remove FirebaseJson lib from firmware (save 17.5k of flash)

we used this library solely to interpret the answer of an HTTP web
server as JSON and find a particular value using a path expression in
the HTTP power meter implementation.

since we ran out of flash memory on non-S3 ESP32, we need to cut some
corners. removing FirebaseJson is the last low-hanging fruit that we
currently know of. we can get rid of it by using ArduinoJson (which is
already integral part of the firmware) and implementing a custom logic
to extract a value based on a path expression.

other than the FirebaseJson path "finder", the new implementation
only knows how to access sub-keys delimited by a forward slash. in
particular, accessing array members is not supported any more. I am
hoping that this is simply not an issue. if so, we will have users
complaining and we can add this functionality in a later release.

include/HttpPowerMeter.h

@@ -26,7 +26,7 @@ class HttpPowerMeterClass {
     String extractParam(String& authReq, const String& param, const char delimit);
     String getcNonce(const int len);
     String getDigestAuth(String& authReq, const String& username, const String& password, const String& method, const String& uri, unsigned int counter);
-    bool tryGetFloatValueForPhase(int phase, const char* jsonPath, Unit_t unit, bool signInverted);
+    bool tryGetFloatValueForPhase(int phase, String jsonPath, Unit_t unit, bool signInverted);
     void prepareRequest(uint32_t timeout, const char* httpHeader, const char* httpValue);
     String sha256(const String& data);
 };

platformio.ini

@@ -47,7 +47,6 @@ lib_deps =
     https://github.com/coryjfowler/MCP_CAN_lib
     plerup/EspSoftwareSerial @ ^8.0.1
     https://github.com/dok-net/ghostl @ ^1.0.1
-    mobizt/FirebaseJson @ ^3.0.6
 	rweather/Crypto@^0.4.0
 
 extra_scripts =

src/HttpPowerMeter.cpp

@@ -3,7 +3,7 @@
 #include "HttpPowerMeter.h"
 #include "MessageOutput.h"
 #include <WiFiClientSecure.h>
-#include <FirebaseJson.h>
+#include <ArduinoJson.h>
 #include <Crypto.h>
 #include <SHA256.h>
 #include <base64.h>
@@ -219,18 +219,43 @@ String HttpPowerMeterClass::getDigestAuth(String& authReq, const String& usernam
     return authorization;
 }
 
-bool HttpPowerMeterClass::tryGetFloatValueForPhase(int phase, const char* jsonPath, Unit_t unit, bool signInverted)
+bool HttpPowerMeterClass::tryGetFloatValueForPhase(int phase, String jsonPath, Unit_t unit, bool signInverted)
 {
-    FirebaseJson json;
-    json.setJsonData(httpResponse);
-    FirebaseJsonData value;
-    if (!json.get(value, jsonPath)) {
-        snprintf_P(httpPowerMeterError, sizeof(httpPowerMeterError), PSTR("[HttpPowerMeter] Couldn't find a value for phase %i with Json query \"%s\""), phase, jsonPath);
+    JsonDocument root;
+    const DeserializationError error = deserializeJson(root, httpResponse);
+    if (error) {
+        snprintf_P(httpPowerMeterError, sizeof(httpPowerMeterError),
+                PSTR("[HttpPowerMeter] Unable to parse server response as JSON"));
+        return false;
+    }
+
+    constexpr char delimiter = '/';
+    int start = 0;
+    int end = jsonPath.indexOf(delimiter);
+    auto value = root.as<JsonVariantConst>();
+
+    // NOTE: "Because ArduinoJson implements the Null Object Pattern, it is
+    // always safe to read the object: if the key doesn't exist, it returns an
+    // empty value."
+    while (end != -1) {
+        String key = jsonPath.substring(start, end);
+        value = value[key];
+        start = end + 1;
+        end = jsonPath.indexOf(delimiter, start);
+    }
+
+    String lastKey = jsonPath.substring(start);
+    value = value[lastKey];
+
+    if (value.isNull()) {
+        snprintf_P(httpPowerMeterError, sizeof(httpPowerMeterError),
+                PSTR("[HttpPowerMeter] Unable to find a value for phase %i with JSON path \"%s\""),
+                phase+1, jsonPath.c_str());
         return false;
     }
 
     // this value is supposed to be in Watts and positive if energy is consumed.
-    power[phase] = value.to<float>();
+    power[phase] = value.as<float>();
 
     switch (unit) {
         case Unit_t::MilliWatts:

webapp/src/locales/de.json

@@ -577,7 +577,7 @@
         "httpHeaderKeyDescription": "Ein individueller HTTP request header kann hier definiert werden. Das kann z.B. verwendet werden um einen eigenen Authorization header mitzugeben.",
         "httpHeaderValue": "Optional: HTTP request header - Wert",
         "httpJsonPath": "JSON Pfad",
-        "httpJsonPathDescription": "JSON Pfad um den Leistungswert zu finden. Es verwendet die Selektions-Syntax von mobizt/FirebaseJson. Beispiele gibt es oben.",
+        "httpJsonPathDescription": "Anwendungsspezifischer JSON-Pfad um den Leistungswert in the HTTP(S) Antwort zu finden, z.B. 'power/total/watts' oder nur 'total'.",
         "httpUnit": "Einheit",
         "httpSignInverted": "Vorzeichen umkehren",
         "httpSignInvertedHint": "Positive Werte werden als Leistungsabnahme aus dem Netz interpretiert. Diese Option muss aktiviert werden, wenn das Vorzeichen des Wertes die gegenteilige Bedeutung hat.",

webapp/src/locales/en.json

@@ -582,7 +582,7 @@
         "httpHeaderKeyDescription": "A custom HTTP request header can be defined. Might be useful if you have to send something like a custom Authorization header.",
         "httpHeaderValue": "Optional: HTTP request header - Value",
         "httpJsonPath": "JSON path",
-        "httpJsonPathDescription": "JSON path to find the power value in the response. This uses the JSON path query syntax from mobizt/FirebaseJson. See above for examples.",
+        "httpJsonPathDescription": "Application specific JSON path to find the power value in the HTTP(S) response, e.g., 'power/total/watts' or simply 'total'.",
         "httpUnit": "Unit",
         "httpSignInverted": "Change Sign",
         "httpSignInvertedHint": "Is is expected that positive values denote power usage from the grid. Check this option if the sign of this value has the opposite meaning.",

webapp/src/views/PowerMeterAdminView.vue

@@ -116,11 +116,9 @@
 
                         <h2>JSON path examples:</h2>
                         <ul>
-                            <li>total_power - { "othervalue": "blah", "total_power": 123.4 }</li>
-                            <li>testarray/[2]/myvalue - { "testarray": [ {}, { "power": 123.4 } ] }</li>
+                            <li><code>power/total/watts</code> - Finds 123.4 in <code>{ "power": { "phase1": { "factor": 0.98, "watts": 42 }, "total": { "watts": 123.4 } } }</code></li>
+                            <li><code>total</code> - Finds 123.4 in <code>{ "othervalue": 66, "total": 123.4 }</code></li>
                         </ul>
-
-                        More info: <a href="https://github.com/mobizt/FirebaseJson">https://github.com/mobizt/FirebaseJson</a>
                     </div>
 
                     <CardElement