Assisted-by: Claude Opus 4.6 via pi

diff --git a/crates/ranger-lib/src/error.rs b/crates/ranger-lib/src/error.rs
new file mode 100644
index 0000000..899704e
--- /dev/null
+++ b/crates/ranger-lib/src/error.rs
@@ -0,0 +1,11 @@
+#[derive(Debug, thiserror::Error)]
+pub enum RangerError {
+    #[error("no key matching prefix '{0}'")]
+    KeyNotFound(String),
+    #[error("ambiguous prefix '{0}' matches multiple keys")]
+    AmbiguousPrefix(String),
+    #[error("database error: {0}")]
+    Db(#[from] sqlx::Error),
+    #[error("io error: {0}")]
+    Io(#[from] std::io::Error),
+}
diff --git a/crates/ranger-lib/src/key.rs b/crates/ranger-lib/src/key.rs
new file mode 100644
index 0000000..3dea937
--- /dev/null
+++ b/crates/ranger-lib/src/key.rs
@@ -0,0 +1,75 @@
+use crate::error::RangerError;
+use rand::Rng;
+
+/// jj-style alphabet for pronounceable keys.
+const ALPHABET: &[char] = &[
+    'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z',
+];
+
+const KEY_LENGTH: usize = 16;
+
+pub fn generate_key() -> String {
+    let mut rng = rand::rng();
+    (0..KEY_LENGTH)
+        .map(|_| {
+            let idx = rng.random_range(0..ALPHABET.len());
+            ALPHABET[idx]
+        })
+        .collect()
+}
+
+pub fn resolve_prefix(prefix: &str, keys: &[String]) -> Result<String, RangerError> {
+    let matches: Vec<&String> = keys.iter().filter(|k| k.starts_with(prefix)).collect();
+    match matches.len() {
+        0 => Err(RangerError::KeyNotFound(prefix.to_string())),
+        1 => Ok(matches[0].clone()),
+        _ => Err(RangerError::AmbiguousPrefix(prefix.to_string())),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn generated_key_has_correct_length() {
+        let key = generate_key();
+        assert_eq!(key.len(), KEY_LENGTH);
+    }
+
+    #[test]
+    fn generated_key_uses_valid_alphabet() {
+        let key = generate_key();
+        for ch in key.chars() {
+            assert!(
+                ALPHABET.contains(&ch),
+                "key contains invalid character: {ch}"
+            );
+        }
+    }
+
+    #[test]
+    fn generated_keys_are_unique() {
+        let keys: Vec<String> = (0..100).map(|_| generate_key()).collect();
+        let unique: std::collections::HashSet<&String> = keys.iter().collect();
+        assert_eq!(keys.len(), unique.len());
+    }
+
+    #[test]
+    fn resolve_prefix_exact_match() {
+        let keys = vec!["romoqtuw".to_string(), "rypqxnkl".to_string()];
+        assert_eq!(resolve_prefix("rom", &keys).unwrap(), "romoqtuw");
+    }
+
+    #[test]
+    fn resolve_prefix_ambiguous() {
+        let keys = vec!["romoqtuw".to_string(), "romxnklp".to_string()];
+        assert!(resolve_prefix("rom", &keys).is_err());
+    }
+
+    #[test]
+    fn resolve_prefix_no_match() {
+        let keys = vec!["romoqtuw".to_string()];
+        assert!(resolve_prefix("xyz", &keys).is_err());
+    }
+}
diff --git a/crates/ranger-lib/src/lib.rs b/crates/ranger-lib/src/lib.rs
index dbc35b1..7915a9a 100644
--- a/crates/ranger-lib/src/lib.rs
+++ b/crates/ranger-lib/src/lib.rs
@@ -1,3 +1,2 @@
-pub fn hello() -> &'static str {
-    "ranger"
-}
+pub mod error;
+pub mod key;