view: add per-remote view types and iterator that reconstructs BranchTarget

I'm planning to add support for untracked remote branches, and under that
model, there will be many remote branches without local counterparts. That's
the main reason why remote branches are grouped by remote, not by branch name.

The added helper functions will be used by simple_op_store and view.

lib/src/op_store.rs

@@ -16,7 +16,9 @@
 
 use std::collections::{BTreeMap, HashMap, HashSet};
 use std::fmt::{Debug, Error, Formatter};
+use std::iter;
 
+use itertools::Itertools as _;
 use once_cell::sync::Lazy;
 use thiserror::Error;
 
@@ -132,6 +134,15 @@ impl RefTarget {
     }
 }
 
+content_hash! {
+    /// Remote branch or tag.
+    #[derive(Clone, Debug, Eq, Hash, PartialEq)]
+    pub struct RemoteRef {
+        pub target: RefTarget,
+        // TODO: add tracking flag or enum
+    }
+}
+
 /// Helper to strip redundant `Option<T>` from `RefTarget` lookup result.
 pub trait RefTargetOptionExt {
     type Value;
@@ -192,6 +203,72 @@ content_hash! {
     }
 }
 
+content_hash! {
+    /// Represents the state of the remote repo.
+    #[derive(Clone, Debug, Default, Eq, PartialEq)]
+    pub struct RemoteView {
+        pub branches: BTreeMap<String, RemoteRef>,
+        // TODO: pub tags: BTreeMap<String, RemoteRef>,
+    }
+}
+
+/// Iterates pair of local and remote branches by branch name.
+#[allow(dead_code)] // TODO
+pub(crate) fn merge_join_branch_views<'a>(
+    local_branches: &'a BTreeMap<String, RefTarget>,
+    remote_views: &'a BTreeMap<String, RemoteView>,
+) -> impl Iterator<Item = (&'a str, BranchTarget)> {
+    let mut local_branches_iter = local_branches
+        .iter()
+        .map(|(branch_name, target)| (branch_name.as_str(), target))
+        .peekable();
+    let mut remote_branches_iter = flatten_remote_branches(remote_views).peekable();
+
+    iter::from_fn(move || {
+        // Pick earlier branch name
+        let (branch_name, local_target) =
+            if let Some(&((remote_branch_name, _), _)) = remote_branches_iter.peek() {
+                local_branches_iter
+                    .next_if(|&(local_branch_name, _)| local_branch_name <= remote_branch_name)
+                    .unwrap_or((remote_branch_name, RefTarget::absent_ref()))
+            } else {
+                local_branches_iter.next()?
+            };
+        // TODO: add borrowed version of BranchTarget?
+        let mut branch_target = BranchTarget {
+            local_target: local_target.clone(),
+            ..Default::default()
+        };
+        while let Some(((_, remote_name), target)) = remote_branches_iter
+            .next_if(|&((remote_branch_name, _), _)| remote_branch_name == branch_name)
+        {
+            branch_target
+                .remote_targets
+                .insert(remote_name.to_owned(), target.clone());
+        }
+        Some((branch_name, branch_target))
+    })
+}
+
+/// Iterates branch `((name, remote_name), target)`s in lexicographical order.
+#[allow(dead_code)] // TODO
+pub(crate) fn flatten_remote_branches(
+    remote_views: &BTreeMap<String, RemoteView>,
+) -> impl Iterator<Item = ((&str, &str), &RefTarget)> {
+    remote_views
+        .iter()
+        .map(|(remote_name, remote_view)| {
+            remote_view
+                .branches
+                .iter()
+                .map(move |(branch_name, remote_ref)| {
+                    let full_name = (branch_name.as_str(), remote_name.as_str());
+                    (full_name, &remote_ref.target)
+                })
+        })
+        .kmerge_by(|(full_name1, _), (full_name2, _)| full_name1 < full_name2)
+}
+
 content_hash! {
     /// Represents an operation (transaction) on the repo view, just like how a
     /// Commit object represents an operation on the tree.
@@ -258,3 +335,109 @@ pub trait OpStore: Send + Sync + Debug {
 
     fn write_operation(&self, contents: &Operation) -> OpStoreResult<OperationId>;
 }
+
+#[cfg(test)]
+mod tests {
+    use maplit::btreemap;
+
+    use super::*;
+
+    #[test]
+    fn test_merge_join_branch_views() {
+        let remote_ref = |target: &RefTarget| RemoteRef {
+            target: target.clone(),
+        };
+        let local_branch1_target = RefTarget::normal(CommitId::from_hex("111111"));
+        let local_branch2_target = RefTarget::normal(CommitId::from_hex("222222"));
+        let git_branch1_target = RefTarget::normal(CommitId::from_hex("333333"));
+        let git_branch2_target = RefTarget::normal(CommitId::from_hex("444444"));
+        let remote1_branch1_target = RefTarget::normal(CommitId::from_hex("555555"));
+        let remote2_branch2_target = RefTarget::normal(CommitId::from_hex("666666"));
+
+        let local_branches = btreemap! {
+            "branch1".to_owned() => local_branch1_target.clone(),
+            "branch2".to_owned() => local_branch2_target.clone(),
+        };
+        let remote_views = btreemap! {
+            "git".to_owned() => RemoteView {
+                branches: btreemap! {
+                    "branch1".to_owned() => remote_ref(&git_branch1_target),
+                    "branch2".to_owned() => remote_ref(&git_branch2_target),
+                },
+            },
+            "remote1".to_owned() => RemoteView {
+                branches: btreemap! {
+                    "branch1".to_owned() => remote_ref(&remote1_branch1_target),
+                },
+            },
+            "remote2".to_owned() => RemoteView {
+                branches: btreemap! {
+                    "branch2".to_owned() => remote_ref(&remote2_branch2_target),
+                },
+            },
+        };
+        assert_eq!(
+            merge_join_branch_views(&local_branches, &remote_views).collect_vec(),
+            vec![
+                (
+                    "branch1",
+                    BranchTarget {
+                        local_target: local_branch1_target.clone(),
+                        remote_targets: btreemap! {
+                            "git".to_owned() => git_branch1_target.clone(),
+                            "remote1".to_owned() => remote1_branch1_target.clone(),
+                        },
+                    },
+                ),
+                (
+                    "branch2",
+                    BranchTarget {
+                        local_target: local_branch2_target.clone(),
+                        remote_targets: btreemap! {
+                            "git".to_owned() => git_branch2_target.clone(),
+                            "remote2".to_owned() => remote2_branch2_target.clone(),
+                        },
+                    },
+                ),
+            ],
+        );
+
+        // Local only
+        let local_branches = btreemap! {
+            "branch1".to_owned() => local_branch1_target.clone(),
+        };
+        let remote_views = btreemap! {};
+        assert_eq!(
+            merge_join_branch_views(&local_branches, &remote_views).collect_vec(),
+            vec![(
+                "branch1",
+                BranchTarget {
+                    local_target: local_branch1_target.clone(),
+                    remote_targets: btreemap! {},
+                },
+            )],
+        );
+
+        // Remote only
+        let local_branches = btreemap! {};
+        let remote_views = btreemap! {
+            "remote1".to_owned() => RemoteView {
+                branches: btreemap! {
+                    "branch1".to_owned() => remote_ref(&remote1_branch1_target),
+                },
+            },
+        };
+        assert_eq!(
+            merge_join_branch_views(&local_branches, &remote_views).collect_vec(),
+            vec![(
+                "branch1",
+                BranchTarget {
+                    local_target: RefTarget::absent(),
+                    remote_targets: btreemap! {
+                        "remote1".to_owned() => remote1_branch1_target.clone(),
+                    },
+                },
+            )],
+        );
+    }
+}