Add topo sort lab

Author: tekknolagi

build-lab.md

@@ -0,0 +1,81 @@
+---
+---
+
+# Graph lab
+
+Build system targets are often modeled as a directed acyclic graph (DAG).
+Each node in the graph represents a build target and each edge represents a
+dependency on another target. A correct build order is a topological sort: an
+order in which each target is only built after all of its dependencies have
+been built.
+
+We're not going to make a whole build system from scratch, but we will
+implement one of the core components in your preferred programming language: a
+topological sort.
+
+## Graph representation
+
+Write a class that satisfies this interface (it need not be Python; that's just
+the syntax for the interface sketch pseudocode):
+
+```python
+class Target:
+    def name(self) -> str: ...
+    def recipe(self) -> str: ...
+    def dependencies(self) -> List[Target]: ...
+```
+
+The `name` method should return the name of the target. The `recipe` method
+should return the command executed to build the target. The `dependencies`
+method should return a list of the target's dependencies.
+
+## A small graph
+
+Now, manually create a small graph of targets and dependencies. Try modeling
+the C example from `build.sh`:
+
+```
+foo: foo.o rng.o
+	cc -o foo foo.o rng.o
+
+foo.o: foo.c rng.h
+	cc -c foo.c
+
+rng.o: rng.c rng.h
+	cc -c rng.c
+
+myls.o: myls.c
+	cc -c myls.c
+
+myls: myls.o
+	cc -o myls myls.o
+```
+
+You may need to change around the order (bottom-up) to get your
+manually-created `Target` objects to make sense.
+
+## Topological sort
+
+Start by writing a function `execute` that, given a target with no
+dependencies, prints the target's recipe.
+
+Then, add another case: if the target has dependencies, execute them first.
+
+Now you have a problem: if targets `A` and `B` both depend on `C`, it will
+execute (print) the recipe for `C` twice. To fix this, add a `set` parameter
+called `visited` to `execute`. Before executing a target, check if its name is
+in the visited set.
+
+## Tests
+
+What are the core properties of your program that you want to test?
+
+Write tests for your topological sort. You can use the example above from
+`build.sh`, but you should also think smaller, and be more targeted. What is
+your specific test case trying to "break" in your implementation? We'll talk
+more about this next module.
+
+## Cycles?
+
+If you have a cycle in your graph, your topological sort will not work. How
+might you detect cycles?