fix(detector): sort previous JSON dirs in descending order for diff

[MaineK00n]

Summary

Fixes #2536 — report --diff-plus keeps re-sending the same unfixed alerts because it compares the current scan against an ancient one (the second-oldest result dir) instead of the most recent previous scan.

Root cause

detector.ListValidJSONDirs sorts ascending, so dirs[0] is the oldest dir. loadPrevious then iterates dirs[1:] and breaks on the first matching JSON, which makes it pick the second-oldest as "previous". The function's own doc comment says the opposite ("recent directories are at the head"), and the sibling reporter.ListValidJSONDirs sorts descending.

This is a regression from #2415 (Go 1.26 bump), where the original

sort.Slice(dirs, func(i, j int) bool { return dirs[j] < dirs[i] })  // descending

was modernized to

slices.Sort(dirs)  // ascending — wrong direction

Fix

Match reporter/util.go's descending sort:

slices.SortFunc(dirs, func(a, b string) int {
    return -cmp.Compare(a, b)
})

Verification — before / after

Added TestListValidJSONDirs_SortedDescending that recreates the directory list from the discussion and asserts dirs[0] is the newest. Running the same test against master vs. this branch:

Before (on master, broken sort):

=== RUN   TestListValidJSONDirs_SortedDescending
    util_test.go:60: dirs[0] = .../2026-04-24T08-22-31+0000, want .../2026-05-01T15-33-21+0000
    util_test.go:60: dirs[1] = .../2026-04-25T08-27-25+0000, want .../2026-05-01T15-17-13+0000
    util_test.go:60: dirs[2] = .../2026-04-25T17-52-14+0000, want .../2026-05-01T12-57-29+0000
    ... (every position wrong, fully reversed)
    util_test.go:60: dirs[11] = .../2026-05-01T15-33-21+0000, want .../2026-04-24T08-22-31+0000
--- FAIL: TestListValidJSONDirs_SortedDescending (0.01s)
FAIL    github.com/future-architect/vuls/detector

Note that dirs[1] = 2026-04-25T08-27-25+0000 — exactly the path the discussion reporter saw in Previous json found: ....

After (this branch):

=== RUN   TestListValidJSONDirs_SortedDescending
--- PASS: TestListValidJSONDirs_SortedDescending (0.00s)
PASS
ok      github.com/future-architect/vuls/detector       0.032s

Test plan

• go build ./...
• go test ./detector/ -run TestListValidJSONDirs_SortedDescending -v — PASS on this branch, FAIL on master (see above)

🤖 Generated with Claude Code

References

• fix(report): sort datetime dir in descending order #2425

[Copilot]

Pull request overview

Fixes report --diff-plus selecting an older “previous” scan by ensuring detector.ListValidJSONDirs returns result directories in descending (newest-first) order, matching the documented behavior and reporter.ListValidJSONDirs.

Changes:

• Sort detector.ListValidJSONDirs results in descending order using slices.SortFunc + cmp.Compare.
• Add a unit test ensuring the returned directory list is newest-first and ignores non-timestamp dirs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
detector/util.go	Changes directory sorting to newest-first so diff logic selects the correct prior scan.
detector/util_test.go	Adds coverage verifying descending sort order and filtering of invalid directory names.

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[shino]

🥂