From 1b691f2d47bf63bda19286b7516aec795545d9f7 Mon Sep 17 00:00:00 2001
From: "Christian W. Zuckschwerdt" <christian@zuckschwerdt.org>
Date: Tue, 1 Mar 2022 10:08:20 +0100
Subject: [PATCH] Add bitbuffer_find_repeated_prefix function

---
 include/bitbuffer.h          | 22 ++++++++++++++++-----
 src/bitbuffer.c              | 37 +++++++++++++++++++++++++++++-------
 src/devices/generic_motion.c |  2 +-
 3 files changed, 48 insertions(+), 13 deletions(-)

diff --git a/include/bitbuffer.h b/include/bitbuffer.h
index cff68d07..581c7486 100644
--- a/include/bitbuffer.h
+++ b/include/bitbuffer.h
@@ -116,15 +116,27 @@ unsigned bitbuffer_manchester_decode(bitbuffer_t *inbuf, unsigned row, unsigned
 unsigned bitbuffer_differential_manchester_decode(bitbuffer_t *inbuf, unsigned row, unsigned start,
         bitbuffer_t *outbuf, unsigned max);
 
-/// Function to compare bitbuffer rows and count repetitions.
-int compare_rows(bitbuffer_t *bits, unsigned row_a, unsigned row_b);
+/// Compares two given rows of a bitbuffer.
+///
+/// If @p max_bits is greater than 0 then only up that many bits are compared.
+int bitbuffer_compare_rows(bitbuffer_t *bits, unsigned row_a, unsigned row_b, unsigned max_bits);
 
-unsigned count_repeats(bitbuffer_t *bits, unsigned row);
+/// Count the number of repeats of row at index @p row.
+///
+/// If @p max_bits is greater than 0 then only up that many bits are compared.
+/// The returned count will include the given row and will be at least 1.
+unsigned bitbuffer_count_repeats(bitbuffer_t *bits, unsigned row, unsigned max_bits);
 
-/// Find a repeated row that has a minimum count of bits.
-/// Return the row index or -1.
+/// Find a row repeated at least @p min_repeats times and with at least @p min_bits bits length,
+/// all bits in the repeats need to match.
+/// @return the row index or -1.
 int bitbuffer_find_repeated_row(bitbuffer_t *bits, unsigned min_repeats, unsigned min_bits);
 
+/// Find a row repeated at least @p min_repeats times and with at least @p min_bits bits length,
+/// a prefix of at most @p min_bits bits will be compared.
+/// @return the row index or -1.
+int bitbuffer_find_repeated_prefix(bitbuffer_t *bits, unsigned min_repeats, unsigned min_bits);
+
 /// Return a single bit from a bitrow at bit_idx position.
 static inline uint8_t bitrow_get_bit(uint8_t const *bitrow, unsigned bit_idx)
 {
diff --git a/src/bitbuffer.c b/src/bitbuffer.c
index be4e1a28..1298c914 100644
--- a/src/bitbuffer.c
+++ b/src/bitbuffer.c
@@ -461,18 +461,30 @@ void bitbuffer_parse(bitbuffer_t *bits, const char *code)
     }
 }
 
-int compare_rows(bitbuffer_t *bits, unsigned row_a, unsigned row_b)
+int bitbuffer_compare_rows(bitbuffer_t *bits, unsigned row_a, unsigned row_b, unsigned max_bits)
 {
-    return (bits->bits_per_row[row_a] == bits->bits_per_row[row_b]
-            && !memcmp(bits->bb[row_a], bits->bb[row_b],
-                    (bits->bits_per_row[row_a] + 7) / 8));
+    if (max_bits == 0 || bits->bits_per_row[row_a] < max_bits || bits->bits_per_row[row_b] < max_bits) {
+        // full compare, no max_bits or rows too short
+        return (bits->bits_per_row[row_a] == bits->bits_per_row[row_b]
+                && !memcmp(bits->bb[row_a], bits->bb[row_b],
+                        (bits->bits_per_row[row_a] + 7) / 8));
+    }
+    else {
+        // prefix-only compare, both rows are at least max_bits long
+        uint8_t *a = bits->bb[row_a];
+        uint8_t *b = bits->bb[row_b];
+        unsigned last = (max_bits - 1) / 8; // max_bits is at least 1
+        unsigned mask = 0xff00 >> (max_bits & 7); // mask off bottom bits
+        return (!memcmp(bits->bb[row_a], bits->bb[row_b], max_bits / 8)
+                && (a[last] & mask) == (b[last] & mask));
+    }
 }
 
-unsigned count_repeats(bitbuffer_t *bits, unsigned row)
+unsigned bitbuffer_count_repeats(bitbuffer_t *bits, unsigned row, unsigned max_bits)
 {
     unsigned cnt = 0;
     for (int i = 0; i < bits->num_rows; ++i) {
-        if (compare_rows(bits, row, i)) {
+        if (bitbuffer_compare_rows(bits, row, i, max_bits)) {
             ++cnt;
         }
     }
@@ -483,7 +495,18 @@ int bitbuffer_find_repeated_row(bitbuffer_t *bits, unsigned min_repeats, unsigne
 {
     for (int i = 0; i < bits->num_rows; ++i) {
         if (bits->bits_per_row[i] >= min_bits &&
-                count_repeats(bits, i) >= min_repeats) {
+                bitbuffer_count_repeats(bits, i, 0) >= min_repeats) {
+            return i;
+        }
+    }
+    return -1;
+}
+
+int bitbuffer_find_repeated_prefix(bitbuffer_t *bits, unsigned min_repeats, unsigned min_bits)
+{
+    for (int i = 0; i < bits->num_rows; ++i) {
+        if (bits->bits_per_row[i] >= min_bits &&
+                bitbuffer_count_repeats(bits, i, min_bits) >= min_repeats) {
             return i;
         }
     }
diff --git a/src/devices/generic_motion.c b/src/devices/generic_motion.c
index 5854515b..824c525a 100644
--- a/src/devices/generic_motion.c
+++ b/src/devices/generic_motion.c
@@ -43,7 +43,7 @@ static int generic_motion_callback(r_device *decoder, bitbuffer_t *bitbuffer)
         if ((bitbuffer->bits_per_row[i] != 20)
                 || ((b[1] == 0) && (b[2] == 0))
                 || ((b[1] == 0xff) && (b[2] == 0xff))
-                || count_repeats(bitbuffer, i) < 3)
+                || bitbuffer_count_repeats(bitbuffer, i, 0) < 3)
             continue; // DECODE_ABORT_EARLY
 
         code = (b[0] << 12) | (b[1] << 4) | (b[2] >> 4);