static CONST1: i64 = 20091;

static CONST2: i64 = 35468;

pub(crate) fn idct4x4(block: &mut [i32]) {

    // The intermediate results may overflow the types, so we stretch the type.

    fn fetch(block: &[i32], idx: usize) -> i64 {

        i64::from(block[idx])

    }

    // Perform one lenght check up front to avoid subsequent bounds checks in this function

    assert!(block.len() >= 16);

    for i in 0usize..4 {

        let a1 = fetch(block, i) + fetch(block, 8 + i);

        let b1 = fetch(block, i) - fetch(block, 8 + i);

        let t1 = (fetch(block, 4 + i) * CONST2) >> 16;

        let t2 = fetch(block, 12 + i) + ((fetch(block, 12 + i) * CONST1) >> 16);

        let c1 = t1 - t2;

        let t1 = fetch(block, 4 + i) + ((fetch(block, 4 + i) * CONST1) >> 16);

        let t2 = (fetch(block, 12 + i) * CONST2) >> 16;

        let d1 = t1 + t2;

        block[i] = (a1 + d1) as i32;

        block[4 + i] = (b1 + c1) as i32;

        block[4 * 3 + i] = (a1 - d1) as i32;

        block[4 * 2 + i] = (b1 - c1) as i32;

    }

    for i in 0usize..4 {

        let a1 = fetch(block, 4 * i) + fetch(block, 4 * i + 2);

        let b1 = fetch(block, 4 * i) - fetch(block, 4 * i + 2);

        let t1 = (fetch(block, 4 * i + 1) * CONST2) >> 16;

        let t2 = fetch(block, 4 * i + 3) + ((fetch(block, 4 * i + 3) * CONST1) >> 16);

        let c1 = t1 - t2;

        let t1 = fetch(block, 4 * i + 1) + ((fetch(block, 4 * i + 1) * CONST1) >> 16);

        let t2 = (fetch(block, 4 * i + 3) * CONST2) >> 16;

        let d1 = t1 + t2;

        block[4 * i] = ((a1 + d1 + 4) >> 3) as i32;

        block[4 * i + 3] = ((a1 - d1 + 4) >> 3) as i32;

        block[4 * i + 1] = ((b1 + c1 + 4) >> 3) as i32;

        block[4 * i + 2] = ((b1 - c1 + 4) >> 3) as i32;

    }

}

// 14.3

pub(crate) fn iwht4x4(block: &mut [i32]) {

    // Perform one length check up front to avoid subsequent bounds checks in this function

    assert!(block.len() >= 16);

    for i in 0usize..4 {

        let a1 = block[i] + block[12 + i];

        let b1 = block[4 + i] + block[8 + i];

        let c1 = block[4 + i] - block[8 + i];

        let d1 = block[i] - block[12 + i];

        block[i] = a1 + b1;

        block[4 + i] = c1 + d1;

        block[8 + i] = a1 - b1;

        block[12 + i] = d1 - c1;

    }

    for block in block.chunks_exact_mut(4) {

        let a1 = block[0] + block[3];

        let b1 = block[1] + block[2];

        let c1 = block[1] - block[2];

        let d1 = block[0] - block[3];

        let a2 = a1 + b1;

        let b2 = c1 + d1;

        let c2 = a1 - b1;

        let d2 = d1 - c1;

        block[0] = (a2 + 3) >> 3;

        block[1] = (b2 + 3) >> 3;

        block[2] = (c2 + 3) >> 3;

        block[3] = (d2 + 3) >> 3;

    }

}