Demo prefilter rule for Nextstrain GISAID build

Adds a prefilter rule to reduce the size of the input metadata for the
GISAID build before running the whole workflow.

nextstrain_profiles/nextstrain-gisaid/builds.yaml

@@ -2,6 +2,7 @@ auspice_json_prefix: ncov_gisaid
 
 # Define custom rules for pre- or post-standard workflow processing of data.
 custom_rules:
+  - workflow/snakemake_rules/prefilter.smk
   - workflow/snakemake_rules/export_for_nextstrain.smk
 
 # These parameters are only used by the `export_for_nextstrain` rule and shouldn't need to be modified.
@@ -25,7 +26,7 @@ files:
 
 inputs:
   - name: gisaid
-    metadata: "s3://nextstrain-ncov-private/metadata.tsv.zst"
+    metadata: "data/prefiltered_metadata.tsv"
     aligned: "s3://nextstrain-ncov-private/aligned.fasta.zst"
     skip_sanitize_metadata: true
 

workflow/snakemake_rules/prefilter.smk

@@ -0,0 +1,26 @@
+rule download_metadata:
+    params:
+        metadata_url="s3://nextstrain-ncov-private/metadata.tsv.zst",
+    output:
+        metadata="data/metadata.tsv.zst",
+    shell:
+        """
+        aws s3 cp {params.metadata_url} {output.metadata}
+        """
+
+rule filter_metadata:
+    input:
+        metadata="data/metadata.tsv.zst",
+    output:
+        metadata="data/prefiltered_metadata.tsv",
+    params:
+        max_sequences=500000,
+        group_by="division year month",
+    shell:
+        """
+        augur filter \
+            --metadata {input.metadata} \
+            --subsample-max-sequences {params.max_sequences} \
+            --group-by {params.group_by} \
+            --output-metadata {output.metadata}
+        """