Johannes Schindelin <Johannes.Schindelin@xxxxxx> writes:
> Satisfied?
Very much.
FWIW almost the same procedure led to the weights 0, 2, 1 and 4 that you
see in help.c. The weights are basically factors with which mistakes are
punished: if you just confuse two adjacent letters, such as "psuh" instead
of "push" (which can be quite common if you use two hands, one on the left
side, and one on the right side of the keyboard, with an en-US layout so
many of us use, myself included) it costs 0.
If you write a different character than what you intended, the cost is 2.
The idea behind it is that you're more likely to miss a key than to hit
the wrong key. With the laptop I am typing this email on, it is
particularly likely that I miss a key, because there are certain
key combinations where only the first key triggers an input event, but the
second only triggers an input event when it is _released_ after the first
one. So when I type "er" real fast and happen to release the "e" key
after the "r" key, no "r" appears on my screen.
Okay, so the weight for adding a character must be smaller than
substituting a character, but why is the cost for deletion so high?
Well, I really rarely type unnecessary characters (except when writing to
the Git mailing list, arguably) so those costs must be substantially
higher than for typing the wrong character.
These are actually very good justifications in the sense that people who
might want to tweak the heuristics can see the reason behind the current
choice and agree or disagree with it.
I somehow suspect that a good mathematician can come up with a rationale
for 6 after the fact that sounds convincing, along the lines of "the
average length of commands being N, and levenshtein penalties being
<0,2,1,4>, you can insert X mistaken keystroke and/or omit Y mistaken
keystroke per every correct keystroke without exceeding this value 6, and
the percentage X and/or Y represents is not too low to be practical but low
enough to reject false positives".
In any case, I'll further squash in the following. Thanks for an amusing
explanation ;-).
diff --git a/help.c b/help.c
index fbf80d9..de1e2ea 100644
--- a/help.c
+++ b/help.c
@@ -297,7 +297,7 @@ static void add_cmd_list(struct cmdnames *cmds, struct cmdnames *old)
old->names = NULL;
}
-/* how did we decide this is a good cutoff??? */
+/* An empirically derived magic number */
#define SIMILAR_ENOUGH(x) ((x) < 6)
const char *help_unknown_cmd(const char *cmd)
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