feat(index): generic SortedIndex (slice + binary search)

sorted_index.go

@@ -0,0 +1,82 @@
+package main
+
+// SortedIndex — a generic sorted container with O(log N) insert and O(N) iteration
+// in sort order. Backed by a slice. For posts/2, this is fine; if we need true
+// O(log N) random access, swap for a btree.BTree from google/btree.
+//
+// The key is extracted from the value via Less. The index is NOT safe for
+// concurrent use; the Store's mu guards all access.
+type SortedIndex[T any] struct {
+	less func(a, b T) bool
+	items []T
+	index map[any]int // value-identity -> position in items (for fast Update/Delete)
+}
+
+func NewSortedIndex[T any](less func(a, b T) bool) *SortedIndex[T] {
+	return &SortedIndex[T]{
+		less:  less,
+		items: make([]T, 0, 64),
+		index: make(map[any]int, 64),
+	}
+}
+
+func (idx *SortedIndex[T]) Insert(v T) {
+	key := indexKey(v)
+	pos := idx.findPos(v)
+	idx.items = append(idx.items[:pos], append([]T{v}, idx.items[pos:]...)...)
+	// Rebuild index for positions >= pos (slice shifted right by 1)
+	for k, p := range idx.index {
+		if p >= pos {
+			idx.index[k] = p + 1
+		}
+	}
+	idx.index[key] = pos
+}
+
+func (idx *SortedIndex[T]) Update(v T) {
+	// Same value, but its sort key may have changed. Delete + re-insert.
+	idx.Delete(v)
+	idx.Insert(v)
+}
+
+func (idx *SortedIndex[T]) Delete(v T) {
+	key := indexKey(v)
+	pos, ok := idx.index[key]
+	if !ok {
+		return
+	}
+	idx.items = append(idx.items[:pos], idx.items[pos+1:]...)
+	delete(idx.index, key)
+	// Rebuild index for positions > pos (slice shifted left by 1)
+	for k, p := range idx.index {
+		if p > pos {
+			idx.index[k] = p - 1
+		}
+	}
+}
+
+func (idx *SortedIndex[T]) Items() []T {
+	out := make([]T, len(idx.items))
+	copy(out, idx.items)
+	return out
+}
+
+func (idx *SortedIndex[T]) Len() int { return len(idx.items) }
+
+// findPos — binary search for insertion point.
+func (idx *SortedIndex[T]) findPos(v T) int {
+	lo, hi := 0, len(idx.items)
+	for lo < hi {
+		mid := (lo + hi) / 2
+		if idx.less(idx.items[mid], v) {
+			lo = mid + 1
+		} else {
+			hi = mid
+		}
+	}
+	return lo
+}
+
+// indexKey — identity key. Pointer types (which *Post, *Comment are) work
+// because we use pointer identity. Value types would need a different scheme.
+func indexKey(v any) any { return v }