Decide decay from the ranks read out of the store

[wyne]

Summary

writeFileStore totalled the final slice, so the paths being added and their rank + 1/rank increments counted toward the decay threshold. fasd accumulates count inside its $2 >= 1 block:

$2 >= 1 {
    if($1 in paths) { rank[$1] = $2 + 1 / $2; time[$1] = now }
    else            { rank[$1] = $2;          time[$1] = $3  }
    count += $2                                   # the rank as read, nothing else
}
END {
    if(count > max) for(i in rank) print i "|" 0.9*rank[i] "|" time[i]
    else            for(i in rank) print i "|" rank[i]     "|" time[i]
}

Three things follow from where that line sits, none of which held before:

• New paths are not counted. They are created in BEGIN and never pass through the body.
• Increments are not counted. count += $2 uses the value as read, not $2 + 1/$2.
• Pruned lines contribute nothing. The accumulator is inside the $2 >= 1 guard.

Follow-up to #48, where this surfaced while fixing the batching bug.

Effect

Measured against binaries built from main at 05fd6c7 and this branch:

case	main	this PR / fasd
high|1999 + -A p1 p2	high|1799.1 p1|0.9 p2|0.9	high|1999 p1|1 p2|1
high|2001 + -A p1 p2	high|1800.9 p1|0.9 p2|0.9	unchanged
high|2000 + -A high	high|1800.00045	high|2000.0005

Row 1 is the practical one: a store sitting just under the threshold got aged early simply because two new paths were added in the same operation. Row 3 shows the same effect from an increment alone — 2000 is not over 2000, but 2000.0005 is.

Row 2 confirms genuine decay still fires, so this narrows when aging happens rather than disabling it.

Validation

• go test ./..., -shuffle=on x3, go vet ./..., gofmt clean
• 3 new tests, one per rule above, verified against two mutations: restoring the final-slice total fails all 3, and moving the total to before the prune fails the pruned-lines case
• TestAddPathsAppliesDecayOnceForTheWholeArgumentList from Drop entries whose rank has decayed below 1 #48 still passes, pinning row 2

Touches filestore.go only.

🤖 Generated with Claude Code