custom dup sort example

heed/examples/custom-dupsort-comparator.rs

@@ -0,0 +1,62 @@
+use std::cmp::Ordering;
+use std::error::Error;
+use std::fs;
+use std::path::Path;
+
+use byteorder::BigEndian;
+use heed::{DatabaseFlags, EnvOpenOptions};
+use heed_traits::Comparator;
+use heed_types::{Str, U128};
+
+enum DescendingIntCmp {}
+
+impl Comparator for DescendingIntCmp {
+    fn compare(a: &[u8], b: &[u8]) -> Ordering {
+        b.cmp(&a)
+    }
+}
+
+fn main() -> Result<(), Box<dyn Error>> {
+    let env_path = Path::new("target").join("custom-dupsort-cmp.mdb");
+
+    let _ = fs::remove_dir_all(&env_path);
+
+    fs::create_dir_all(&env_path)?;
+    let env = unsafe {
+        EnvOpenOptions::new()
+            .map_size(10 * 1024 * 1024) // 10MB
+            .max_dbs(3)
+            .open(env_path)?
+    };
+
+    let mut wtxn = env.write_txn()?;
+    let db = env
+        .database_options()
+        .types::<Str, U128<BigEndian>>()
+        .flags(DatabaseFlags::DUP_SORT)
+        .dup_sort_comparator::<DescendingIntCmp>()
+        .create(&mut wtxn)?;
+    wtxn.commit()?;
+
+    let mut wtxn = env.write_txn()?;
+
+    // We fill our database with entries.
+    db.put(&mut wtxn, "1", &1)?;
+    db.put(&mut wtxn, "1", &2)?;
+    db.put(&mut wtxn, "1", &3)?;
+    db.put(&mut wtxn, "2", &4)?;
+    db.put(&mut wtxn, "1", &5)?;
+    db.put(&mut wtxn, "0", &0)?;
+
+    // We check that the keys are in lexicographic and values in descending order.
+    let mut iter = db.iter(&wtxn)?;
+    assert_eq!(iter.next().transpose()?, Some(("0", 0)));
+    assert_eq!(iter.next().transpose()?, Some(("1", 5)));
+    assert_eq!(iter.next().transpose()?, Some(("1", 3)));
+    assert_eq!(iter.next().transpose()?, Some(("1", 2)));
+    assert_eq!(iter.next().transpose()?, Some(("1", 1)));
+    assert_eq!(iter.next().transpose()?, Some(("2", 4)));
+    drop(iter);
+
+    Ok(())
+}