Add a function to fingerprint flag/offset info.

Uses SipHasher implementation added to the project. Ideally the output
would remain consistent between builds (though the worst case of an
updated algorithm would be a string lookup for cross-container reads
only).

Sort the flags first so everything is always added to the hasher in the
same order - note this code is only run at build time, not runtime.

Next step is to write this fingerprint into the package.map file and
introduce a flag to guard this change.

Bug: 316357686
Test: atest aconfig.test
Change-Id: Ie5f34541d982dfa120ffb05fc0790603689d47c0

tools/aconfig/aconfig/src/commands.rs

@@ -17,7 +17,8 @@
 use anyhow::{bail, ensure, Context, Result};
 use itertools::Itertools;
 use protobuf::Message;
-use std::collections::HashMap;
+use std::collections::{BTreeMap, HashMap};
+use std::hash::Hasher;
 use std::io::Read;
 use std::path::PathBuf;
 
@@ -31,6 +32,7 @@ use aconfig_protos::{
     ParsedFlagExt, ProtoFlagMetadata, ProtoFlagPermission, ProtoFlagState, ProtoParsedFlag,
     ProtoParsedFlags, ProtoTracepoint,
 };
+use aconfig_storage_file::sip_hasher13::SipHasher13;
 use aconfig_storage_file::StorageFileType;
 
 pub struct Input {
@@ -410,11 +412,42 @@ where
     Ok(flag_ids)
 }
 
+#[allow(dead_code)] // TODO: b/316357686 - Use fingerprint in codegen to
+                    // protect hardcoded offset reads.
+pub fn compute_flag_offsets_fingerprint(flags_map: &HashMap<String, u16>) -> Result<u64> {
+    let mut hasher = SipHasher13::new();
+
+    // Need to sort to ensure the data is added to the hasher in the same order
+    // each run.
+    let sorted_map: BTreeMap<&String, &u16> = flags_map.iter().collect();
+
+    for (flag, offset) in sorted_map {
+        // See https://docs.rs/siphasher/latest/siphasher/#note for use of write
+        // over write_i16. Similarly, use to_be_bytes rather than to_ne_bytes to
+        // ensure consistency.
+        hasher.write(flag.as_bytes());
+        hasher.write(&offset.to_be_bytes());
+    }
+    Ok(hasher.finish())
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
     use aconfig_protos::ProtoFlagPurpose;
 
+    #[test]
+    fn test_offset_fingerprint() {
+        let parsed_flags = crate::test::parse_test_flags();
+        let package = find_unique_package(&parsed_flags.parsed_flag).unwrap().to_string();
+        let flag_ids = assign_flag_ids(&package, parsed_flags.parsed_flag.iter()).unwrap();
+        let expected_fingerprint = 10709892481002252132u64;
+
+        let hash_result = compute_flag_offsets_fingerprint(&flag_ids);
+
+        assert_eq!(hash_result.unwrap(), expected_fingerprint);
+    }
+
     #[test]
     fn test_parse_flags() {
         let parsed_flags = crate::test::parse_test_flags(); // calls parse_flags