Urgh I made some wrong amends and I'm too tired to separate the commits out this time

ancestor: Split out of reachability
mergebase: Add merge base routines
internal/commitquery: Add commit query context engine thingy
internal/peel: Shared tag peeling
errors: Shared object query errors
internal/testgit: Add rooted repo helpers; remove raw path access
objectstore/memory: Add in-memory object store
objectid: Add Compare helper

1 files changed, 56 insertions, 0 deletions

diff --git a/mergebase/compute.go b/mergebase/compute.go
new file mode 100644
index 00000000..0fae0aed
--- /dev/null
+++ b/mergebase/compute.go
@@ -0,0 +1,56 @@
+package mergebase
+
+import (
+	"slices"
+
+	"codeberg.org/lindenii/furgit/internal/commitquery"
+	"codeberg.org/lindenii/furgit/internal/peel"
+	"codeberg.org/lindenii/furgit/objectid"
+)
+
+func (query *Bases) compute() ([]objectid.ObjectID, error) {
+	leftCommit, err := peel.ToCommit(query.store, query.left)
+	if err != nil {
+		return nil, err
+	}
+
+	rightCommit, err := peel.ToCommit(query.store, query.right)
+	if err != nil {
+		return nil, err
+	}
+
+	ctx := commitquery.NewContext(query.store, query.graph)
+
+	leftIdx, err := ctx.ResolveOID(leftCommit)
+	if err != nil {
+		return nil, err
+	}
+
+	rightIdx, err := ctx.ResolveOID(rightCommit)
+	if err != nil {
+		return nil, err
+	}
+
+	candidates, err := commitquery.MergeBases(ctx, leftIdx, rightIdx)
+	if err != nil {
+		return nil, err
+	}
+
+	slices.SortFunc(candidates, func(left, right commitquery.NodeIndex) int {
+		switch {
+		case ctx.CommitTime(left) > ctx.CommitTime(right):
+			return -1
+		case ctx.CommitTime(left) < ctx.CommitTime(right):
+			return 1
+		default:
+			return objectid.Compare(ctx.ID(left), ctx.ID(right))
+		}
+	})
+
+	out := make([]objectid.ObjectID, 0, len(candidates))
+	for _, idx := range candidates {
+		out = append(out, ctx.ID(idx))
+	}
+
+	return out, nil
+}