Suggest valid options when the configuration contains unknown keys

This changeset is purely to improve the user experience. When the
configuration parsing fails with an unknown key we'll test all possible
keys and suggest one that looks a lot like it. This way we can be
helpful in case someone made a typo or misremembered the precise name.

FlashMQTests/configtests.cpp

@@ -117,3 +117,56 @@ void MainTests::test_parsing_numbers()
         }
     }
 }
+
+void MainTests::testStringDistances()
+{
+    FMQ_COMPARE(distanceBetweenStrings("", ""), (unsigned int)0);
+    FMQ_COMPARE(distanceBetweenStrings("dog", ""), (unsigned int)3);
+    FMQ_COMPARE(distanceBetweenStrings("", "dog"), (unsigned int)3);
+    FMQ_COMPARE(distanceBetweenStrings("dog", "horse"), (unsigned int)4);
+    FMQ_COMPARE(distanceBetweenStrings("horse", "dog"), (unsigned int)4);
+    FMQ_COMPARE(distanceBetweenStrings("industry", "interest"), (unsigned int)6);
+    FMQ_COMPARE(distanceBetweenStrings("kitten", "sitting"), (unsigned int)3);
+    FMQ_COMPARE(distanceBetweenStrings("uninformed", "uniformed"), (unsigned int)1);
+}
+
+void MainTests::testConfigSuggestion()
+{
+    // User made a small typo: 'session' instead of 'sessions'
+    {
+        ConfFileTemp config;
+        config.writeLine("expire_session_after_seconds 180");
+        config.closeFile();
+
+        ConfigFileParser parser(config.getFilePath());
+
+        try
+        {
+            parser.loadFile(false);
+            FMQ_FAIL("The parser is too liberal");
+        }
+        catch (ConfigFileException &ex)
+        {
+            FMQ_COMPARE(ex.what(), "Config key 'expire_session_after_seconds' is not valid (here). Did you mean: expire_sessions_after_seconds ?");
+        }
+    }
+
+    // User entered gibberish. Let's not suggest gibberish back
+    {
+        ConfFileTemp config;
+        config.writeLine("foobarbaz 180");
+        config.closeFile();
+
+        ConfigFileParser parser(config.getFilePath());
+
+        try
+        {
+            parser.loadFile(false);
+            FMQ_FAIL("The parser is too liberal");
+        }
+        catch (ConfigFileException &ex)
+        {
+            FMQ_COMPARE(ex.what(), "Config key 'foobarbaz' is not valid (here).");
+        }
+    }
+}

FlashMQTests/maintests.cpp

@@ -231,6 +231,8 @@ MainTests::MainTests()
     REGISTER_FUNCTION(testOverrideWillDelayOnSessionDestructionByTakeOver);
     REGISTER_FUNCTION(testDisabledWills);
     REGISTER_FUNCTION(testMqtt5DelayedWillsDisabled);
+    REGISTER_FUNCTION(testStringDistances);
+    REGISTER_FUNCTION(testConfigSuggestion);
     REGISTER_FUNCTION(testIncomingTopicAlias);
     REGISTER_FUNCTION(testOutgoingTopicAlias);
     REGISTER_FUNCTION(testOutgoingTopicAliasBeyondMax);

FlashMQTests/maintests.h

@@ -120,6 +120,8 @@ class MainTests
     void testOverrideWillDelayOnSessionDestructionByTakeOver();
     void testDisabledWills();
     void testMqtt5DelayedWillsDisabled();
+    void testStringDistances();
+    void testConfigSuggestion();
 
 
     void testIncomingTopicAlias();

FlashMQTests/testhelpers.cpp

@@ -1,6 +1,7 @@
 #include "testhelpers.h"
 
 #include <iostream>
+#include <cstring>
 
 int assert_count;
 int assert_fail_count;
@@ -16,8 +17,21 @@ bool fmq_assert(bool b, const char *failmsg, const char *actual, const char *exp
 
         if (asserts_print)
         {
-            std::cout << RED << "FAIL" << COLOR_END << ": '" << failmsg << "', " << actual << " != " << expected << std::endl
-                      << " in " << file << ", line " << line << std::endl;
+            // There are two types of failmsg: unformatted ones and formatted ones.
+            // By testing for a newline we can detect formatted ones.
+            if (strchr(failmsg, '\n') == nullptr)
+            {
+                // unformatted
+                std::cout << RED << "FAIL" << COLOR_END << ": '" << failmsg << "', " << actual << " != " << expected << std::endl
+                          << " in " << file << ", line " << line << std::endl;
+            }
+            else
+            {
+                // formatted
+                std::cout << RED << "FAIL" << COLOR_END << " in " << file << ", line " << line << std::endl;
+                std::cout << failmsg << std::endl;
+                std::cout << "Comparison: " << actual << " != " << expected << std::endl;
+            }
         }
     }
 

FlashMQTests/testhelpers.h

@@ -61,7 +61,17 @@ inline bool fmq_compare(const char *c1, const char *c2, const char *actual, cons
     std::string s2(c2);
 
     std::ostringstream oss;
-    oss << s1 << " != " << s2;
+
+    if (s1.length() + s2.length() < 100)
+    {
+        // short form
+        oss << s1 << " != " << s2;
+    }
+    else
+    {
+        oss << "Actual:     " << s1 << std::endl;
+        oss << "Expected:   " << s2;
+    }
 
     return fmq_assert(s1 == s2, oss.str().c_str(), actual, expected, file, line);
 }

configfileparser.cpp

@@ -93,6 +93,33 @@ bool ConfigFileParser::testKeyValidity(const std::string &key, const std::string
     {
         std::ostringstream oss;
         oss << "Config key '" << key << "' is not valid (here).";
+
+        // Maybe the user made a typo
+        auto alternative = validKeys.end();
+        unsigned int alternative_distance = UINT_MAX;
+
+        for (auto possible_key = validKeys.begin(); possible_key != validKeys.end(); ++possible_key)
+        {
+            unsigned int distance = distanceBetweenStrings(key.c_str(), *possible_key);
+            // We only want to suggest options that look a bit like the unknown
+            // one. Experimentally I found 50% of the total length a decent
+            // cutoff.
+            //
+            // The mathemathical formula "distance/length < 0.5" can be
+            // approximated with integers as "distance*2/length < 1"
+            if ((distance * 2) / key.length() < 1 && distance < alternative_distance)
+            {
+                alternative = possible_key;
+                alternative_distance = distance;
+            }
+        }
+        if (alternative != validKeys.end())
+        {
+            // It might indeed have been a typo.
+            // The space before the question mark is to make copying using mouse-double-click possible.
+            oss << " Did you mean: " << alternative->c_str() << " ?";
+        }
+
         throw ConfigFileException(oss.str());
     }
 

utils.cpp

@@ -724,6 +724,90 @@ std::string protocolVersionString(ProtocolVersion p)
     }
 }
 
+/**
+ * @brief Returns the Levenshtein distance between the two given strings
+ *
+ * This function uses the Wagner–Fischer algorithm to calculate the edit
+ * distance between two strings: the total number of insertions, swaps, and
+ * deletions are needed to transform the one into the other.
+ */
+unsigned int distanceBetweenStrings(const std::string &stringA, const std::string &stringB)
+{
+    // The matrix contains the distances between the substrings.
+    // You can find a description of the algorithm online.
+    //
+    // Roughly:
+    //
+    // line_a: "dog"
+    // line_b: "horse"
+    //
+    //   -->
+    //  |   | # | d | o | g
+    //  V --+---+---+---+---+---+---+---
+    //    # | P
+    //    h |
+    //    o |
+    //    r | Q   X       Y
+    //    s |
+    //    e |             Z
+    //
+    // P = [0, 0] = the distance from ""    to ""       (which is 0)
+    // Q = [0, 3] = the distance from ""    to "hor"    (which is 3, all inserts)
+    // X = [1, 3] = the distance from "d"   to "hor"    (which is 3, 1 swap and 2 inserts)
+    // Y = [3, 3] = the distance from "dog" to "hor"    (which is 2, both swaps)
+    // Z = [3, 5] = the distance from "dog" to "horse"  (which is 4, two swaps and 2 inserts)
+    //
+    // The matrix does not have to be square, the dimensions depends on the inputs
+    //
+    // the position within stringA should always be referred to as x
+    // the position within stringB should always be referred to as y
+
+    using mymatrix = std::vector<std::vector<int>>;
+    // +1 because we also need to store the length from the empty strings
+    int width = stringA.size() + 1;
+    int height = stringB.size() + 1;
+    mymatrix distances(width, std::vector<int>(height));
+
+    // We know that the distance from the substrings of line_a to ""
+    // is equal to the length of the substring of line_a
+    for (int x = 0; x < width; x++)
+    {
+        distances[x][0] = x;
+    }
+
+    // We know that the distance from "" to the substrings of line_b
+    // is equal to the length of the substring of line_b
+    for (int y = 0; y < height; y++)
+    {
+        distances[0][y] = y;
+    }
+
+    // Now all we do is to fill out the rest of the matrix, easy peasy
+    // note we start at 1 because the top row and left column have already been calculated
+    for (int x = 1; x < width; x++)
+    {
+        for (int y = 1; y < height; y++)
+        {
+            if (stringA[x - 1] == stringB[y - 1])
+            {
+                // the letters in both words are the same: we can travel from the top-left for free to the current state
+                distances[x][y] = distances[x - 1][y - 1];
+            }
+            else
+            {
+                // let's calculate the different costs and pick the cheapest option
+                // We use "+1" for all costs since they are all equally likely in our case
+                int dinstance_with_deletion = distances[x][y - 1] + 1;
+                int dinstance_with_insertion = distances[x - 1][y] + 1;
+                int dinstance_with_substitution = distances[x - 1][y - 1] + 1;
+                distances[x][y] = std::min({dinstance_with_deletion, dinstance_with_insertion, dinstance_with_substitution});
+            }
+        }
+    }
+
+    return distances[width - 1][height - 1]; // the last cell contains our answer
+}
+
 uint32_t ageFromTimePoint(const std::chrono::time_point<std::chrono::steady_clock> &point)
 {
     auto duration = std::chrono::steady_clock::now() - point;

utils.h

@@ -124,6 +124,8 @@ std::string websocketCloseCodeToString(uint16_t code);
 
 std::string protocolVersionString(ProtocolVersion p);
 
+unsigned int distanceBetweenStrings(const std::string &stringA, const std::string &stringB);
+
 uint32_t ageFromTimePoint(const std::chrono::time_point<std::chrono::steady_clock> &point);
 std::chrono::time_point<std::chrono::steady_clock> timepointFromAge(const uint32_t age);