swap matrix along diagonal

00 01 02 03 04
10 11 12 13 14
20 21 22 23 24
30 31 32 33 34
40 41 42 43 44

swapRightDiagonal(m):
    // no throw for n = 0
    if m == null ||
        (m.length() > 0 && m.length() != m[0].length()):
        throw
    n = m.length()
    ni = n -1
    for layer = 0, layer < n/2, layer++:
        swap(m, layer, layer, ni -layer, ni -layer)
        for i = layer +1, i < ni -layer, i++:
            swap(m, layer, i, ni -i, ni -layer)
            swap(m, i, layer, ni -layer, ni -i)

swapLeftDiagonal(m):
    // no throw for n = 0
    if m == null ||
        (m.length() > 0 && m.length() != m[0].length()):
        throw
    n = m.length()
    ni = n -1
    for layer = 0, layer < n/2, layer++:
        swap(m, ni -layer, layer, layer, ni -layer)
        for i = layer +1, i < ni -layer, i++:
            swap(m, i, layer, layer, i)
            swap(m, ni -layer, i, i, ni -layer)

swap(m, ai, aj, bi, bj):
    temp = m[ai][aj]
    m[ai][aj] = m[bi][bj]
    m[bi][bj] = temp