Don’t compute unreachable outputs.

[mbostock]

Previously, a variable that was unreachable but which referenced exactly one reachable variable would be computed. This was not the intent! We set variable._indegree to -1 to prevent the variable from being added to the compute queue here:

function postqueue(variable) {
  if (--variable._indegree === 0) {
    queue.push(variable);
  }
}

However, if the unreachable variable was an output of exactly one other reachable variable, its indegree would be erroneously incremented back up to 0 here:

// Compute the indegree of updating variables.
variables.forEach(function(variable) {
  variable._outputs.forEach(variable_increment);
});

By setting variable._indegree to NaN instead of -1, we ensure that this incrementing and decrementing has no effect, and thus the unreachable variable is never computed.

Not computing unreachable variables is important when these variables have side-effects.

[jashkenas]

This looks like a good fix ... but just to fully strap on our belt and suspenders, should we also change the definition of postqueue to be:

function postqueue(variable) {
  if (--variable._indegree === 0 && variable._reachable) {
    queue.push(variable);
  }
}

[mbostock]

That shouldn’t be necessary if the code is correct; just slower. I’ve added a test to confirm the correctness of the fix, and realized it takes exactly two reachable references, not one. (We already had a test for one.)

[mbostock]

Merged in b86e7e9.