--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hg-core/src/revset.rs	Tue Jan 26 18:31:46 2021 +0100
@@ -0,0 +1,53 @@
+//! The revset query language
+//!
+//! <https://www.mercurial-scm.org/repo/hg/help/revsets>
+
+use crate::repo::Repo;
+use crate::revlog::changelog::Changelog;
+use crate::revlog::revlog::{Revlog, RevlogError};
+use crate::revlog::NodePrefix;
+use crate::revlog::{Revision, NULL_REVISION};
+
+/// Resolve a query string into a single revision.
+///
+/// Only some of the revset language is implemented yet.
+pub fn resolve_single(
+    input: &str,
+    repo: &Repo,
+) -> Result<Revision, RevlogError> {
+    let changelog = Changelog::open(repo)?;
+
+    match resolve_rev_number_or_hex_prefix(input, &changelog.revlog) {
+        Err(RevlogError::InvalidRevision) => {} // Try other syntax
+        result => return result,
+    }
+
+    if input == "null" {
+        return Ok(NULL_REVISION);
+    }
+
+    // TODO: support for the rest of the language here.
+
+    Err(RevlogError::InvalidRevision)
+}
+
+/// Resolve the small subset of the language suitable for revlogs other than
+/// the changelog, such as in `hg debugdata --manifest` CLI argument.
+///
+/// * A non-negative decimal integer for a revision number, or
+/// * An hexadecimal string, for the unique node ID that starts with this
+///   prefix
+pub fn resolve_rev_number_or_hex_prefix(
+    input: &str,
+    revlog: &Revlog,
+) -> Result<Revision, RevlogError> {
+    if let Ok(integer) = input.parse::<i32>() {
+        if integer >= 0 && revlog.has_rev(integer) {
+            return Ok(integer);
+        }
+    }
+    if let Ok(prefix) = NodePrefix::from_hex(input) {
+        return revlog.get_node_rev(prefix);
+    }
+    Err(RevlogError::InvalidRevision)
+}