Digging quarry timelapse
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.+
$*_
(_+)¶(_+)
$2¶$1¶$1
r`__\G
$%`$&amp;
¶
;
(?&lt;=(_+);.*)(?&lt;=;_+;\1*)_
¶$`_
m`^_+;
__
+`(&gt;*)_(_+)(_+&lt;?;)\2
$1_$.2$* $3¶$1&gt;$2&lt;;
T`&gt;\_` \\`&gt;+_
T`\\\_;&lt;`V/_`.&lt;|;


Try it online! Explanation:

.+
$*_


Convert the inputs to unary. This gives us W¶S.

(_+)¶(_+)
$2¶$1¶$1


Swap the inputs and duplicate the width. This gives us S¶W¶W.

r`__\G
$%`$&amp;


Calculate the volume of the quarry. This gives us S¶W¶V.

¶
;


Join the inputs into one line. This gives us S;W;V.

(?&lt;=(_+);.*)(?&lt;=;_+;\1*)_
¶$`_


Calculate the amount of progress for each day on its own line. Each day has the format S;W;D, where D is 0 on the first line and increments by S each day until it reaches V.

m`^_+;
__


Delete S and increase W by 2 on each line. This gives us G;D for each day.

+`(&gt;*)_(_+)(_+&lt;?;)\2
$1_$.2$* $3¶$1&gt;$2&lt;;


While D is nonzero, dig either D or G-2 from the line (so the first and last characters are always left), moving the depth to the next line. Each line is indented with one more &gt; than the previous. Newly dug lines also include a &lt;.

T`&gt;\_` \\`&gt;+_


Turn the indent into spaces and the following _ into a \.

T`\\\_;&lt;`V/_`.&lt;|;


If a &lt; is following a \ then turn it into a V, if it's following a _ then turn it into a /. Delete all the &lt;s and ;s.