# The Vehicle Routing Problem with Time Windows¶

This example illustrates a model for the vehicle routing problem with time windows (VRPTW).

In the VRPTW the objective is to minimize the total cost of routing vehicles from a central depot to a set of customers. Each customer must be visited exactly once within a specified time window to deliver their required demand, each customer has a service time it takes to unload the vehicle, and each vehicle has a maximum capacity of goods to deliver. If a vehicle arrives early it is allowed to wait for the customer's time window to start.

Let $G(V,E)$ be graph with vertices $V = C \cup \{0, n + 1 \}$ where $C = \{1, \dots, n \}$ are the customers such that the depot is split in a 'start' and and 'end' depot and $E$ is the edges connecting all customers with each other and the depot vertices. Edge $(i,j)$ is associated with a cost $c_{ij}$ and a travel time $t_{ij}$. Vertex $i$ is associated with a time window $[a_i,b_i]$ and a demand $d_i$. The vehicle capacity is denoted $Q$.

Usually the problem is formulated as a path based model

$\begin{aligned} \min{ } & \sum_{p \in P} c_p \lambda_p \\ \text{s.t. } & \sum_{p \in P} \alpha_i \lambda_p = 1 & & i \in C \\ & \lambda_p \in \mathbb{B} && p \in P \end{aligned}$

where paths $p \in P$ are feasible paths in $G(V,E)$ with accumulated edge cost $c_p = \sum_{(i,j) \in E} c_{ij} \Big ( \sum_{ s \in p: s = (i,j)} \mathbf{1} \Big )$ and $\alpha_i = \sum_{(i,j) \in \delta^+(i)} \Big ( \sum_{ s \in p: s = (i,j)} \mathbf{1} \Big )$ expresses if path $p$ visits customer $i$.

The corresponding network model can be formulated as

$\begin{aligned} \min{ } & \sum_{(i,j) \in E} c_{ij} x_{ij} \\ \text{s.t. } & \sum_{(i,j) \in \delta^+(i)} x_{ij} = 1 & & i \in C \\ & x_{ij} = \sum_{p \in P} \Big ( \sum_{ s \in p: s = (i,j)} \mathbf{1} \Big ) \lambda_p & & (i,j) \in E \\ & 1 \leq \sum_{p \in P} \lambda_p \leq |C| \\ & \lambda_p \in \mathbb{B} && p \in P \\ & x_{i,j} \in \mathbb{B} && (i,j) \in E \end{aligned}$

where $p \in P$ are paths in a graph $G(V,E)$ subject to time and demand constraints modeled as disposable resources. Demand with vertex consumption $d_i$ and bounds $[0,Q]$ for all $i \in V$ and time with edge consumption $t_{ij}$ and vertex bounds $[a_i,b_i]$ for all $i \in V$.

The problem is implemented as

$\begin{aligned} \min{ } & \sum_{(i,j) \in E} c_{ij} x_{ij} \\ \text{s.t. } & \sum_{(i,j) \in \delta^+(i)} x_{ij} = 1 & & i \in C \\ & x_{i,j} \in \mathbb{B} && (i,j) \in E \end{aligned}$

with the graph and resource constraints described above. See the modeling section for details.

```
# Vehicle Routing Problem with Time Windows
from flowty import Model, xsum
from or_datasets import vrp_rep
bunch = vrp_rep.fetch_vrp_rep("solomon-1987-r1", instance="R102_025")
name, n, E, c, d, Q, t, a, b, x, y = bunch["instance"]
m = Model()
# one graph, it is identical for all vehicles
g = m.addGraph(obj=c, edges=E, source=0, sink=n - 1, L=1, U=n - 2, type="B")
# adds resources variables to the graph.
# travel time and customer time windows
m.addResourceDisposable(
graph=g, consumptionType="E", weight=t, boundsType="V", lb=a, ub=b, name="t"
)
# demand and capacity
m.addResourceDisposable(
graph=g, consumptionType="V", weight=d, boundsType="V", lb=0, ub=Q, name="d"
)
# set partition constriants
for i in range(n)[1:-1]:
m += xsum(x * 1 for x in g.vars if i == x.source) == 1
# packing set
for i in range(n)[1:-1]:
m.addPackingSet([x for x in g.vars if i == x.source])
status = m.optimize()
```