Add unit tests for the IDCT function

This adds unit tests with various inputs for the IDCT function.
This should make it easier to then optimize the function while making sure we are not altering its functionality.

Author: lovasoa

src/idct.rs

@@ -307,3 +307,74 @@ fn stbi_f2f(x: f32) -> i32 {
 fn stbi_fsh(x: i32) -> i32 {
     x << 12
 }
+
+#[test]
+fn test_dequantize_and_idct_block_8x8() {
+    let coefficients: [i16; 8 * 8] = [
+        -14, -39, 58, -2, 3, 3, 0, 1,
+        11, 27, 4, -3, 3, 0, 1, 0,
+        -6, -13, -9, -1, -2, -1, 0, 0,
+        -4, 0, -1, -2, 0, 0, 0, 0,
+        3, 0, 0, 0, 0, 0, 0, 0,
+        -3, -2, 0, 0, 0, 0, 0, 0,
+        0, 0, 0, 0, 0, 0, 0, 0,
+        0, 0, 0, 0, 0, 0, 0, 0];
+
+    let quantization_table: [u16; 8 * 8] = [
+        8, 6, 5, 8, 12, 20, 26, 31,
+        6, 6, 7, 10, 13, 29, 30, 28,
+        7, 7, 8, 12, 20, 29, 35, 28,
+        7, 9, 11, 15, 26, 44, 40, 31,
+        9, 11, 19, 28, 34, 55, 52, 39,
+        12, 18, 28, 32, 41, 52, 57, 46,
+        25, 32, 39, 44, 52, 61, 60, 51,
+        36, 46, 48, 49, 56, 50, 52, 50];
+    let output_linestride: usize = 8;
+    let mut output = [0u8; 8 * 8];
+    dequantize_and_idct_block_8x8(
+        &coefficients,
+        &quantization_table,
+        output_linestride,
+        &mut output);
+    let expected_output = [
+        118, 92, 110, 83, 77, 93, 144, 198,
+        172, 116, 114, 87, 78, 93, 146, 191,
+        194, 107, 91, 76, 71, 93, 160, 198,
+        196, 100, 80, 74, 67, 92, 174, 209,
+        182, 104, 88, 81, 68, 89, 178, 206,
+        105, 64, 59, 59, 63, 94, 183, 201,
+        35, 27, 28, 37, 72, 121, 203, 204,
+        37, 45, 41, 47, 98, 154, 223, 208];
+    assert_eq!(&output[..], &expected_output[..]);
+}
+
+#[test]
+fn test_dequantize_and_idct_block_8x8_all_zero() {
+    let mut output = [0u8; 8 * 8];
+    dequantize_and_idct_block_8x8(
+        &[0; 8*8],
+        &[666; 8*8],
+        8,
+        &mut output);
+    assert_eq!(&output[..], &[128; 8*8][..]);
+}
+
+#[test]
+fn test_dequantize_and_idct_block_8x8_saturated() {
+    let mut output = [0u8; 8 * 8];
+    dequantize_and_idct_block_8x8(
+        &[std::i16::MAX; 8*8],
+        &[std::u16::MAX; 8*8],
+        8,
+        &mut output);
+    let expected = [
+        0, 0, 0, 255, 255, 0, 0, 255,
+        0, 0, 215, 0, 0, 255, 255, 0,
+        255, 255, 255, 255, 255, 0, 0, 255,
+        0, 0, 255, 0, 255, 0, 255, 255,
+        0, 0, 255, 255, 0, 255, 0, 0,
+        255, 255, 0, 255, 255, 255, 170, 0,
+        0, 255, 0, 0, 0, 0, 0, 255,
+        255, 255, 0, 255, 0, 255, 0, 0];
+    assert_eq!(&output[..], &expected[..]);
+}