shallow zip-file copy from codec2 e9d726bf20

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diff --git a/src/gp_interleaver.c b/src/gp_interleaver.c
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+/*---------------------------------------------------------------------------*\
+
+  FILE........: gp_interleaver.c
+  AUTHOR......: David Rowe
+  DATE CREATED: April 2018
+
+  Golden Prime Interleaver. My interpretation of "On the Analysis and
+  Design of Good Algebraic Interleavers", Xie et al,eq (5).
+
+  See also octave/gp_interleaver.m
+
+\*---------------------------------------------------------------------------*/
+
+/*
+  Copyright (C) 2018 David Rowe
+
+  All rights reserved.
+
+  This program is free software; you can redistribute it and/or modify
+  it under the terms of the GNU Lesser General Public License version 2.1, as
+  published by the Free Software Foundation.  This program is
+  distributed in the hope that it will be useful, but WITHOUT ANY
+  WARRANTY; without even the implied warranty of MERCHANTABILITY or
+  FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+  License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with this program; if not, see <http://www.gnu.org/licenses/>.
+*/
+
+#include <assert.h>
+#include <stdio.h>
+#include "gp_interleaver.h"
+
+/*
+  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.
+
+  Implemented with a LUT in C for convenience, Octave version
+  has a more complete implementation.  If you find you need some more
+  numbers head back to the Octave choose_interleaver_b() function.
+*/
+
+static const int b_table[] = {
+     56, 37,    /* 700E:   HRA_56_56                              */
+    106, 67,    /* 2020B:  (112,56) partial protection            */
+    112, 71,    /* 700D:   HRA_112_112                            */
+    128, 83,    /* datac0: H_128_256_5                            */
+    192, 127,   /* datac13: H_256_512_4, 128 data bits used       */
+    210, 131,   /* 2020:   HRAb_396_504 with 312 data bits used   */
+    736, 457,   /* datac4: H_1024_2048_4f, 448 data bits used     */
+    1024, 641,  /* datac3: H_1024_2048_4f                         */
+    1290, 797,  /* datac2: H2064_516_sparse                       */
+    4096, 2531  /* datac1: H_4096_8192_3d                         */
+};
+
+int choose_interleaver_b(int Nbits)
+{
+    int i;
+    for(i=0; i<sizeof(b_table)/sizeof(int); i+=2) {
+        if (b_table[i] == Nbits) {
+            return b_table[i+1];
+        }
+    }
+
+    /* if we get to here it means a Nbits we don't have in our table so choke */
+
+    fprintf(stderr, "gp_interleaver: Nbits: %d, b not found!\n", Nbits);
+    assert(0);
+    return -1;
+}
+
+
+void gp_interleave_comp(COMP interleaved_frame[], COMP frame[], int Nbits) {
+  int b = choose_interleaver_b(Nbits);
+  int i,j;
+  for (i=0; i<Nbits; i++) {
+    j = (b*i) % Nbits;
+    interleaved_frame[j] = frame[i];
+  }
+}
+
+void gp_deinterleave_comp(COMP frame[], COMP interleaved_frame[], int Nbits) {
+  int b = choose_interleaver_b(Nbits);
+  int i,j;
+  for (i=0; i<Nbits; i++) {
+    j = (b*i) % Nbits;
+    frame[i] =  interleaved_frame[j];
+  }
+}
+
+void gp_interleave_float(float interleaved_frame[], float frame[], int Nbits) {
+  int b = choose_interleaver_b(Nbits);
+  int i,j;
+
+  for (i=0; i<Nbits; i++) {
+    j = (b*i) % Nbits;
+    interleaved_frame[j] = frame[i];
+  }
+}
+
+void gp_deinterleave_float(float frame[], float interleaved_frame[], int Nbits) {
+  int b = choose_interleaver_b(Nbits);
+  int i,j;
+
+  for (i=0; i<Nbits; i++) {
+    j = (b*i) % Nbits;
+    frame[i] = interleaved_frame[j];
+  }
+}
+
+// The above work on complex numbers (e.g. OFDM symbols), so the below work on
+// groups of two bits at a time to remain compatible with the above.
+void gp_interleave_bits(char interleaved_frame[], char frame[], int Nbits)
+{
+    char temp[Nbits];
+    int b = choose_interleaver_b(Nbits);
+    int i,j;
+
+    for (i=0; i<Nbits; i++) {
+      j = (b*i) % Nbits;
+      temp[j] = ((frame[i*2] & 1) << 1) | (frame[i*2+1] & 1);
+    }
+    
+    for (i=0; i<Nbits; i++) {
+        interleaved_frame[i*2] = temp[i] >> 1;
+        interleaved_frame[i*2+1] = temp[i] & 1;
+    }
+}
+
+void gp_deinterleave_bits(char frame[], char interleaved_frame[], int Nbits)
+{
+    char temp[Nbits];
+    int b = choose_interleaver_b(Nbits);
+    int i,j;
+
+    for (i=0; i<Nbits; i++) {
+        j = (b*i) % Nbits;
+        temp[i] = ((interleaved_frame[j*2] & 1) << 1) | (interleaved_frame[j*2+1] & 1);
+    }
+    
+    for (i=0; i<Nbits; i++) {
+      frame[i*2] = temp[i] >> 1;
+      frame[i*2 + 1] = temp[i] & 1;
+    }
+}