Add pseudo-random bit sequence generator to libosmcoore

These PRBS sequences are specified in ITU-T O.150.  They are typically
used as test data to be transmitted for BER (bit error rate) testing.

Change-Id: I227b6a6e86a251460ecb816afa9a7439d5fb94d1

1 files changed, 74 insertions, 0 deletions

diff --git a/src/prbs.c b/src/prbs.c
new file mode 100644
index 00000000..be52fd4c
--- /dev/null
+++ b/src/prbs.c
@@ -0,0 +1,74 @@
+/* Osmocom implementation of pseudo-random bit sequence generation */
+/* (C) 2017 by Harald Welte <laforge@gnumonks.org> */
+
+#include <stdint.h>
+#include <string.h>
+#include <osmocom/core/bits.h>
+#include <osmocom/core/prbs.h>
+
+/*! \brief PRBS-7 according ITU-T O.150 */
+const struct osmo_prbs osmo_prbs7 = {
+	/* x^7 + x^6 + 1 */
+	.name = "PRBS-7",
+	.len = 7,
+	.coeff = (1<<6) | (1<<5),
+};
+
+/*! \brief PRBS-9 according ITU-T O.150 */
+const struct osmo_prbs osmo_prbs9 = {
+	/* x^9 + x^5 + 1 */
+	.name = "PRBS-9",
+	.len = 9,
+	.coeff = (1<<8) | (1<<4),
+};
+
+/*! \brief PRBS-11 according ITU-T O.150 */
+const struct osmo_prbs osmo_prbs11 = {
+	/* x^11 + x^9 + 1 */
+	.name = "PRBS-11",
+	.len = 11,
+	.coeff = (1<<10) | (1<<8),
+};
+
+/*! \brief PRBS-15 according ITU-T O.150 */
+const struct osmo_prbs osmo_prbs15 = {
+	/* x^15 + x^14+ 1 */
+	.name = "PRBS-15",
+	.len = 15,
+	.coeff = (1<<14) | (1<<13),
+};
+
+/*! \brief Initialize the given caller-allocated PRBS state */
+void osmo_prbs_state_init(struct osmo_prbs_state *st, const struct osmo_prbs *prbs)
+{
+	memset(st, 0, sizeof(*st));
+	st->prbs = prbs;
+	st->state = 1;
+}
+
+static void osmo_prbs_process_bit(struct osmo_prbs_state *state, ubit_t bit)
+{
+	state->state >>= 1;
+	if (bit)
+		state->state ^= state->prbs->coeff;
+}
+
+/*! \brief Get the next bit out of given PRBS instance */
+ubit_t osmo_prbs_get_ubit(struct osmo_prbs_state *state)
+{
+	ubit_t result = state->state & 0x1;
+	osmo_prbs_process_bit(state, result);
+
+	return result;
+}
+
+/*! \brief Fill buffer of unpacked bits with next bits out of given PRBS instance */
+int osmo_prbs_get_ubits(ubit_t *out, unsigned int out_len, struct osmo_prbs_state *state)
+{
+	unsigned int i;
+
+	for (i = 0; i < out_len; i++)
+		out[i] = osmo_prbs_get_ubit(state);
+
+	return i;
+}