Add simple transaction ID prefix collision length calculator

examples/txid_collisions.rs

@@ -0,0 +1,49 @@
+extern crate electrs;
+
+extern crate hex;
+extern crate log;
+
+use electrs::{config::Config, store::DBStore};
+
+fn max_collision(store: DBStore, prefix: &[u8]) {
+    let prefix_len = prefix.len();
+    let mut prev: Option<Vec<u8>> = None;
+    let mut collision_max = 0;
+
+    for row in store.iter_scan(prefix) {
+        assert!(row.key.starts_with(prefix));
+        if let Some(prev) = prev {
+            let collision_len = prev
+                .iter()
+                .zip(row.key.iter())
+                .take_while(|(a, b)| a == b)
+                .count();
+            if collision_len > collision_max {
+                eprintln!(
+                    "{} bytes collision found:\n{:?}\n{:?}\n",
+                    collision_len - prefix_len,
+                    revhex(&prev[prefix_len..]),
+                    revhex(&row.key[prefix_len..]),
+                );
+                collision_max = collision_len;
+            }
+        }
+        prev = Some(row.key.to_vec());
+    }
+}
+
+fn revhex(value: &[u8]) -> String {
+    hex::encode(&value.iter().cloned().rev().collect::<Vec<u8>>())
+}
+
+fn run(config: Config) {
+    if !config.db_path.exists() {
+        panic!("DB {:?} must exist when running this tool!", config.db_path);
+    }
+    let store = DBStore::open(&config.db_path, /*low_memory=*/ false);
+    max_collision(store, b"T");
+}
+
+fn main() {
+    run(Config::from_args());
+}