Allow rusterize-specific merge algorithms

- rasterio and exactextract use "replace" and "add"
- rusterize additionally accepts its native algorithms:
  "first", "min", "max", "count", "any"
- "replace" and "add" are translated to "last" and "sum" for rusterize
- rusterize native names can also be used directly with engine="rusterize"

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dcherian

src/rasterix/rasterize/core.py

@@ -95,22 +95,35 @@ def _get_mask_funcs(engine: Engine):
 
 
 def _normalize_merge_alg(merge_alg: str, engine: Engine) -> Any:
-    """Normalize merge_alg string to engine-specific value."""
-    valid_values = ("replace", "add")
-    if merge_alg not in valid_values:
-        raise ValueError(f"Invalid merge_alg {merge_alg!r}. Must be one of: {list(valid_values)}")
+    """Normalize merge_alg string to engine-specific value.
 
+    rasterio and exactextract use "replace" and "add".
+    rusterize uses "last" and "sum" (plus "first", "min", "max", "count", "any").
+    We translate "replace" -> "last" and "add" -> "sum" for rusterize.
+    """
     if engine == "rasterio":
         from rasterio.features import MergeAlg
 
         mapping = {
             "replace": MergeAlg.replace,
             "add": MergeAlg.add,
         }
+        if merge_alg not in mapping:
+            raise ValueError(
+                f"Invalid merge_alg {merge_alg!r} for rasterio. Must be one of: {list(mapping.keys())}"
+            )
         return mapping[merge_alg]
-    else:
-        # rusterize and exactextract handle the translation internally
+    elif engine == "exactextract":
+        valid_values = ("replace", "add")
+        if merge_alg not in valid_values:
+            raise ValueError(
+                f"Invalid merge_alg {merge_alg!r} for exactextract. Must be one of: {list(valid_values)}"
+            )
         return merge_alg
+    else:
+        # rusterize: translate common names, pass through native names
+        translation = {"replace": "last", "add": "sum"}
+        return translation.get(merge_alg, merge_alg)
 
 
 def replace_values(array: np.ndarray, to, *, from_=0) -> np.ndarray:
@@ -157,10 +170,11 @@ def rasterize(
         If True, all pixels touched by geometries will be burned in.
         If False, only pixels whose center is within the geometry are burned.
         Note: Not supported by rusterize or exactextract engines.
-    merge_alg : {"replace", "add"}
+    merge_alg : str
         Merge algorithm when geometries overlap.
         - "replace": later geometries overwrite earlier ones
         - "add": values are summed where geometries overlap
+        The rusterize engine also accepts: "first", "min", "max", "count", "any".
     geoms_rechunk_size : int or None
         Size to rechunk the geometry array to *after* conversion from dataframe.
     clip : bool

src/rasterix/rasterize/rusterize.py

@@ -44,7 +44,7 @@ def rasterize_geometries(
     affine: Affine,
     offset: int,
     all_touched: bool = False,
-    merge_alg: str = "replace",
+    merge_alg: str = "last",
     fill: Any = 0,
     **kwargs,
 ) -> np.ndarray:
@@ -67,7 +67,7 @@ def rasterize_geometries(
         If True, all pixels touched by geometries will be burned in.
         Note: rusterize may not support this parameter directly.
     merge_alg : str
-        Merge algorithm: "replace" or "add".
+        Merge algorithm. Supported values: "last", "sum", "first", "min", "max", "count", "any".
     fill : Any
         Fill value for pixels not covered by any geometry.
     **kwargs
@@ -94,18 +94,13 @@ def rasterize_geometries(
 
     extent, (xres, yres) = _affine_to_extent_and_res(affine, shape)
 
-    # Translate merge_alg to rusterize's native names
-    rusterize_merge_alg = {"replace": "last", "add": "sum"}.get(merge_alg)
-    if rusterize_merge_alg is None:
-        raise ValueError(f"Unsupported merge_alg: {merge_alg}. Must be 'replace' or 'add'.")
-
     result = rusterize(
         gdf,
         res=(xres, yres),
         extent=extent,
         out_shape=shape,
         field="value",
-        fun=rusterize_merge_alg,
+        fun=merge_alg,
         background=fill,
         encoding="numpy",
         dtype=str(dtype),