refactor around deltas, add configurable decay coefficient and smarter halting

impatient

@@ -12,11 +12,21 @@ parser = argparse.ArgumentParser(description='Display progress and time estimate
 parser.add_argument('-f', '--final', type=str, help='Expected final size/value at completion, optionally with a K/M/G/T suffix')
 parser.add_argument('-i', '--interval', type=int, default=10, help='Interval in seconds between samples (default 10)')
 parser.add_argument('-w', '--window', type=int, default=100, help='Number of samples to keep for the sliding window (default 100)')
+parser.add_argument('-d', '--decay', type=float, default=1, help='Decay coefficient for older samples (default 1). Must be between 0 and 1 inclusive. The lower this is, the more responsive/swingy the estimate will be.')
 tracker_types = parser.add_mutually_exclusive_group(required=True)
 tracker_types.add_argument('-p', '--path', type=str, help='Track total disk usage of a given path')
 tracker_types.add_argument('-c', '--command', type=str, help='Track value returned by a shell command; this should return a single number, optionally with a K/M/G/T suffix')
 
 args = parser.parse_args()
+if args.interval < 1:
+    print('interval must be at least 1', file=sys.stderr)
+    exit(1)
+if args.window < 1:
+    print('window size must be at least 1', file=sys.stderr)
+    exit(1)
+if args.decay < 0 or args.decay > 1:
+    print('decay coefficient must be between 0 and 1', file=sys.stderr)
+    exit(1)
 
 def parse_value(v):
     suffix = v[-1].upper()
@@ -57,19 +67,26 @@ if args.final:
 else:
     final = None
 
-samples = [current_val()]
+deltas = []
+current = current_val()
 while True:
     time.sleep(args.interval)
 
-    current = current_val()
+    new = current_val()
 
-    samples.append(current)
+    deltas.append(new - current)
-    if len(samples) < 2:
+    current = new
-        continue
+    if len(deltas) > args.window:
-    if len(samples) > args.window:
+        deltas = deltas[-args.window:]
-        samples = samples[-args.window:]
 
-    rate = (current - samples[0])/((len(samples)-1)*args.interval)
+    total = 0
+    divisor = 0
+    coeff = 1
+    for d in deltas[::-1]:
+        total += coeff*d
+        divisor += coeff
+        coeff *= args.decay
+    rate = total/(divisor*args.interval)
 
     print('\033[2K\r', end='')
     print('{} - {}/s'.format(display_value(current), display_value(rate)), end='')
@@ -86,6 +103,6 @@ while True:
             ), end='')
     sys.stdout.flush()
 
-    if final and current >= final:
+    if final and current >= final and all(d == 0 for d in deltas[-10:]):
         print()
         break