2 files changed, 184 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)
+}
diff --git a/lib/iterator/index_test.go b/lib/iterator/index_test.go
new file mode 100644
index 00000000..c9b79304
--- /dev/null
+++ b/lib/iterator/index_test.go
@@ -0,0 +1,133 @@
+package iterator_test
+
+import (
+	"testing"
+
+	"git.sr.ht/~rjarry/aerc/lib/iterator"
+)
+
+type indexer struct {
+	start int
+	end   int
+}
+
+func (ip *indexer) StartIndex() int {
+	return ip.start
+}
+
+func (ip *indexer) EndIndex() int {
+	return ip.end
+}
+
+func TestMoveIndex(t *testing.T) {
+	tests := []struct {
+		idx      int
+		delta    int
+		start    int
+		end      int
+		cb       iterator.BoundsCheckFunc
+		expected int
+	}{
+		{
+			idx:      0,
+			delta:    1,
+			start:    0,
+			end:      2,
+			cb:       iterator.FixBounds,
+			expected: 1,
+		},
+		{
+			idx:      0,
+			delta:    5,
+			start:    0,
+			end:      2,
+			cb:       iterator.FixBounds,
+			expected: 2,
+		},
+		{
+			idx:      0,
+			delta:    -1,
+			start:    0,
+			end:      2,
+			cb:       iterator.FixBounds,
+			expected: 0,
+		},
+		{
+			idx:      0,
+			delta:    2,
+			start:    0,
+			end:      2,
+			cb:       iterator.WrapBounds,
+			expected: 2,
+		},
+		{
+			idx:      0,
+			delta:    3,
+			start:    0,
+			end:      2,
+			cb:       iterator.WrapBounds,
+			expected: 0,
+		},
+		{
+			idx:      0,
+			delta:    -1,
+			start:    0,
+			end:      2,
+			cb:       iterator.WrapBounds,
+			expected: 2,
+		},
+		{
+			idx:      2,
+			delta:    2,
+			start:    0,
+			end:      2,
+			cb:       iterator.WrapBounds,
+			expected: 1,
+		},
+		{
+			idx:      0,
+			delta:    -2,
+			start:    0,
+			end:      2,
+			cb:       iterator.WrapBounds,
+			expected: 1,
+		},
+		{
+			idx:      1,
+			delta:    1,
+			start:    2,
+			end:      0,
+			cb:       iterator.FixBounds,
+			expected: 0,
+		},
+		{
+			idx:      0,
+			delta:    1,
+			start:    2,
+			end:      0,
+			cb:       iterator.FixBounds,
+			expected: 0,
+		},
+		{
+			idx:      0,
+			delta:    1,
+			start:    2,
+			end:      0,
+			cb:       iterator.WrapBounds,
+			expected: 2,
+		},
+	}
+
+	for i, test := range tests {
+		idx := iterator.MoveIndex(
+			test.idx,
+			test.delta,
+			&indexer{test.start, test.end},
+			test.cb,
+		)
+		if idx != test.expected {
+			t.Errorf("test %d [%#v] failed: got %d but expected %d",
+				i, test, idx, test.expected)
+		}
+	}
+}