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+% gp_interleaver.m
+%
+% David Rowe May 2017
+%
+% Golden Prime Interleaver. My interprestation of "On the Analysis and
+% Design of Good Algebraic Interleavers", Xie et al,eq (5).
+
+1;
+
+% return 1 if prime
+function ret = is_prime(x)
+  for i=2:x-1
+    if mod(x,i) == 0
+      ret = 0;
+      return;
+    end
+  end
+  ret = 1;
+end
+
+function x = next_prime(x)
+  x++;
+  while is_prime(x) == 0
+    x++;
+  end
+end
+
+% Choose b for Golden Prime Interleaver.  b is chosen to be the
+% closest integer, which is relatively prime to N, to the Golden
+% section of N.
+
+function b = choose_interleaver_b(Nbits)
+  b = floor(Nbits/1.62);
+  b = next_prime(b);
+  assert(gcd(b,Nbits) == 1, "b and Nbits must be co-prime");
+end
+
+
+function interleaved_frame = gp_interleave(frame)
+  Nbits = length(frame);
+  b = choose_interleaver_b(Nbits);
+  interleaved_frame = zeros(1,Nbits);
+  for i=1:Nbits
+    j = mod((b*(i-1)), Nbits);
+    interleaved_frame(j+1) = frame(i);
+  end
+endfunction
+
+
+function frame = gp_deinterleave(interleaved_frame)
+  Nbits = length(interleaved_frame);
+  b = choose_interleaver_b(Nbits); 
+  frame = zeros(1,Nbits);
+  for i=1:Nbits
+    j = mod((b*(i-1)), Nbits);
+    frame(i) = interleaved_frame(j+1);
+  end
+endfunction
+