renormalizer , 11 days ago

I’ve been a four-star programmer a few times. Imagine a blocked symmetric matrix where the rows and columns are indexed by triples (u,v,w). The entries are zero whenever u != u’ or v != v’, and because of symmetry you only store entries with w <= w’. But the range of v depends on the value of u and the range of w on the value of v. So you do

<span style="font-weight:bold;color:#a71d5d;">double ****</span><span style="color:#323232;">mat </span><span style="font-weight:bold;color:#a71d5d;">= </span><span style="color:#62a35c;">calloc </span><span style="color:#323232;">(UMAX, </span><span style="font-weight:bold;color:#a71d5d;">sizeof</span><span style="color:#323232;">(</span><span style="font-weight:bold;color:#a71d5d;">*</span><span style="color:#323232;">mat));
</span><span style="font-weight:bold;color:#a71d5d;">for </span><span style="color:#323232;">(</span><span style="font-weight:bold;color:#a71d5d;">int</span><span style="color:#323232;"> u </span><span style="font-weight:bold;color:#a71d5d;">= </span><span style="color:#0086b3;">0</span><span style="color:#323232;">; u </span><span style="font-weight:bold;color:#a71d5d;"><</span><span style="color:#323232;"> UMAX; </span><span style="font-weight:bold;color:#a71d5d;">++</span><span style="color:#323232;">u) {
</span><span style="color:#323232;">  mat[u] </span><span style="font-weight:bold;color:#a71d5d;">= </span><span style="color:#62a35c;">calloc </span><span style="color:#323232;">(u </span><span style="font-weight:bold;color:#a71d5d;">+ </span><span style="color:#0086b3;">1</span><span style="color:#323232;">, </span><span style="font-weight:bold;color:#a71d5d;">sizeof</span><span style="color:#323232;">(</span><span style="font-weight:bold;color:#a71d5d;">**</span><span style="color:#323232;">mat));
</span><span style="color:#323232;">  </span><span style="font-weight:bold;color:#a71d5d;">for </span><span style="color:#323232;">(</span><span style="font-weight:bold;color:#a71d5d;">int</span><span style="color:#323232;"> v </span><span style="font-weight:bold;color:#a71d5d;">= </span><span style="color:#0086b3;">0</span><span style="color:#323232;">; v </span><span style="font-weight:bold;color:#a71d5d;"><=</span><span style="color:#323232;"> u; </span><span style="font-weight:bold;color:#a71d5d;">++</span><span style="color:#323232;">v) {
</span><span style="color:#323232;">    mat[u][v] </span><span style="font-weight:bold;color:#a71d5d;">= </span><span style="color:#62a35c;">calloc </span><span style="color:#323232;">(v </span><span style="font-weight:bold;color:#a71d5d;">+ </span><span style="color:#0086b3;">1</span><span style="color:#323232;">, </span><span style="font-weight:bold;color:#a71d5d;">sizeof</span><span style="color:#323232;">(</span><span style="font-weight:bold;color:#a71d5d;">***</span><span style="color:#323232;">mat));
</span><span style="color:#323232;">    </span><span style="font-weight:bold;color:#a71d5d;">for </span><span style="color:#323232;">(</span><span style="font-weight:bold;color:#a71d5d;">int</span><span style="color:#323232;"> w </span><span style="font-weight:bold;color:#a71d5d;">= </span><span style="color:#0086b3;">0</span><span style="color:#323232;">; w </span><span style="font-weight:bold;color:#a71d5d;"><=</span><span style="color:#323232;"> v; </span><span style="font-weight:bold;color:#a71d5d;">++</span><span style="color:#323232;">w) {
</span><span style="color:#323232;">      mat[u][v][w] </span><span style="font-weight:bold;color:#a71d5d;">= </span><span style="color:#62a35c;">calloc </span><span style="color:#323232;">(w </span><span style="font-weight:bold;color:#a71d5d;">+ </span><span style="color:#0086b3;">1</span><span style="color:#323232;">, </span><span style="font-weight:bold;color:#a71d5d;">sizeof</span><span style="color:#323232;">(</span><span style="font-weight:bold;color:#a71d5d;">****</span><span style="color:#323232;">mat));
</span><span style="color:#323232;">      </span><span style="font-weight:bold;color:#a71d5d;">for </span><span style="color:#323232;">(</span><span style="font-weight:bold;color:#a71d5d;">int</span><span style="color:#323232;"> ww </span><span style="font-weight:bold;color:#a71d5d;">= </span><span style="color:#0086b3;">0</span><span style="color:#323232;">; ww </span><span style="font-weight:bold;color:#a71d5d;"><=</span><span style="color:#323232;"> w; </span><span style="font-weight:bold;color:#a71d5d;">++</span><span style="color:#323232;">ww)
</span><span style="color:#323232;">        mat[u][v][w][ww] </span><span style="font-weight:bold;color:#a71d5d;">= </span><span style="color:#323232;">some_function (u, v, w, ww);
</span><span style="color:#323232;">    }
</span><span style="color:#323232;">  }
</span><span style="color:#323232;">}
</span>

and weep a little. In reality, this gets a bit optimized by allocating a single chunk of memory and carving that up into the pointer and data arrays, so everything is reasonably close together in memory.