Intial somewhat working version

src/JsonUtil.cpp

@@ -0,0 +1,48 @@
+#include "JsonUtil.h"
+
+#include <charconv>
+
+std::string jsonEscape(const std::string& value)
+{
+    std::string out;
+    out.reserve(value.size());
+    for (char ch : value)
+    {
+        switch (ch)
+        {
+        case '\\': out += "\\\\"; break;
+        case '"': out += "\\\""; break;
+        case '\n': out += "\\n"; break;
+        case '\r': out += "\\r"; break;
+        case '\t': out += "\\t"; break;
+        default: out += ch; break;
+        }
+    }
+    return out;
+}
+
+std::optional<float> findJsonFloat(const std::string& body, const std::string& key)
+{
+    const std::string quotedKey = "\"" + key + "\"";
+    size_t pos = body.find(quotedKey);
+    if (pos == std::string::npos)
+        return std::nullopt;
+
+    pos = body.find(':', pos + quotedKey.size());
+    if (pos == std::string::npos)
+        return std::nullopt;
+    ++pos;
+
+    while (pos < body.size() && (body[pos] == ' ' || body[pos] == '\t' || body[pos] == '\r' || body[pos] == '\n'))
+        ++pos;
+
+    size_t end = pos;
+    while (end < body.size() && (body[end] == '-' || body[end] == '+' || body[end] == '.' || (body[end] >= '0' && body[end] <= '9') || body[end] == 'e' || body[end] == 'E'))
+        ++end;
+
+    float value = 0.0f;
+    const auto result = std::from_chars(body.data() + pos, body.data() + end, value);
+    if (result.ec != std::errc())
+        return std::nullopt;
+    return value;
+}