Simplify stride calculations in loglocator.

anntzer · anntzer · commit a06f343dee3c · 2018-11-22T19:53:26.000+01:00
These are just algebraic manipulations:

In classic mode, we want the smallest stride so that

    numdec/stride+1 &lt;= numticks

i.e.

    stride &gt;= numdec/(numticks-1)

so stride is the ceil() of the RHS (and at least 1, to handle the case
vmin==vmax).

In nonclassic mode, we want the smallest stride so that

    numdec//stride+1 &lt;= numticks, i.e. numdec//stride &lt; numticks

(everything is integer), i.e.

    numdec &lt; numticks*stride- 1

i.e.

    stride &gt; (numdec+1)/numticks

so stride is 1 + the floor() of the LHS.
diff --git a/lib/matplotlib/ticker.py b/lib/matplotlib/ticker.py
@@ -2182,15 +2182,10 @@ def tick_values(self, vmin, vmax):
         else:
             subs = self._subs
 
-        # get decades between major ticks.
-        stride = 1
-        if rcParams['_internal.classic_mode']:
-            # Leave the bug left over from the PY2-PY3 transition.
-            while numdec / stride + 1 > numticks:
-                stride += 1
-        else:
-            while numdec // stride + 1 > numticks:
-                stride += 1
+        # Get decades between major ticks.
+        stride = (max(math.ceil(numdec / (numticks - 1)), 1)
+                  if rcParams['_internal.classic_mode'] else
+                  (numdec + 1) // numticks + 1)
 
         # Does subs include anything other than 1?
         have_subs = len(subs) > 1 or (len(subs) == 1 and subs[0] != 1.0)
