TST: add test for function coverage

Author: eigenfoo

tests/requirements.txt

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+requests==2.22.0

tests/test_function_coverage.py

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+import re
+import requests
+
+FUNCTION_SIGNATURES_FILE = "https://raw.githubusercontent.com/stan-dev/stan/develop/src/stan/lang/function_signatures.h"
+SYNTAX_FILE = "syntax/stan.vim"
+STAN_ADD_FUNCTIONS = [
+    "add",
+    "add_unary",
+    "add_unary_vectorized",
+    "add_binary",
+    "add_ternary",
+    "add_nullary",
+]
+
+# Request Stan file
+req = requests.get(FUNCTION_SIGNATURES_FILE)
+if req.status_code == requests.codes.ok:
+    stan_file = req.text
+else:
+    msg = "Could not request Stan file."
+    raise RuntimeError(msg)
+
+# Read syntax file
+with open(SYNTAX_FILE, "r") as f:
+    syntax_file = f.read()
+
+# Find all functions from Stan file
+add_functions = "|".join(STAN_ADD_FUNCTIONS)
+regex = r"(?<=[{}]\(['\"])(\w+)(?=['\"])".format(add_functions)
+stan_functions = set(re.findall(regex, stan_file))
+# Remove _log functions
+stan_functions = {
+    function for function in stan_functions if not function.endswith("_log")
+}
+# Add bare distribution names
+stan_functions = stan_functions | {
+    function[:-4] for function in stan_functions if function.endswith("_rng")
+}
+
+regex1 = r"(?<=syntax keyword stanFunction )(.*)"
+regex2 = r"(?<=syntax keyword stanConstant )(.*)"
+matches = re.findall(regex1, syntax_file) + re.findall(regex2, syntax_file)
+
+# Flatten
+highlighted_functions = set([word for match in matches for word in match.split()])
+
+missing = stan_functions - highlighted_functions
+unnecessary = highlighted_functions - stan_functions
+
+print("Missing:")
+print(missing)
+print()
+print("Unnecessary:")
+print(unnecessary)