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    // Malicious JPEG files can cause operations in the idct to overflow.
    // One example is tests/crashtest/images/imagetestsuite/b0b8914cc5f7a6eff409f16d8cc236c5.jpg
    // That's why wrapping operators are needed.

    // This is based on stb_image's 'stbi__idct_block'.
    pub fn dequantize_and_idct_block(coefficients: &[i16], quantization_table: &[u16; 64], output_linestride: usize, output: &mut [u8]) {
        debug_assert_eq!(coefficients.len(), 64);

        let mut temp = [0i32; 64];

        // columns
        for i in 0 .. 8 {
            // if all zeroes, shortcut -- this avoids dequantizing 0s and IDCTing
            if coefficients[i + 8] == 0 && coefficients[i + 16] == 0 && coefficients[i + 24] == 0 &&
                    coefficients[i + 32] == 0 && coefficients[i + 40] == 0 && coefficients[i + 48] == 0 &&
                    coefficients[i + 56] == 0 {
                let dcterm = (coefficients[i] as i32 * quantization_table[i] as i32).wrapping_shl(2);
                temp[i]      = dcterm;
                temp[i + 8]  = dcterm;
                temp[i + 16] = dcterm;
                temp[i + 24] = dcterm;
                temp[i + 32] = dcterm;
                temp[i + 40] = dcterm;
                temp[i + 48] = dcterm;
                temp[i + 56] = dcterm;
            }
            else {
                let s0 = coefficients[i] as i32      * quantization_table[i] as i32;
                let s1 = coefficients[i + 8] as i32  * quantization_table[i + 8] as i32;
                let s2 = coefficients[i + 16] as i32 * quantization_table[i + 16] as i32;
                let s3 = coefficients[i + 24] as i32 * quantization_table[i + 24] as i32;
                let s4 = coefficients[i + 32] as i32 * quantization_table[i + 32] as i32;
                let s5 = coefficients[i + 40] as i32 * quantization_table[i + 40] as i32;
                let s6 = coefficients[i + 48] as i32 * quantization_table[i + 48] as i32;
                let s7 = coefficients[i + 56] as i32 * quantization_table[i + 56] as i32;

                let p2 = s2;
                let p3 = s6;
                let p1 = p2.wrapping_add(p3).wrapping_mul(stbi_f2f(0.5411961));
                let t2 = p1.wrapping_add(p3.wrapping_mul(stbi_f2f(-1.847759065)));
                let t3 = p1.wrapping_add(p2.wrapping_mul(stbi_f2f(0.765366865)));
                let p2 = s0;
                let p3 = s4;
                let t0 = stbi_fsh(p2.wrapping_add(p3));
                let t1 = stbi_fsh(p2.wrapping_sub(p3));
                let x0 = t0.wrapping_add(t3);
                let x3 = t0.wrapping_sub(t3);
                let x1 = t1.wrapping_add(t2);
                let x2 = t1.wrapping_sub(t2);
                let t0 = s7;
                let t1 = s5;
                let t2 = s3;
                let t3 = s1;
                let p3 = t0.wrapping_add(t2);
                let p4 = t1.wrapping_add(t3);
                let p1 = t0.wrapping_add(t3);
                let p2 = t1.wrapping_add(t2);
                let p5 = p3.wrapping_add(p4).wrapping_mul(stbi_f2f(1.175875602));
                let t0 = t0.wrapping_mul(stbi_f2f(0.298631336));
                let t1 = t1.wrapping_mul(stbi_f2f(2.053119869));
                let t2 = t2.wrapping_mul(stbi_f2f(3.072711026));
                let t3 = t3.wrapping_mul(stbi_f2f(1.501321110));
                let p1 = p5.wrapping_add(p1.wrapping_mul(stbi_f2f(-0.899976223)));
                let p2 = p5.wrapping_add(p2.wrapping_mul(stbi_f2f(-2.562915447)));
                let p3 = p3.wrapping_mul(stbi_f2f(-1.961570560));
                let p4 = p4.wrapping_mul(stbi_f2f(-0.390180644));
                let t3 = t3.wrapping_add(p1.wrapping_add(p4));
                let t2 = t2.wrapping_add(p2.wrapping_add(p3));
                let t1 = t1.wrapping_add(p2.wrapping_add(p4));
                let t0 = t0.wrapping_add(p1.wrapping_add(p3));

                // constants scaled things up by 1<<12; let's bring them back
                // down, but keep 2 extra bits of precision
                let x0 = x0.wrapping_add(512);
                let x1 = x1.wrapping_add(512);
                let x2 = x2.wrapping_add(512);
                let x3 = x3.wrapping_add(512);

                temp[i]      = x0.wrapping_add(t3).wrapping_shr(10);
                temp[i + 56] = x0.wrapping_sub(t3).wrapping_shr(10);
                temp[i + 8]  = x1.wrapping_add(t2).wrapping_shr(10);
                temp[i + 48] = x1.wrapping_sub(t2).wrapping_shr(10);
                temp[i + 16] = x2.wrapping_add(t1).wrapping_shr(10);
                temp[i + 40] = x2.wrapping_sub(t1).wrapping_shr(10);
                temp[i + 24] = x3.wrapping_add(t0).wrapping_shr(10);
                temp[i + 32] = x3.wrapping_sub(t0).wrapping_shr(10);
            }
        }

        for i in 0 .. 8 {
            // no fast case since the first 1D IDCT spread components out
            let s0 = temp[i * 8];
            let s1 = temp[i * 8 + 1];
            let s2 = temp[i * 8 + 2];
            let s3 = temp[i * 8 + 3];
            let s4 = temp[i * 8 + 4];
            let s5 = temp[i * 8 + 5];
            let s6 = temp[i * 8 + 6];
            let s7 = temp[i * 8 + 7];

            let p2 = s2;
            let p3 = s6;
            let p1 = p2.wrapping_add(p3).wrapping_mul(stbi_f2f(0.5411961));
            let t2 = p1.wrapping_add(p3.wrapping_mul(stbi_f2f(-1.847759065)));
            let t3 = p1.wrapping_add(p2.wrapping_mul(stbi_f2f(0.765366865)));
            let p2 = s0;
            let p3 = s4;
            let t0 = stbi_fsh(p2.wrapping_add(p3));
            let t1 = stbi_fsh(p2.wrapping_sub(p3));
            let x0 = t0.wrapping_add(t3);
            let x3 = t0.wrapping_sub(t3);
            let x1 = t1.wrapping_add(t2);
            let x2 = t1.wrapping_sub(t2);
            let t0 = s7;
            let t1 = s5;
            let t2 = s3;
            let t3 = s1;
            let p3 = t0.wrapping_add(t2);
            let p4 = t1.wrapping_add(t3);
            let p1 = t0.wrapping_add(t3);
            let p2 = t1.wrapping_add(t2);
            let p5 = p3.wrapping_add(p4).wrapping_mul(stbi_f2f(1.175875602));
            let t0 = t0.wrapping_mul(stbi_f2f(0.298631336));
            let t1 = t1.wrapping_mul(stbi_f2f(2.053119869));
            let t2 = t2.wrapping_mul(stbi_f2f(3.072711026));
            let t3 = t3.wrapping_mul(stbi_f2f(1.501321110));
            let p1 = p5.wrapping_add(p1.wrapping_mul(stbi_f2f(-0.899976223)));
            let p2 = p5.wrapping_add(p2.wrapping_mul(stbi_f2f(-2.562915447)));
            let p3 = p3.wrapping_mul(stbi_f2f(-1.961570560));
            let p4 = p4.wrapping_mul(stbi_f2f(-0.390180644));
            let t3 = t3.wrapping_add(p1.wrapping_add(p4));
            let t2 = t2.wrapping_add(p2.wrapping_add(p3));
            let t1 = t1.wrapping_add(p2.wrapping_add(p4));
            let t0 = t0.wrapping_add(p1.wrapping_add(p3));

            // constants scaled things up by 1<<12, plus we had 1<<2 from first
            // loop, plus horizontal and vertical each scale by sqrt(8) so together
            // we've got an extra 1<<3, so 1<<17 total we need to remove.
            // so we want to round that, which means adding 0.5 * 1<<17,
            // aka 65536. Also, we'll end up with -128 to 127 that we want
            // to encode as 0..255 by adding 128, so we'll add that before the shift
            let x0 = x0.wrapping_add(65536 + (128 << 17));
            let x1 = x1.wrapping_add(65536 + (128 << 17));
            let x2 = x2.wrapping_add(65536 + (128 << 17));
            let x3 = x3.wrapping_add(65536 + (128 << 17));

            output[i * output_linestride]     = stbi_clamp(x0.wrapping_add(t3).wrapping_shr(17));
            output[i * output_linestride + 7] = stbi_clamp(x0.wrapping_sub(t3).wrapping_shr(17));
            output[i * output_linestride + 1] = stbi_clamp(x1.wrapping_add(t2).wrapping_shr(17));
            output[i * output_linestride + 6] = stbi_clamp(x1.wrapping_sub(t2).wrapping_shr(17));
            output[i * output_linestride + 2] = stbi_clamp(x2.wrapping_add(t1).wrapping_shr(17));
            output[i * output_linestride + 5] = stbi_clamp(x2.wrapping_sub(t1).wrapping_shr(17));
            output[i * output_linestride + 3] = stbi_clamp(x3.wrapping_add(t0).wrapping_shr(17));
            output[i * output_linestride + 4] = stbi_clamp(x3.wrapping_sub(t0).wrapping_shr(17));
        }
    }

    // take a -128..127 value and stbi__clamp it and convert to 0..255
    fn stbi_clamp(x: i32) -> u8
    {
       // trick to use a single test to catch both cases
       if x as u32 > 255 {
          if x < 0 { return 0; }
          if x > 255 { return 255; }
       }

       x as u8
    }

    fn stbi_f2f(x: f32) -> i32 {
        (x * 4096.0 + 0.5) as i32
    }

    fn stbi_fsh(x: i32) -> i32 {
        x << 12
    }

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