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