diff --git a/src/db.rs b/src/db.rs
index 24a2687..d56a4a4 100644
--- a/src/db.rs
+++ b/src/db.rs
@@ -30,6 +30,20 @@ fn encode_volumes(v: &[String]) -> String {
     serde_json::to_string(v).unwrap()
 }
 
+/// Pure: compute exclusive upper bound for prefix range queries.
+/// Increments the last byte that isn't 0xFF.
+/// Examples: "abc" -> Some("abd"), "ab\xff" -> Some("ac"), "\xff\xff" -> None
+pub fn prefix_upper_bound(prefix: &str) -> Option<String> {
+    let mut bytes = prefix.as_bytes().to_vec();
+    while let Some(last) = bytes.pop() {
+        if last < 0xFF {
+            bytes.push(last + 1);
+            return Some(String::from_utf8_lossy(&bytes).into_owned());
+        }
+    }
+    None
+}
+
 /// A single SQLite connection behind a mutex, used for both reads and writes.
 #[derive(Clone)]
 pub struct Db {
@@ -93,19 +107,7 @@ impl Db {
                     .collect::<Result<Vec<String>, _>>()?;
                 return Ok(keys);
             }
-            // Compute exclusive upper bound: increment last non-0xFF byte
-            let upper = {
-                let mut bytes = prefix.as_bytes().to_vec();
-                let mut result = None;
-                while let Some(last) = bytes.pop() {
-                    if last < 0xFF {
-                        bytes.push(last + 1);
-                        result = Some(String::from_utf8_lossy(&bytes).into_owned());
-                        break;
-                    }
-                }
-                result
-            };
+            let upper = prefix_upper_bound(&prefix);
             let keys = match &upper {
                 Some(end) => {
                     let mut stmt = conn.prepare_cached(
@@ -198,3 +200,55 @@ impl Db {
         Ok(records)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_prefix_upper_bound_simple() {
+        assert_eq!(prefix_upper_bound("abc"), Some("abd".to_string()));
+        assert_eq!(prefix_upper_bound("a"), Some("b".to_string()));
+        assert_eq!(prefix_upper_bound("foo/bar"), Some("foo/bas".to_string()));
+    }
+
+    #[test]
+    fn test_prefix_upper_bound_paths() {
+        // '/' is 0x2F, +1 = '0' (0x30)
+        assert_eq!(prefix_upper_bound("users/"), Some("users0".to_string()));
+        // '_' is 0x5F, +1 = '`' (0x60)
+        assert_eq!(prefix_upper_bound("img_"), Some("img`".to_string()));
+    }
+
+    #[test]
+    fn test_prefix_upper_bound_empty() {
+        assert_eq!(prefix_upper_bound(""), None);
+    }
+
+    #[test]
+    fn test_prefix_upper_bound_single_char() {
+        assert_eq!(prefix_upper_bound("z"), Some("{".to_string())); // 'z' + 1 = '{'
+        assert_eq!(prefix_upper_bound("9"), Some(":".to_string())); // '9' + 1 = ':'
+    }
+
+    #[test]
+    fn test_prefix_upper_bound_range_correctness() {
+        // Verify the bound works for range queries:
+        // All strings starting with "foo" should be >= "foo" and < "fop"
+        let prefix = "foo";
+        let upper = prefix_upper_bound(prefix).unwrap();
+        assert_eq!(upper, "fop");
+
+        let upper = upper.as_str();
+
+        // These should be in range [foo, fop)
+        assert!("foo" >= prefix && "foo" < upper);
+        assert!("foo/bar" >= prefix && "foo/bar" < upper);
+        assert!("foobar" >= prefix && "foobar" < upper);
+        assert!("foo\x7f" >= prefix && "foo\x7f" < upper); // high ASCII
+
+        // These should be out of range
+        assert!("fop" >= upper);
+        assert!("fon" < prefix);
+    }
+}
diff --git a/src/rebuild.rs b/src/rebuild.rs
index f0f3121..5db2527 100644
--- a/src/rebuild.rs
+++ b/src/rebuild.rs
@@ -12,6 +12,25 @@ struct NginxEntry {
     size: Option<i64>,
 }
 
+/// Pure: merge volume scan results into a unified index.
+/// Each scan is (volume_url, list of (key, size) pairs).
+/// Returns a map of key -> (volumes containing it, max size seen).
+pub fn merge_volume_scans(
+    scans: &[(String, Vec<(String, i64)>)],
+) -> HashMap<String, (Vec<String>, i64)> {
+    let mut index: HashMap<String, (Vec<String>, i64)> = HashMap::new();
+    for (vol_url, keys) in scans {
+        for (key, size) in keys {
+            let entry = index.entry(key.clone()).or_insert_with(|| (Vec::new(), *size));
+            entry.0.push(vol_url.clone());
+            if *size > entry.1 {
+                entry.1 = *size;
+            }
+        }
+    }
+    index
+}
+
 async fn list_volume_keys(volume_url: &str) -> Result<Vec<(String, i64)>, String> {
     let http = reqwest::Client::new();
     let mut keys = Vec::new();
@@ -57,25 +76,23 @@ pub async fn run(args: &Args) {
     let _ = std::fs::remove_file(format!("{db_path}-shm"));
 
     let db = db::Db::new(db_path);
-    let mut index: HashMap<String, (Vec<String>, i64)> = HashMap::new();
 
+    // I/O: scan each volume
+    let mut scans = Vec::new();
     for vol_url in &args.volumes {
         eprintln!("Scanning {vol_url}...");
         match list_volume_keys(vol_url).await {
             Ok(keys) => {
                 eprintln!("  Found {} keys", keys.len());
-                for (key, size) in keys {
-                    let entry = index.entry(key).or_insert_with(|| (Vec::new(), size));
-                    entry.0.push(vol_url.clone());
-                    if size > entry.1 {
-                        entry.1 = size;
-                    }
-                }
+                scans.push((vol_url.clone(), keys));
             }
             Err(e) => eprintln!("  Error scanning {vol_url}: {e}"),
         }
     }
 
+    // Pure: merge scan results
+    let index = merge_volume_scans(&scans);
+
     let records: Vec<_> = index
         .into_iter()
         .map(|(k, (v, s))| (k, v, Some(s)))
@@ -84,3 +101,72 @@ pub async fn run(args: &Args) {
     db.bulk_put(records).await.expect("bulk_put failed");
     eprintln!("Rebuilt index with {count} keys");
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_merge_empty_scans() {
+        let scans: Vec<(String, Vec<(String, i64)>)> = vec![];
+        let index = merge_volume_scans(&scans);
+        assert!(index.is_empty());
+    }
+
+    #[test]
+    fn test_merge_single_volume() {
+        let scans = vec![(
+            "http://vol1".to_string(),
+            vec![
+                ("key1".to_string(), 100),
+                ("key2".to_string(), 200),
+            ],
+        )];
+        let index = merge_volume_scans(&scans);
+        assert_eq!(index.len(), 2);
+        assert_eq!(index.get("key1"), Some(&(vec!["http://vol1".to_string()], 100)));
+        assert_eq!(index.get("key2"), Some(&(vec!["http://vol1".to_string()], 200)));
+    }
+
+    #[test]
+    fn test_merge_key_on_multiple_volumes() {
+        let scans = vec![
+            ("http://vol1".to_string(), vec![("shared".to_string(), 100)]),
+            ("http://vol2".to_string(), vec![("shared".to_string(), 100)]),
+            ("http://vol3".to_string(), vec![("shared".to_string(), 100)]),
+        ];
+        let index = merge_volume_scans(&scans);
+        assert_eq!(index.len(), 1);
+        let (volumes, size) = index.get("shared").unwrap();
+        assert_eq!(volumes.len(), 3);
+        assert!(volumes.contains(&"http://vol1".to_string()));
+        assert!(volumes.contains(&"http://vol2".to_string()));
+        assert!(volumes.contains(&"http://vol3".to_string()));
+        assert_eq!(*size, 100);
+    }
+
+    #[test]
+    fn test_merge_takes_max_size() {
+        // Same key with different sizes on different volumes (corruption or update race)
+        let scans = vec![
+            ("http://vol1".to_string(), vec![("key".to_string(), 50)]),
+            ("http://vol2".to_string(), vec![("key".to_string(), 200)]),
+            ("http://vol3".to_string(), vec![("key".to_string(), 100)]),
+        ];
+        let index = merge_volume_scans(&scans);
+        let (_, size) = index.get("key").unwrap();
+        assert_eq!(*size, 200, "should take maximum size across volumes");
+    }
+
+    #[test]
+    fn test_merge_disjoint_keys() {
+        let scans = vec![
+            ("http://vol1".to_string(), vec![("a".to_string(), 10)]),
+            ("http://vol2".to_string(), vec![("b".to_string(), 20)]),
+        ];
+        let index = merge_volume_scans(&scans);
+        assert_eq!(index.len(), 2);
+        assert_eq!(index.get("a").unwrap().0, vec!["http://vol1".to_string()]);
+        assert_eq!(index.get("b").unwrap().0, vec!["http://vol2".to_string()]);
+    }
+}