port arfcn range encode support from osmo-bsc

As part of fixing issue OS#3075, we want to migrate support
for encoding system information from osmo-bsc to libosmocore.

This change ports one of the prerequisites for doing so:
osmo-bsc code for range-encoding ARFCNs, including tests.

An osmo_gsm48_ prefix has been prepended to public symbols in
order to avoid clashes with existing symbols in osmo-bsc code.

Change-Id: Ia220764fba451be5e975ae7c5eefb1a25ac2bf2c
Related: OS#3075

3 files changed, 332 insertions, 1 deletions

diff --git a/src/gsm/Makefile.am b/src/gsm/Makefile.am
index 3d2c5606..c4b5fce3 100644
--- a/src/gsm/Makefile.am
+++ b/src/gsm/Makefile.am
@@ -21,7 +21,8 @@ lib_LTLIBRARIES = libosmogsm.la
 BUILT_SOURCES = gsm0503_conv.c
 
 libgsmint_la_SOURCES =  a5.c rxlev_stat.c tlv_parser.c comp128.c comp128v23.c \
-			gsm_utils.c rsl.c gsm48.c gsm48_ie.c gsm0808.c sysinfo.c \
+			gsm_utils.c rsl.c gsm48.c gsm48_arfcn_range_encode.c \
+			gsm48_ie.c gsm0808.c sysinfo.c \
 			gprs_cipher_core.c gprs_rlc.c gsm0480.c abis_nm.c gsm0502.c \
 			gsm0411_utils.c gsm0411_smc.c gsm0411_smr.c gsm0414.c \
 			lapd_core.c lapdm.c kasumi.c gsm29205.c gsm_04_08_gprs.c \
diff --git a/src/gsm/gsm48_arfcn_range_encode.c b/src/gsm/gsm48_arfcn_range_encode.c
new file mode 100644
index 00000000..6423a9a8
--- /dev/null
+++ b/src/gsm/gsm48_arfcn_range_encode.c
@@ -0,0 +1,321 @@
+/* gsm 04.08 system information (si) encoding and decoding
+ * 3gpp ts 04.08 version 7.21.0 release 1998 / etsi ts 100 940 v7.21.0 */
+
+/*
+ * (C) 2012 Holger Hans Peter Freyther
+ * (C) 2012 by On-Waves
+ * All Rights Reserved
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * 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 Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <osmocom/gsm/protocol/gsm_04_08.h>
+#include <osmocom/gsm/gsm48_arfcn_range_encode.h>
+
+#include <osmocom/core/utils.h>
+
+#include <errno.h>
+
+static inline int greatest_power_of_2_lesser_or_equal_to(int index)
+{
+	int power_of_2 = 1;
+
+	do {
+		power_of_2 *= 2;
+	} while (power_of_2 <= index);
+
+	/* now go back one step */
+	return power_of_2 / 2;
+}
+
+static inline int mod(int data, int range)
+{
+	int res = data % range;
+	while (res < 0)
+		res += range;
+	return res;
+}
+
+/**
+ * Determine at which index to split the ARFCNs to create an
+ * equally size partition for the given range. Return -1 if
+ * no such partition exists.
+ */
+int osmo_gsm48_range_enc_find_index(enum osmo_gsm48_range range, const int *freqs, const int size)
+{
+	int i, j, n;
+
+	const int RANGE_DELTA = (range - 1) / 2;
+
+	for (i = 0; i < size; ++i) {
+		n = 0;
+		for (j = 0; j < size; ++j) {
+			if (mod(freqs[j] - freqs[i], range) <= RANGE_DELTA)
+				n += 1;
+		}
+
+		if (n - 1 == (size - 1) / 2)
+			return i;
+	}
+
+	return -1;
+}
+
+/* Worker for range_enc_arfcns(), do not call directly. */
+static int _range_enc_arfcns(enum osmo_gsm48_range range,
+		const int *arfcns, int size, int *out,
+		const int index)
+{
+	int split_at;
+	int i;
+
+	/*
+	 * The below is a GNU extension and we can remove it when
+	 * we move to a quicksort like in-situ swap with the pivot.
+	 */
+	int arfcns_left[size / 2];
+	int arfcns_right[size / 2];
+	int l_size;
+	int r_size;
+	int l_origin;
+	int r_origin;
+
+	/* Now do the processing */
+	split_at = osmo_gsm48_range_enc_find_index(range, arfcns, size);
+	if (split_at < 0)
+		return -EINVAL;
+
+	/* we now know where to split */
+	out[index] = 1 + arfcns[split_at];
+
+	/* calculate the work that needs to be done for the leafs */
+	l_origin = mod(arfcns[split_at] + ((range - 1) / 2) + 1, range);
+	r_origin = mod(arfcns[split_at] + 1, range);
+	for (i = 0, l_size = 0, r_size = 0; i < size; ++i) {
+		if (mod(arfcns[i] - l_origin, range) < range / 2)
+			arfcns_left[l_size++] = mod(arfcns[i] - l_origin, range);
+		if (mod(arfcns[i] - r_origin, range) < range / 2)
+			arfcns_right[r_size++] = mod(arfcns[i] - r_origin, range);
+	}
+
+	/*
+	 * Now recurse and we need to make this iterative... but as the
+	 * tree is balanced the stack will not be too deep.
+	 */
+	if (l_size)
+		osmo_gsm48_range_enc_arfcns(range / 2, arfcns_left, l_size,
+			out, index + greatest_power_of_2_lesser_or_equal_to(index + 1));
+	if (r_size)
+		osmo_gsm48_range_enc_arfcns((range - 1) / 2, arfcns_right, r_size,
+			 out, index + (2 * greatest_power_of_2_lesser_or_equal_to(index + 1)));
+	return 0;
+}
+
+/**
+ * Range encode the ARFCN list.
+ * \param range The range to use.
+ * \param arfcns The list of ARFCNs
+ * \param size The size of the list of ARFCNs
+ * \param out Place to store the W(i) output.
+ */
+int osmo_gsm48_range_enc_arfcns(enum osmo_gsm48_range range,
+		const int *arfcns, int size, int *out,
+		const int index)
+{
+	if (size <= 0)
+		return 0;
+
+	if (size == 1) {
+		out[index] = 1 + arfcns[0];
+		return 0;
+	}
+
+	return _range_enc_arfcns(range, arfcns, size, out, index);
+}
+
+/*
+ * The easiest is to use f0 == arfcns[0]. This means that under certain
+ * circumstances we can encode less ARFCNs than possible with an optimal f0.
+ *
+ * TODO: Solve the optimisation problem and pick f0 so that the max distance
+ * is the smallest. Taking into account the modulo operation. I think picking
+ * size/2 will be the optimal arfcn.
+ */
+/**
+ * This implements the range determination as described in GSM 04.08 J4. The
+ * result will be a base frequency f0 and the range to use. Note that for range
+ * 1024 encoding f0 always refers to ARFCN 0 even if it is not an element of
+ * the arfcns list.
+ *
+ * \param[in] arfcns The input frequencies, they must be sorted, lowest number first
+ * \param[in] size The length of the array
+ * \param[out] f0 The selected F0 base frequency. It might not be inside the list
+ */
+int osmo_gsm48_range_enc_determine_range(const int *arfcns, const int size, int *f0)
+{
+	int max = 0;
+
+	/*
+	 * Go for the easiest. And pick arfcns[0] == f0.
+	 */
+	max = arfcns[size - 1] - arfcns[0];
+	*f0 = arfcns[0];
+
+	if (max < 128 && size <= 29)
+		return OSMO_GSM48_ARFCN_RANGE_128;
+	if (max < 256 && size <= 22)
+		return OSMO_GSM48_ARFCN_RANGE_256;
+	if (max < 512 && size <= 18)
+		return OSMO_GSM48_ARFCN_RANGE_512;
+	if (max < 1024 && size <= 17) {
+		*f0 = 0;
+		return OSMO_GSM48_ARFCN_RANGE_1024;
+	}
+
+	return OSMO_GSM48_ARFCN_RANGE_INVALID;
+}
+
+static void write_orig_arfcn(uint8_t *chan_list, int f0)
+{
+	chan_list[0] |= (f0 >> 9) & 1;
+	chan_list[1] = (f0 >> 1);
+	chan_list[2] = (f0 & 1) << 7;
+}
+
+static void write_all_wn(uint8_t *chan_list, int bit_offs,
+			 int *w, int w_size, int w1_len)
+{
+	int octet_offs = 0; /* offset into chan_list */
+	int wk_len = w1_len; /* encoding size in bits of w[k] */
+	int k; /* 1 based */
+	int level = 0; /* tree level, top level = 0 */
+	int lvl_left = 1; /* nodes per tree level */
+
+	/* W(2^i) to W(2^(i+1)-1) are on w1_len-i bits when present */
+
+	for (k = 1; k <= w_size; k++) {
+		int wk_left = wk_len;
+
+		while (wk_left > 0) {
+			int cur_bits = 8 - bit_offs;
+			int cur_mask;
+			int wk_slice;
+
+			if (cur_bits > wk_left)
+				cur_bits = wk_left;
+
+			cur_mask = ((1 << cur_bits) - 1);
+
+			/* advance */
+			wk_left -= cur_bits;
+			bit_offs += cur_bits;
+
+			/* right aligned wk data for current out octet */
+			wk_slice = (w[k-1] >> wk_left) & cur_mask;
+
+			/* cur_bits now contains the number of bits
+			 * that are to be copied from wk to the chan_list.
+			 * wk_left is set to the number of bits that must
+			 * not yet be copied.
+			 * bit_offs points after the bit area that is going to
+			 * be overwritten:
+			 *
+			 *          wk_left
+			 *             |
+			 *             v
+			 * wk: WWWWWWWWWWW
+			 *        |||||<-- wk_slice, cur_bits=5
+			 *      --WWWWW-
+			 *             ^
+			 *             |
+			 *           bit_offs
+			 */
+
+			chan_list[octet_offs] &= ~(cur_mask << (8 - bit_offs));
+			chan_list[octet_offs] |= wk_slice << (8 - bit_offs);
+
+			/* adjust output */
+			if (bit_offs == 8) {
+				bit_offs = 0;
+				octet_offs += 1;
+			}
+		}
+
+		/* adjust bit sizes */
+		lvl_left -= 1;
+		if (!lvl_left) {
+			/* completed tree level, advance to next */
+			level += 1;
+			lvl_left = 1 << level;
+			wk_len -= 1;
+		}
+	}
+}
+
+int osmo_gsm48_range_enc_128(uint8_t *chan_list, int f0, int *w)
+{
+	chan_list[0] = 0x8C;
+	write_orig_arfcn(chan_list, f0);
+
+	write_all_wn(&chan_list[2], 1, w, 28, 7);
+	return 0;
+}
+
+int osmo_gsm48_range_enc_256(uint8_t *chan_list, int f0, int *w)
+{
+	chan_list[0] = 0x8A;
+	write_orig_arfcn(chan_list, f0);
+
+	write_all_wn(&chan_list[2], 1, w, 21, 8);
+	return 0;
+}
+
+int osmo_gsm48_range_enc_512(uint8_t *chan_list, int f0, int *w)
+{
+	chan_list[0] = 0x88;
+	write_orig_arfcn(chan_list, f0);
+
+	write_all_wn(&chan_list[2], 1, w, 17, 9);
+	return 0;
+}
+
+int osmo_gsm48_range_enc_1024(uint8_t *chan_list, int f0, int f0_included, int *w)
+{
+	chan_list[0] = 0x80 | (f0_included << 2);
+
+	write_all_wn(&chan_list[0], 6, w, 16, 10);
+	return 0;
+}
+
+int osmo_gsm48_range_enc_filter_arfcns(int *arfcns, const int size, const int f0, int *f0_included)
+{
+	int i, j = 0;
+	*f0_included = 0;
+
+	for (i = 0; i < size; ++i) {
+		/*
+		 * Appendix J.4 says the following:
+		 * All frequencies except F(0), minus F(0) + 1.
+		 * I assume we need to exclude it here.
+		 */
+		if (arfcns[i] == f0) {
+			*f0_included = 1;
+			continue;
+		}
+
+		arfcns[j++] = mod(arfcns[i] - (f0 + 1), 1024);
+	}
+
+	return j;
+}
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index cdcbcc98..922ac01f 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -518,6 +518,15 @@ osmo_apn_qualify_from_imsi;
 osmo_apn_to_str;
 osmo_apn_from_str;
 
+osmo_gsm48_range_enc_determine_range;
+osmo_gsm48_range_enc_arfcns;
+osmo_gsm48_range_enc_find_index;
+osmo_gsm48_range_enc_filter_arfcns;
+osmo_gsm48_range_enc_128;
+osmo_gsm48_range_enc_256;
+osmo_gsm48_range_enc_512;
+osmo_gsm48_range_enc_1024;
+
 osmo_gsup_encode;
 osmo_gsup_decode;
 osmo_gsup_message_type_names;