prepare range parsing (#427)

git-revision/Cargo.toml

@@ -19,6 +19,7 @@ serde1 = [ "serde", "git-hash/serde1", "git-object/serde1" ]
 [dependencies]
 git-hash = { version = "^0.9.4", path = "../git-hash" }
 git-object = { version = "^0.19.0", path = "../git-object" }
+
 hash_hasher = "2.0.3"
 thiserror = "1.0.26"
 serde = { version = "1.0.114", optional = true, default-features = false, features = ["derive"] }

git-revision/src/spec.rs

@@ -1,3 +1,21 @@
+/// How to interpret a revision specification, or `revspec`.
+#[derive(Debug, Copy, Clone, PartialOrd, PartialEq, Ord, Eq, Hash)]
+#[cfg_attr(feature = "serde1", derive(serde::Serialize, serde::Deserialize))]
+pub enum Kind {
+    /// A single revision specification, pointing at one reference.
+    Single,
+    /// Two revision specifications `a` and `b` where we want all commits from `b` that are not also in `a`.
+    Range,
+    /// Everything in `a` and `b` but no commit from any of their merge bases.
+    MergeBase,
+}
+
+impl Default for Kind {
+    fn default() -> Self {
+        Kind::Single
+    }
+}
+
 pub mod parse {
     #![allow(missing_docs)]
     use git_object::bstr::BStr;
@@ -8,12 +26,25 @@ pub mod parse {
         Delegate,
     }
 
+    /// A delegate to be informed about parse events, with methods split into three categories.
+    ///
+    /// - **Revisions** - which revision to use as starting point for…
+    /// - **Navigation** - where to go once from the initial revision.
+    /// - **range** - to learn if the specification is for a single or multiple references.
     pub trait Delegate {
         fn resolve_ref(&mut self, input: &BStr) -> Option<()>;
         fn find_by_prefix(&mut self, input: &BStr) -> Option<()>;
 
         fn nth_ancestor(&mut self, n: usize) -> Option<()>;
         fn nth_parent(&mut self, n: usize) -> Option<()>;
+
+        /// Set the kind of the specification, which happens only once if it happens at all.
+        /// In case this method isn't called, assume `Single`.
+        ///
+        /// Note that ranges don't necessarily assure that a second specification will be parsed.
+        /// If `^rev` is given, this method is called with [`spec::Kind::Range`][crate::spec::Kind::Range]
+        /// and no second specification is provided.
+        fn kind(&mut self, kind: crate::spec::Kind);
     }
 
     pub(crate) mod function {

git-revision/tests/spec/mod.rs

@@ -1,9 +1,11 @@
 mod parse {
     use git_object::bstr::{BStr, BString};
     use git_revision::spec;
+
     #[derive(Default, Debug)]
     struct Recorder {
         resolve_ref_input: Option<BString>,
+        kind: Option<spec::Kind>,
     }
     impl spec::parse::Delegate for Recorder {
         fn resolve_ref(&mut self, input: &BStr) -> Option<()> {
@@ -27,6 +29,10 @@ mod parse {
         fn nth_parent(&mut self, _n: usize) -> Option<()> {
             todo!()
         }
+
+        fn kind(&mut self, kind: spec::Kind) {
+            self.kind = Some(kind);
+        }
     }
 
     fn parse(spec: &str) -> Recorder {
@@ -35,15 +41,25 @@ mod parse {
         rec
     }
 
+    #[test]
+    #[ignore]
+    fn leading_caret_is_range_kind() {
+        let rec = parse("^HEAD");
+        assert_eq!(rec.kind.unwrap(), spec::Kind::Range);
+        assert_eq!(rec.resolve_ref_input.unwrap(), "HEAD");
+    }
+
     #[test]
     fn at_by_iteself_is_shortcut_for_head() {
         let rec = parse("@");
+        assert!(rec.kind.is_none());
         assert_eq!(rec.resolve_ref_input.unwrap(), "HEAD");
     }
 
     #[test]
     fn refname_head() {
         let rec = parse("HEAD");
+        assert!(rec.kind.is_none());
         assert_eq!(rec.resolve_ref_input.unwrap(), "HEAD");
     }
 }