dft_rast_classify(): classify factor input from its raw codes (#91)

[NewGraphEnvironment]

Summary

• dft_rast_classify() returned a factor with no levels and no colours when given a raster that was already a factor, for example a published floodplain raster with a RAT, or a raster re-classified. The cause: terra::unique() returns a factor's labels, not its codes. The fix strips the levels on a copy (strip_copy(), which already existed) before reading the codes. The caller's raster is untouched.
• The strip runs after the remap. terra::classify() already reads raw codes, so a remap that matches pays nothing extra. A remap that matches nothing had the same bug on main, and now works.
• is.factor(x)[1]: the plan review and code-check round 1 each found, independently, that a bare is.factor(x) errors on a multi-layer stack. main classified a stack's layer 1 without error, and that behaviour is kept.

Scale check (BULK classified_2017.tif, 169M cells, published RAT, /usr/bin/time -l)

input	main peak	branch peak	levels out (main → branch)
file-backed factor	1.05 GiB	1.05 GiB	0 → 9
in-memory factor	4.20 GiB	5.47 GiB	0 → 9
in-memory factor + matching remap	5.47 GiB	5.47 GiB	8 → 8

The one cost is an in-memory factor with no matching remap: one full extra copy. No R-level terra call reads a factor's codes without a copy (activeCat<- and levels<- both deepcopy()).

Related Issues

• Fixes dft_rast_classify() returns empty levels when its input is already a factor #91
• Relates to NewGraphEnvironment/sred-2025-2026#16

Test plan

• devtools::test(): 1344 pass / 0 fail / 15 skip (network and bfast, opt-in)
• New tests fail on main (9 assertions) and pass on the branch: re-classify, file-backed RAT with its own labels and palette, remap on a factor (matched and unmatched), caller unmodified, active category ≠ 1, 2-layer stacks
• Mutation checks: an in-place strip turns both caller-unmodified tests red; dropping [1] turns the stack test red
• /code-check: 3 rounds (the stack error, then Clean, Clean). Round 3 checked every NEWS claim against a copy of main.

Notes

Planning record: planning/archive/2026-09-issue-91-classify-factor-input/, which includes the plan review and the three review rounds.

🤖 Generated with Claude Code

https://claude.ai/code/session_01PRhUJsuKABLfpBGktPoiBN