Use the GTR mode to detect sidemount tanks

Firmware v84 introduced support for sidemount diving. Users can now
configure the two sidemount tanks as the source for the GTR (Gas Time
Remaining) estimations. We can take advantage of this feature to detect
the sidemount tanks. This is more reliable than using the tank name.

Author: jefdriesen

src/array.c

@@ -404,3 +404,14 @@ signextend (unsigned int value, unsigned int nbits)
 	else
 		return value & mask;
 }
+
+unsigned int
+popcount (unsigned int value)
+{
+	unsigned int count = 0;
+	while (value) {
+		value &= value - 1;
+		count++;
+	}
+	return count;
+}

src/array.h

@@ -126,6 +126,9 @@ dec2bcd (unsigned char value);
 unsigned int
 signextend (unsigned int value, unsigned int nbits);
 
+unsigned int
+popcount (unsigned int value);
+
 #ifdef __cplusplus
 }
 #endif /* __cplusplus */

src/shearwater_predator_parser.c

@@ -118,6 +118,7 @@ typedef struct shearwater_predator_tank_t {
 	unsigned int pressure_reserve;
 	unsigned int serial;
 	char name[2];
+	dc_usage_t usage;
 } shearwater_predator_tank_t;
 
 struct shearwater_predator_parser_t {
@@ -254,6 +255,7 @@ shearwater_common_parser_create (dc_parser_t **out, dc_context_t *context, const
 		parser->tank[i].pressure_reserve = 0;
 		parser->tank[i].serial = 0;
 		memset (parser->tank[i].name, 0, sizeof (parser->tank[i].name));
+		parser->tank[i].usage = DC_USAGE_NONE;
 		parser->tankidx[i] = i;
 	}
 	parser->aimode = AI_OFF;
@@ -547,6 +549,14 @@ shearwater_predator_parser_cache (shearwater_predator_parser_t *parser)
 						}
 					}
 				}
+				unsigned int gtrmode = data[offset + 29];
+				if (popcount(gtrmode) >= 2) {
+					for (unsigned int i = 0; i < 4; ++i) {
+						if (gtrmode & (1 << i)) {
+							tank[i].usage = DC_USAGE_SIDEMOUNT;
+						}
+					}
+				}
 			} else if (type == LOG_RECORD_OPENING_5) {
 				if (logversion >= 9) {
 					tank[0].serial = array_convert_bcd2dec (data + offset + 1, 3);
@@ -759,11 +769,7 @@ shearwater_predator_parser_get_field (dc_parser_t *abstract, dc_field_type_t typ
 					break;
 				}
 			} else {
-				if (parser->tank[flags].name[0] == 'S') {
-					tank->usage = DC_USAGE_SIDEMOUNT;
-				} else {
-					tank->usage = DC_USAGE_NONE;
-				}
+				tank->usage = parser->tank[flags].usage;
 			}
 			break;
 		case DC_FIELD_SALINITY: