diff --git a/cmMath.c b/cmMath.c
index f740b1b..72ce7fd 100644
--- a/cmMath.c
+++ b/cmMath.c
@@ -154,6 +154,20 @@ unsigned cmPrevOddU(  unsigned v ) { return cmIsOddU(v)  ? v : v-1; }
 unsigned cmNextEvenU( unsigned v ) { return cmIsEvenU(v) ? v : v+1; }
 unsigned cmPrevEvenU( unsigned v ) { return cmIsEvenU(v) ? v : v-1; }
 
+unsigned cmModIncr(int idx, int delta, int maxN )
+{
+  int sum = idx + delta;
+
+  if( sum >= maxN )
+    return sum - maxN;
+
+  if( sum < 0 )
+    return maxN + sum;
+
+  return sum;
+}
+
+
 // modified bessel function of first kind, order 0
 // ref: orfandis appendix B io.m
 double	cmBessel0( double x )
diff --git a/cmMath.h b/cmMath.h
index 57d96ea..b80716e 100644
--- a/cmMath.h
+++ b/cmMath.h
@@ -15,6 +15,12 @@ unsigned cmPrevOddU(  unsigned v );
 unsigned cmNextEvenU( unsigned v );
 unsigned cmPrevEvenU( unsigned v );
 
+/// Increment or decrement 'idx' by 'delta' always wrapping the result into the range
+/// 0 to (maxN-1).
+/// 'idx': initial value 
+/// 'delta':  incremental amount
+/// 'maxN' - 1 : maximum return value.
+unsigned cmModIncr(int idx, int delta, int maxN );
 
 // modified bessel function of first kind, order 0
 // ref: orfandis appendix B io.m