Add command suggestions

src/formula/string_utils.cpp

@@ -356,3 +356,42 @@ std::string vngettext_impl(const char* domain,
 	const std::string msg = utils::interpolate_variables_into_string(orig, &symbols);
 	return msg;
 }
+
+int edit_distance_approx(const std::string &str_1, const std::string &str_2)
+{
+	int edit_distance = 0;
+	if(str_1.length() == 0) {
+		return str_2.length();
+	}
+	else if(str_2.length() == 0) {
+		return str_1.length();
+	}
+	else {
+		int j = 0;
+		int len_max = std::max(str_1.length(), str_2.length());
+		for(int i = 0; i < len_max; i++) {
+			if(str_1[i] != str_2[j]) {
+				//SWAP
+				if(str_1[i+1] == str_2[j] && str_1[i] == str_2[j+1]) {
+					// No need to test the next letter
+					i++;j++;
+				}
+				//ADDITION
+				else if(str_1[i+1] == str_2[j]) {
+					j--;
+				}
+				//DELETION
+				else if(str_1[i] == str_2[j+1]) {
+					i--;
+				}
+				// CHANGE (no need to do anything, next letter MAY be successful).
+				edit_distance++;
+				if(edit_distance * 100 / std::min(str_1.length(), str_2.length()) > 33) {
+					break;
+				}
+			}
+			j++;
+		}
+	}
+	return edit_distance;
+}

src/formula/string_utils.hpp

@@ -129,3 +129,19 @@ std::string vngettext_impl(const char* domain,
 
 #define VNGETTEXT(msgid, msgid_plural, count, ...) \
 	vngettext_impl(GETTEXT_DOMAIN, msgid, msgid_plural, count, __VA_ARGS__)
+
+/**
+ * Approximately calculates the distance between two strings
+ *
+ * Inspired in the Levenshtein distance, but made simpler
+ * to avoid using recursion and wasting resources.
+ *
+ * The consequence is that the function gets "lost"
+ * after two consecutive differences.
+ *
+ * @param str_1 First string to compare
+ * @param str_2 Second string to compare
+ */
+
+int edit_distance_approx(const std::string &str_1, const std::string &str_2);
+

src/map_command_handler.hpp

@@ -190,10 +190,39 @@ class map_command_handler
 		}
 		else if (help_on_unknown_) {
 			utils::string_map symbols;
+			std::string string_user = get_cmd();
+			int distance = 0;
+			// Minimum length of the two compared strings.
+			int len_min = 0;
+			bool has_command_proposal = false;
+			// Compare the input with every command (excluding alias).
+			for(const auto& [key, index] : command_map_) {
+				// No need to test commands that are not enabled.
+				if(is_enabled(index)) {
+					distance = edit_distance_approx(string_user, key);
+					len_min = std::min(string_user.length(), key.length());
+					// Maximum of a third of the letters are wrong. The ratio
+					// between the edit distance and the minimum length, multiplied
+					// by a hundred gives us the  percentage of errors.
+					if(distance * 100 / len_min < 34) {
+						symbols["command_proposal"] = key;
+						has_command_proposal = true;
+						// If a good enough candidate is found, exit the loop.
+						break;
+					}
+				}
+			}
 			symbols["command"] = get_cmd();
 			symbols["help_command"] = cmd_prefix_ + "help";
-			print("help", VGETTEXT("Unknown command '$command', try $help_command "
-				"for a list of available commands.", symbols));
+			// If a proposal for a command is found, print it
+			if(has_command_proposal) {
+				print("help", VGETTEXT("Unknown command '$command', did you mean '$command_proposal'? try $help_command "
+					"for a list of available commands.", symbols));
+			}
+			else {
+				print("help", VGETTEXT("Unknown command '$command', try $help_command "
+					"for a list of available commands.", symbols));
+			}
 		}
 	}