codec/vp9/parser: add helpers for SegmentationParams

These short helpers reduce the number of casts we need to do and also
constraint the possible input values to an acceptable range for safe
array access.

src/codec/vp9/parser.rs

@@ -195,9 +195,26 @@ pub struct SegmentationParams {
     /// If unset, indicates that the corresponding feature is unused and has
     /// value equal to 0. if set, indicates that the feature value is coded in
     /// the bitstream.
-    pub feature_enabled: [[bool; SEG_LVL_MAX]; MAX_SEGMENTS],
+    feature_enabled: [[bool; SEG_LVL_MAX]; MAX_SEGMENTS],
     /// Specifies the magnitude of the feature data for a segment feature.
-    pub feature_data: [[i16; SEG_LVL_MAX]; MAX_SEGMENTS],
+    feature_data: [[i16; SEG_LVL_MAX]; MAX_SEGMENTS],
+}
+
+impl SegmentationParams {
+    /// Returns whether `feature` is enabled for `segment_id`.
+    fn is_feature_enabled(&self, segment_id: u8, feature: SegLvl) -> bool {
+        self.feature_enabled[segment_id as usize][feature as usize]
+    }
+
+    /// An implementation of seg_feature_active as per "6.4.9 Segmentation feature active syntax"
+    fn is_feature_active(&self, segment_id: u8, feature: SegLvl) -> bool {
+        self.enabled && self.is_feature_enabled(segment_id, feature)
+    }
+
+    /// Returns the data for `feature` on `segment_id`.
+    fn feature_data(&self, segment_id: u8, feature: SegLvl) -> i16 {
+        self.feature_data[segment_id as usize][feature as usize]
+    }
 }
 
 #[derive(Clone, Debug, Default, PartialEq, Eq)]
@@ -244,15 +261,11 @@ impl Segmentation {
                 let mut lvl_seg = i32::from(lf.level);
 
                 // 8.8.1 Loop filter frame init process
-                if hdr.seg_feature_active(segment_id, SegLvl::AltL) {
+                if hdr.seg.is_feature_active(segment_id, SegLvl::AltL) {
                     if seg.abs_or_delta_update {
-                        lvl_seg = i32::from(
-                            seg.feature_data[usize::from(segment_id)][SegLvl::AltL as usize],
-                        );
+                        lvl_seg = i32::from(seg.feature_data(segment_id, SegLvl::AltL));
                     } else {
-                        lvl_seg += i32::from(
-                            seg.feature_data[usize::from(segment_id)][SegLvl::AltL as usize],
-                        );
+                        lvl_seg += i32::from(seg.feature_data(segment_id, SegLvl::AltL));
                     }
                 }
 
@@ -291,12 +304,9 @@ impl Segmentation {
                 luma_dc_quant_scale,
                 chroma_ac_quant_scale,
                 chroma_dc_quant_scale,
-                reference_frame_enabled: seg.feature_enabled[segment_id as usize]
-                    [SegLvl::RefFrame as usize],
-                reference_frame: seg.feature_data[usize::from(segment_id)]
-                    [SegLvl::RefFrame as usize],
-                reference_skip_enabled: seg.feature_enabled[segment_id as usize]
-                    [SegLvl::LvlSkip as usize],
+                reference_frame_enabled: seg.is_feature_enabled(segment_id, SegLvl::RefFrame),
+                reference_frame: seg.feature_data(segment_id, SegLvl::RefFrame),
+                reference_skip_enabled: seg.is_feature_enabled(segment_id, SegLvl::LvlSkip),
             }
         }
     }
@@ -434,17 +444,12 @@ pub struct Header {
 }
 
 impl Header {
-    /// An implementation of seg_feature_active as per "6.4.9 Segmentation feature active syntax"
-    fn seg_feature_active(&self, segment_id: u8, feature: SegLvl) -> bool {
-        self.seg.enabled && self.seg.feature_enabled[segment_id as usize][feature as usize]
-    }
-
     /// An implementation of get_qindex as per "8.6.1 Dequantization functions"
     fn get_qindex(&self, segment_id: u8) -> u8 {
         let base_q_idx = self.quant.base_q_idx;
 
-        if self.seg_feature_active(segment_id, SegLvl::AltQ) {
-            let mut data = self.seg.feature_data[segment_id as usize][0] as i32;
+        if self.seg.is_feature_active(segment_id, SegLvl::AltQ) {
+            let mut data = self.seg.feature_data(segment_id, SegLvl::AltQ) as i32;
 
             if !self.seg.abs_or_delta_update {
                 data += base_q_idx as i32;