.

wcfs/internal/xbtree/δbtail.go

@@ -36,6 +36,7 @@ type Bucket = btree.LOBucket
 type Node   = btree.LONode
 type Key    = int64
 type Value  = zodb.Oid // XXX assumes key points to IPersistent
+                       // XXX how to represent deletion? InvalidOid?
 
 // ΔTail represents tail of revisional changes to BTrees.
 //
@@ -98,10 +99,10 @@ type ΔRoots struct {
 //
 // See ΔTail documentation for details.
 type ΔTreeTail struct {
-	δTtail []ΔTree // changes to tree keys; covers keys ∈ tracket subset
+	δTtail []ΔTree // changes to tree keys; rev↑. covers keys ∈ tracket subset
 
 	// {}k/v @tail for keys that are changed in (tail, head].
-	atTail map[*Tree]map[Key]Value
+	KVAtTail map[Key]Value
 }
 
 // ΔTree describes changes to one BTree in one revision.
@@ -238,8 +239,37 @@ func (btail *ΔTail) Update(δZ *zodb.EventCommit) []ΔTree {
 //
 // XXX at must ∈ (tail, head]	XXX [tail ?
 // XXX key must be tracked?
+//
+// XXX -> meth of ΔTreeTail ?
 func (btail *ΔTail) GetAt(root *Tree, key Key, at zodb.Tid) (value Value, ok bool, rev zodb.Tid, revExact bool) {
-	panic("TODO")
+	// XXX key not tracked		-> panic
+	// XXX at not ∈ (tail, head]	-> panic
+
+	// XXX -> index lastXXXOf(key)
+	δt := btail.byRoot[root]
+	for i := len(δt.δTtail)-1; i >= 0; i-- {
+		δ := δt.δTtail[i]
+		if at < δ.Rev {
+			continue
+		}
+		value, ok = δ.KV[key]
+		if ok {
+			rev = δ.Rev
+			revExact = true
+			break
+		}
+	}
+
+	// found key in history tail
+	if ok {
+		return
+	}
+
+	// key not in history tail
+	value, ok = δt.KVAtTail[key]
+	rev = btail.Tail()
+	revExact = false
+	return
 }
 
 func (btail *ΔTail) LastRevOf(root *Tree, key Key, at zodb.Tid) (_ zodb.Tid, exact bool) {

wcfs/wcfs.go

@@ -1095,7 +1095,6 @@ func (f *BigFile) readBlk(ctx context.Context, blk int64, dest []byte) (err erro
 	}
 
 	// noone was loading - we became responsible to load this block
-//	blkdata, treepath, pathRevMax, err := f.zfile.LoadBlk(ctx, blk)
 	blkdata, treepath, zblkrev, err := f.zfile.LoadBlk(ctx, blk)
 	loading.blkdata = blkdata
 	loading.err = err