Fix choice of delta sign.

Ensure that the delta goes in the correct direction (i.e. use positive delta for prev <= sample, and negative delta for prev > sample)

1 files changed, 22 insertions, 1 deletions

diff --git a/tools/aif2pcm/main.c b/tools/aif2pcm/main.c
index b9709bee8..af24c417f 100644
--- a/tools/aif2pcm/main.c
+++ b/tools/aif2pcm/main.c
@@ -351,6 +351,12 @@ const int gDeltaEncodingTable[] = {
 	-64, -49, -36, -25, -16, -9, -4, -1,
 };
 
+#define POSITIVE_DELTAS_START 0
+#define POSITIVE_DELTAS_END 8
+
+#define NEGATIVE_DELTAS_START 8
+#define NEGATIVE_DELTAS_END 16
+
 struct Bytes *delta_decompress(struct Bytes *delta, unsigned int expected_length)
 {
 	struct Bytes *pcm = malloc(sizeof(struct Bytes));
@@ -418,12 +424,27 @@ struct Bytes *delta_decompress(struct Bytes *delta, unsigned int expected_length
 	return pcm;
 }
 
+#define U8_TO_S8(value) ((value) < 128 ? (value) : (value) - 256)
+
 int get_delta_index(uint8_t sample, uint8_t prev_sample)
 {
 	int best_error = INT_MAX;
 	int best_index = -1;
+	int delta_table_start_index;
+	int delta_table_end_index;
+	int sample_signed = U8_TO_S8(sample);
+	int prev_sample_signed = U8_TO_S8(prev_sample);
+
+    // if we're going up (or equal), only choose positive deltas
+	if (prev_sample_signed <= sample_signed) {
+		delta_table_start_index = POSITIVE_DELTAS_START;
+		delta_table_end_index = POSITIVE_DELTAS_END;
+	} else {
+		delta_table_start_index = NEGATIVE_DELTAS_START;
+		delta_table_end_index = NEGATIVE_DELTAS_END;
+	}
 
-	for (int i = 0; i < 16; i++)
+	for (int i = delta_table_start_index; i < delta_table_end_index; i++)
 	{
 		uint8_t new_sample = prev_sample + gDeltaEncodingTable[i];
 		uint8_t sample_diff_1 = (sample - new_sample) & 0xff;