improve display of estimated number of lines

fixes #6506

for large files the number of lines is estimated and shown
as a rounded number (using two significant digits), e.g
`~8,700,000 lines`.

with this change it will be: `~87 10^5 lines`

this commit also makes roundify really round numbers (as the name
suggests) and not simply cut at two digits, but this could be
reverted if there are concerns wrt speed due to using more math

lib/galaxy/datatypes/data.py

@@ -1004,7 +1004,7 @@ def set_peek(self, dataset, line_count=None, WIDTH=256, skipchars=None, line_wra
                     else:
                         est_lines = self.estimate_file_lines(dataset)
                         if est_lines is not None:
-                            dataset.blurb = f"~{util.commaify(util.roundify(str(est_lines)))} {inflector.cond_plural(est_lines, self.line_class)}"
+                            dataset.blurb = f"~{util.trailing_zeros_to_powerof10(util.roundify(str(est_lines)))} {inflector.cond_plural(est_lines, self.line_class)}"
                         else:
                             dataset.blurb = "Error: Cannot estimate lines in dataset"
             else:

lib/galaxy/util/__init__.py

@@ -29,6 +29,10 @@
 from email.mime.multipart import MIMEMultipart
 from email.mime.text import MIMEText
 from hashlib import md5
+from math import (
+    floor,
+    log10,
+)
 from os.path import relpath
 from urllib.parse import (
     urlencode,
@@ -1065,14 +1069,48 @@ def commaify(amount):
         return commaify(new)
 
 
+def trailing_zeros_to_powerof10(amount):
+    """
+    >>> trailing_zeros_to_powerof10(23000)
+    '23000'
+    >>> trailing_zeros_to_powerof10(2300000)
+    '23 10^5'
+    >>> trailing_zeros_to_powerof10(23000000)
+    >>> trailing_zeros_to_powerof10(1)
+    '1'
+    >>> trailing_zeros_to_powerof10(0)
+    '0'
+    >>> trailing_zeros_to_powerof10(100)
+    '100'
+    >>> trailing_zeros_to_powerof10(-100)
+    '-100'
+    """
+    amount = str(amount)
+    zeros = 0
+    i = len(amount) - 1
+    while i >= 0 and amount[i] == "0":
+        zeros += 1
+        i -= 1
+    if len(amount) < len(f"{amount[:i+1]} 10^{zeros}"):
+        return amount
+    else:
+        return f"{amount[:i+1]} 10^{zeros}"
+
+
 def roundify(amount, sfs=2):
     """
-    Take a number in string form and truncate to 'sfs' significant figures.
+    Take a number and round it to 'sfs' significant figures.
+
+    >>> roundify(99)
+    99
+    >>> roundify(-99)
+    -99
+    >>> roundify(1111)
+    1100
+    >>> roundify(1999)
+    2000
     """
-    if len(amount) <= sfs:
-        return amount
-    else:
-        return amount[0:sfs] + "0" * (len(amount) - sfs)
+    return round(amount, -int(floor(log10(abs(amount)))) + sfs - 1)
 
 
 def unicodify(value, encoding=DEFAULT_ENCODING, error="replace", strip_null=False, log_exception=True):