[Bug] r.category removes map title

[petrasovaa]

Describe the bug

When you run r.support setting a raster title and then run r.category setting categories with labels, the title is empty afterwards

To reproduce

tools.r_proj(project="nlcd", mapset="PERMANENT", input="nlcd", output=landuse, resolution=30)
tools.r_support(map=landuse, title="USGS National Land Cover Data 2024")
categories = \
"""
0:Unclassified
11:Open Water
12:Perennial Snow/Ice
21:Developed, Open Space
22:Developed, Low Intensity
23:Developed, Medium Intensity
24:Developed, High Intensity
31:Barren Land
41:Deciduous Forest
42:Evergreen Forest
43:Mixed Forest
52:Shrub/Scrub
71:Grasslands/Herbaceous
81:Pasture/Hay
82:Cultivated Crops
90:Woody Wetlands
95:Emergent Herbaceous Wetlands
"""
tools.r_category(map=landuse, rules=io.StringIO(categories), separator=":")
tools.r_info(map=landuse, format="json")["title"]

and the result is empty string.

Additional context

I briefly looked into it and it seems there are 2 different titles - for categories and for the raster - Rast_set_cats_title and Rast_put_cell_title and they are both writing into the same file so that's the source of this. But I don't know whether both make sense, or which one is legacy.

[Vedant-Bachhav]

Hi,
I have submitted a PR addressing this issue #7210

[HUN-sp]

Hi @petrasovaa @wenzeslaus,

I've been looking into this bug and wanted to share my findings before opening a PR,.

Root cause

In raster/r.category/main.c, the rules= path initializes the category struct with an empty title:

Rast_init_cats("", &cats);

After parsing the rules, Rast_write_cats rewrites the entire cats file from scratch — and since cats->title is "", the title set earlier by r.support is permanently lost.

My approach

I use a separate temporary struct solely to extract the title string, free it immediately, then initialize the working struct fresh with only the preserved title:

struct Categories existing_cats;
char *existing_title;

if (Rast_read_cats(name, G_mapset(), &existing_cats) >= 0) {
existing_title = G_store(existing_cats.title);
Rast_free_cats(&existing_cats);
}
else {
existing_title = G_store("");
}
Rast_init_cats(existing_title, &cats);
G_free(existing_title);

Since Rast_init_cats resets labels, ncats, and all quantization rules to empty, new rules are applied to a clean slate ,no bleed-through.

Test

I've added a pytest test test_r_category_rules_preserves_title to raster/r.category/tests/r_category_test.py that:

Creates a raster map
Sets a title via r.support title=...
Sets categories via r.category rules=-
Reads the cats file directly and asserts the title is still the original value
All 12 tests pass (11 pre-existing + 1 new).

[petrasovaa]

It's hard to read it like that, please use markdown code formatting, but it sounds like a good approach, so a PR would be helpful.

[HUN-sp]

@petrasovaa I'll wrap these updates into a PR and send it over for your review now. And apologies for bad formatting.