iterator: add functionality to move indices

Extract the index acrobatics from the message store and move it to
iterator package for re-use and to add unit tests.

Signed-off-by: Koni Marti <koni.marti@gmail.com>
Acked-by: Robin Jarry <robin@jarry.cc>

1 files changed, 51 insertions, 0 deletions

diff --git a/lib/iterator/index.go b/lib/iterator/index.go
new file mode 100644
index 00000000..dd660052
--- /dev/null
+++ b/lib/iterator/index.go
@@ -0,0 +1,51 @@
+package iterator
+
+// IndexProvider implements a subset of the Interator interface
+type IndexProvider interface {
+	StartIndex() int
+	EndIndex() int
+}
+
+// FixBounds will force the index i to either its lower- or upper-bound value
+// if out-of-bound
+func FixBounds(i, lower, upper int) int {
+	switch {
+	case i > upper:
+		i = upper
+	case i < lower:
+		i = lower
+	}
+	return i
+}
+
+// WrapBounds will wrap the index i around its upper- or lower-bound if
+// out-of-bound
+func WrapBounds(i, lower, upper int) int {
+	switch {
+	case i > upper:
+		i = lower + (i-upper-1)%upper
+	case i < lower:
+		i = upper - (lower-i-1)%upper
+	}
+	return i
+}
+
+type BoundsCheckFunc func(int, int, int) int
+
+// MoveIndex moves the index variable idx forward by delta steps and ensures
+// that the boundary policy as defined by the CheckBoundsFunc is enforced.
+//
+// If CheckBoundsFunc is nil, fix boundary checks are performed.
+func MoveIndex(idx, delta int, indexer IndexProvider, cb BoundsCheckFunc) int {
+	lower, upper := indexer.StartIndex(), indexer.EndIndex()
+	sign := 1
+	if upper < lower {
+		lower, upper = upper, lower
+		sign = -1
+	}
+	result := idx + sign*delta
+	if cb == nil {
+		return FixBounds(result, lower, upper)
+	}
+	return cb(result, lower, upper)
+}