We have 4 cameras “camA”, “camB”, “camC”, “camD” installed at every junction along a road and in this sequence. The flow of traffic is such “camA” is at the beginning, and “camD” is at the end.
| | | | | | | | -------- -------- -------- -------- ------- ===> camA camB camC camD ---------------------------------------------------------------------
Whenever a vehicle passes a junction or makes a turn at the junction, the camera is positioned to record down the license plate number. A working camera captures the plate number MOST of the time, but when it misses a certain % of cars in the past N cars, we consider it a “bad” camera.
For example, if a car with plate “plateAlice” drives forward without turning, and a second car with plate "plateTom" makes a turn at "camC" junction, we will record down the captures:
{
"camA":[
(1655529900, "plateAlice"),
(1655530010, "plateTom"),
],
"camB":[
(1655529920, "plateAlice"),
(1655530020, "plateTom"),
],
"camC":[
(1655529970, "plateAlice"),
(1655530050, "plateTom"),
],
"camD":[
(1655529980, "plateAlice"),
]
}
Is there a way to take the past X records from all 4 cameras and identify when a camera has gone bad? For example, if a camera misses 5 out of the past 10 cars, we consider it as a bad camera.
Assumptions
- Multiple cameras can be bad, but at least 1 camera will always be working.
- A vehicle can return back to “camA” again, so plate numbers can be recorded multiple times by a camera when the vehicle returns.
- A vehicle can only make a turn at a junction if its plate number has been recorded by the camera (strange assumption to simplify things)
Python solution preferred, I have a non-working attempt in Python below. Thanks!
My non-working attempt in Python:
from collections import namedtuple
Record = namedtuple("record", ["time", "plate"])
# 10 vehicles (counting repeated vehicles that circle back again)
data = {
"camA":[
Record(1655530000, "plateAlice"),
Record(1655530110, "plateTom"),
Record(1655530210, "plateMissing"),
Record(1655530310, "plateCharlie"),
Record(1655530410, "plateDan"),
Record(1655530510, "plateEdward"),
Record(1655530610, "plateMissing"), # looped back for a 2nd time
Record(1655530700, "plateAlice"), # looped back for a 2nd time
Record(1655530800, "plateAlice"), # looped back for a 3rd time
# missing "plateTom" (looped back for a 2nd time) because bad camera
# missing "plateFrank" because bad camera
],
"camB":[
Record(1655530020, "plateAlice"),
Record(1655530120, "plateTom"),
# missing "plateMissing" because bad camera
# missing "plateCharlie" because bad camera
# missing "plateDan" because bad camera
# missing "plateEdward" because bad camera
Record(1655530620, "plateMissing"),
# missing "plateAlice" because bad camera
Record(1655530820, "plateAlice"),
# missing "plateFrank" because bad camera
],
"camC":[
Record(1655530070, "plateAlice"),
Record(1655530150, "plateTom"), # Makes a turn at this junction
Record(1655530230, "plateMissing"),
Record(1655530330, "plateCharlie"),
Record(1655530430, "plateDan"),
Record(1655530530, "plateEdward"),
Record(1655530630, "plateMissing"),
Record(1655530730, "plateAlice"),
Record(1655530830, "plateAlice"),
Record(1655530930, "plateTom"), # looped back for a 2nd time
Record(1655530930, "plateFrank"), # Makes a turn at this junction
],
"camD":[
Record(1655530080, "plateAlice"),
# missing "plateTom" because it turned at "camC"
Record(1655530240, "plateMissing"),
Record(1655530340, "plateCharlie"),
Record(1655530440, "plateDan"),
# missing "plateEdward" because bad camera
Record(1655530640, "plateMissing"),
Record(1655530740, "plateAlice"),
Record(1655530840, "plateAlice"),
Record(1655530940, "plateTom"), # looped back for a 2nd time
]
}
def is_first_cam(cam):
return cam == "camA"
def check_for_bad_cameras(data):
misses = {
"camA": 0,
"camB": 0,
"camC": 0,
"camD": 0,
}
for r in data["camA"][::-1]:
for i, cam in enumerate(data):
if not is_first_cam(cam):
for s in data[cam][::-1]:
if s.time > r.time and s.plate == r.plate:
break
else:
misses[cam] += 1
print(misses)
# Expected {'camA': 2, 'camB': 6, 'camC': 0, 'camD': 1}
# Obtained {'camA': 0, 'camB': 3, 'camC': 0, 'camD': 1}
check_for_bad_cameras(data)
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Answer
Plates should be seen by cameras in order ABCD, ABC, AB or A. Anything else misses at least the previous camera.
Note: Missing D is invisible as it is equivalent to a turn-off at C.
from collections import namedtuple, defaultdict
from heapq import merge
Record = namedtuple("record", ["time", "plate"])
# data from your example.
# data = ... # Later CPythons key order preservation assumed.
cam2idx = {cam: i for i, cam in enumerate(data)} # order cams should appear
idx2cam = {i: cam for i, cam in enumerate(data)}
last_idx = defaultdict(lambda :-1)
time_plate_cam = [[(rec.time, rec.plate, cam) for rec in records]
for cam, records in data.items()]
for time, plate, cam in merge(*time_plate_cam): # sort by time
idx = cam2idx[cam]
prev = last_idx[plate]
if idx != prev + 1 and idx != 0:
print(f"{time}: {plate} appears suddenly at {cam} "
f"missing {idx2cam[idx-1]}")
last_idx[plate] = idx
Result:
1655530230: plateMissing appears suddenly at camC missing camB 1655530330: plateCharlie appears suddenly at camC missing camB 1655530430: plateDan appears suddenly at camC missing camB 1655530530: plateEdward appears suddenly at camC missing camB 1655530730: plateAlice appears suddenly at camC missing camB 1655530930: plateTom appears suddenly at camC missing camB 1655530930: plateFrank appears suddenly at camC missing camB