get_log_tx_scale: remove mutable variable (#757)

* get_log_tx_scale: remove mutable variable

No functional change.

* quantize: add test for get_log_tx_scale()

src/quantize.rs

@@ -15,12 +15,8 @@ use std::mem;
 
 pub fn get_log_tx_scale(tx_size: TxSize) -> i32 {
   let num_pixels = tx_size.area();
-  let mut shift_bits = 0;
 
-  if num_pixels > 256 { shift_bits += 1; };
-  if num_pixels > 1024 { shift_bits += 1; };
-
-  shift_bits
+  i32::from(num_pixels > 256) + i32::from(num_pixels > 1024)
 }
 
 pub fn dc_q(qindex: u8, bit_depth: usize) -> i16 {
@@ -93,6 +89,7 @@ fn divu_pair(x: i32, d: (u32, u32, u32)) -> i32 {
 #[cfg(test)]
 mod test {
   use super::*;
+  use partition::TxSize::*;
   #[test]
   fn test_divu_pair() {
     for d in 1..1024 {
@@ -109,6 +106,33 @@ mod test {
 
     println!("{:?}", b);
   }
+  #[test]
+  fn test_tx_log_scale() {
+    let tx_sizes = [
+      (TX_4X4, 0),
+      (TX_8X8, 0),
+      (TX_16X16, 0),
+      (TX_32X32, 1),
+      (TX_64X64, 2),
+      (TX_4X8, 0),
+      (TX_8X4, 0),
+      (TX_8X16, 0),
+      (TX_16X8, 0),
+      (TX_16X32, 1),
+      (TX_32X16, 1),
+      (TX_32X64, 2),
+      (TX_64X32, 2),
+      (TX_4X16, 0),
+      (TX_16X4, 0),
+      (TX_8X32, 0),
+      (TX_32X8, 0),
+      (TX_16X64, 1),
+      (TX_64X16, 1),
+    ];
+    for &tx_size in tx_sizes.iter() {
+      assert!(tx_size.1 == get_log_tx_scale(tx_size.0));
+    }
+  }
 }
 
 impl QuantizationContext {