[MRG] fix Signature.minhash API during sourmash sketch (#2329)

Addresses #1167 by fixing `signature_first_mh` in the signature FFI to
support both `KmerMinHash` and `KmerMinHashBTree`; this way, sketches
generated by the Python `_signatures_for_sketch_factory` (which are
BTree sketches in Rust) can be passed back to
Python.

Specifically, this PR:
* implements `From<&KmerMinHashBTree>` for `KmerMinHash`;
* adds an arm to the match statement in the FFI function to catch
`KmerMinHashBTree` structs and convert them into `KmerMinHash` structs
suitable for returning to Python;
* provides a default match arm to catch (e.g.) `HyperLogLog` and raise
an error back to Python reporting that it found an unsupported sketch
type;
* adds a little to the Python tests to prevent backsliding.

Note that I don't _think_ this does any more copying of sketches than
the current codebase does.

And, on that halcyonic day when the FFI properly wraps
`KmerMinHashBTree`, this code can be replaced by returning the proper
Rust object without conversion.

Fixes #1167.

src/core/src/ffi/signature.rs

@@ -4,6 +4,8 @@ use std::io;
 use std::os::raw::c_char;
 use std::slice;
 
+use crate::errors::SourmashError;
+
 use crate::encodings::HashFunctions;
 use crate::signature::Signature;
 use crate::sketch::Sketch;
@@ -165,11 +167,16 @@ ffi_fn! {
 unsafe fn signature_first_mh(ptr: *const SourmashSignature) -> Result<*mut SourmashKmerMinHash> {
     let sig = SourmashSignature::as_rust(ptr);
 
-    if let Some(Sketch::MinHash(mh)) = sig.signatures.get(0) {
-        Ok(SourmashKmerMinHash::from_rust(mh.clone()))
-    } else {
-        // TODO: need to select the correct one
-        unimplemented!()
+    match sig.signatures.get(0) {
+        Some(Sketch::MinHash(mh)) => {
+            Ok(SourmashKmerMinHash::from_rust(mh.clone()))
+        },
+        Some(Sketch::LargeMinHash(mh_btree)) => {
+            Ok(SourmashKmerMinHash::from_rust(mh_btree.into()))
+        },
+        _ => Err(SourmashError::Internal {
+            message: "found unsupported sketch type".to_string()
+        }),
     }
 }
 }

src/core/src/sketch/minhash.rs

@@ -1617,6 +1617,30 @@ impl From<KmerMinHashBTree> for KmerMinHash {
     }
 }
 
+impl From<&KmerMinHashBTree> for KmerMinHash {
+    fn from(other: &KmerMinHashBTree) -> KmerMinHash {
+        let mut new_mh = KmerMinHash::new(
+            other.scaled(),
+            other.ksize() as u32,
+            other.hash_function(),
+            other.seed(),
+            other.track_abundance(),
+            other.num(),
+        );
+
+        let mins = other.mins.iter().copied().collect();
+        let abunds = other
+            .abunds
+            .as_ref()
+            .map(|abunds| abunds.values().cloned().collect());
+
+        new_mh.mins = mins;
+        new_mh.abunds = abunds;
+
+        new_mh
+    }
+}
+
 impl From<KmerMinHash> for KmerMinHashBTree {
     fn from(other: KmerMinHash) -> KmerMinHashBTree {
         let mut new_mh = KmerMinHashBTree::new(

tests/test_sourmash_sketch.py

@@ -213,6 +213,10 @@ def test_dna_defaults():
     assert not params.hp
     assert not params.protein
 
+    siglist = factory()
+    sig = siglist[0]
+    sig.minhash
+
 
 def test_dna_override_1():
     factory = _signatures_for_sketch_factory(['k=21,scaled=2000,abund'],