add rounding

rounding.py

@@ -0,0 +1,55 @@
+_FLOAT = float
+
+def float(string, length):
+    """Round the base-10 number spelled out by `string` to `length`-many digits
+       after the decimal point."""
+
+    #if `string` has no decimal tail at all, just kick back the original
+    try:
+        int(string)
+        return string
+    except ValueError:
+        pass
+
+    #If given a negative number, round its positive counterpart and return
+    #that rounded string prepended with a minus sign
+    if _FLOAT(string) < 0:
+        return '-' + float(string[1:], length)
+
+    #Split the string into an int head and a decimal tail
+    #raise an exception if there's more than 1 . in the string
+    head, tail = string.split('.')
+
+    #If the decimal tail fits within the prescribed length, return the original
+    if len(tail) <= length:
+        return string
+
+    #Split off the parts of the tail before and beyond the length limit
+    trunk, branch = tail[:length], tail[length:]
+
+    #set state to 1 if we need to round up or -1 if we need to round down
+    #for ties, round to the evens
+    stick = _FLOAT('0.' + branch)
+    if stick < 0.5:
+        state = -1
+    elif stick > 0.5:
+        state = 1
+    else:
+        state = (-1
+                 if int(trunk[-1]) % 2 == 0
+                 else 1)
+
+    #If rounding down, that's easy
+    if state < 0:
+        return f'{head}.{trunk}'
+    #otherwise, we're definitely rounding up
+
+    #cram the int head and the trunk part of the decimal tail together,
+    #parse that as an int, 
+    #add 1 to round up
+    #convert back to string
+    #split into two pieces for the int part and the decimal part
+    #put the decimal point into the string between those two pieces and return
+    stuff = str(int(head + trunk) + 1)
+    new_head, new_trunk = stuff[:-length], stuff[-length:]
+    return f'{new_head}.{new_trunk}'