Add example with custom checker

Author: kiritofeng

docs/judge/problem_examples.md

@@ -4,3 +4,4 @@
 - [Interactive Grading (conditional IO-based)](https://github.com/DMOJ/docs/tree/master/problem_examples/interactive/seed2) - implements [https://dmoj.ca/problem/seed2](https://dmoj.ca/problem/seed2)
 - [Signature Grading (IOI-style)](https://github.com/DMOJ/docs/tree/master/problem_examples/signature/siggrade) - implements [https://dmoj.ca/problem/siggrade](https://dmoj.ca/problem/siggrade)
 - [Generating IO Data on the Fly (large testcase generation)](https://github.com/DMOJ/docs/tree/master/problem_examples/generator/ds3) - implements [https://dmoj.ca/problem/ds3](https://dmoj.ca/problem/ds3)
+- [Custom Checker (IO-based)](https://github.com/DMOJ/docs/tree/master/problem_examples/checker/seq3) - implements [https://dmoj.ca/problem/seq3](https://dmoj.ca/problem/seq3)

problem_examples/checker/seq3/check.py

@@ -0,0 +1,50 @@
+from dmoj.result import CheckerResult
+
+
+def check(process_output, judge_output, judge_input, point_value, **kwargs):
+    # process_output contains the user's output
+    # Firstly, we split the user's output into lines
+    process_lines = filter(None, process_output.split('\n'))
+
+    # We check that they only output 1 line of output
+    if len(process_lines) != 1:
+        # They did not follow output specifications
+        # Thus they get a WA verdict, 0 points, and a message telling them their output is malformed
+        return CheckerResult(False, 0, "Expected 1 line of output, got %d" % len(process_lines))
+
+    # Next we check that they printed only integers
+    try:
+        process_tokens = map(int, process_lines[0].split())
+    except (TypeError, ValueError):
+        # We again tell them they did not follow output specifications
+        return CheckerResult(False, 0, "Sequence contains non-numeric values!")
+
+    # We check that the sequence is of the correct length
+    # Firstly, we split the input into lines
+    input_lines = filter(None, judge_input.split('\n'))
+    # Then we parse N and K from the first line of input
+    N, K = map(int, input_lines[0].split())
+
+    if len(process_tokens) != N:
+        # We inform them that they did not output N numbers
+        return CheckerResult(False, 0, "Sequence's length is incorrect!")
+
+    # We check all numbers in the sequence are non-negative
+    if any(process_token < 0 for process_token in process_tokens):
+        # We again tell them they did not follow output specifications
+        return CheckerResult(False, 0, "Sequence contains negative numbers!")
+
+    # We check that the sequence sums to K
+    conditions_met = 0
+
+    if sum(process_tokens) == K:
+        conditions_met += 1
+    else:
+        return CheckerResult(False, 0, "Sequence's sum is incorrect!")
+
+    # The minimal possible product is 0, so we check if there exists a 0 in the sequence
+    if not all(process_tokens):
+        conditions_met += 1
+
+    # Finally, return the verdict
+    return CheckerResult(True, point_value * conditions_met / 2.0)

problem_examples/checker/seq3/in1.txt

@@ -0,0 +1 @@
+2 926

problem_examples/checker/seq3/in2.txt

@@ -0,0 +1 @@
+1002 243013241

problem_examples/checker/seq3/in3.txt

@@ -0,0 +1 @@
+34124 10

problem_examples/checker/seq3/in4.txt

@@ -0,0 +1 @@
+5 100000000

problem_examples/checker/seq3/in5.txt

@@ -0,0 +1 @@
+2 1

problem_examples/checker/seq3/init.yml

@@ -0,0 +1,7 @@
+checker: check.py
+test_cases:
+- {in: in1.txt, points: 20}
+- {in: in2.txt, points: 20}
+- {in: in3.txt, points: 20}
+- {in: in4.txt, points: 20}
+- {in: in5.txt, points: 20}