rpc: Accept scientific notation for monetary amounts in JSON

Add a function `ParseFixedPoint` that parses numbers according
to the JSON number specification and returns a 64-bit integer.

Then this in `AmountFromValue`, rather than `ParseMoney`.

Also add lots of tests (thanks to @jonasschnelli for some of them).

Fixes issue #6297.

src/utilstrencodings.cpp

@@ -538,3 +538,123 @@ int atoi(const std::string& str)
 {
     return atoi(str.c_str());
 }
+
+/** Upper bound for mantissa.
+ * 10^18-1 is the largest arbitrary decimal that will fit in a signed 64-bit integer.
+ * Larger integers cannot consist of arbitrary combinations of 0-9:
+ *
+ *   999999999999999999  1^18-1
+ *  9223372036854775807  (1<<63)-1  (max int64_t)
+ *  9999999999999999999  1^19-1     (would overflow)
+ */
+static const int64_t UPPER_BOUND = 1000000000000000000LL - 1LL;
+
+/** Helper function for ParseFixedPoint */
+static inline bool ProcessMantissaDigit(char ch, int64_t &mantissa, int &mantissa_tzeros)
+{
+    if(ch == '0')
+        ++mantissa_tzeros;
+    else {
+        for (int i=0; i<=mantissa_tzeros; ++i) {
+            if (mantissa > (UPPER_BOUND / 10LL))
+                return false; /* overflow */
+            mantissa *= 10;
+        }
+        mantissa += ch - '0';
+        mantissa_tzeros = 0;
+    }
+    return true;
+}
+
+bool ParseFixedPoint(const std::string &val, int decimals, int64_t *amount_out)
+{
+    int64_t mantissa = 0;
+    int64_t exponent = 0;
+    int mantissa_tzeros = 0;
+    bool mantissa_sign = false;
+    bool exponent_sign = false;
+    int ptr = 0;
+    int end = val.size();
+    int point_ofs = 0;
+
+    if (ptr < end && val[ptr] == '-') {
+        mantissa_sign = true;
+        ++ptr;
+    }
+    if (ptr < end)
+    {
+        if (val[ptr] == '0') {
+            /* pass single 0 */
+            ++ptr;
+        } else if (val[ptr] >= '1' && val[ptr] <= '9') {
+            while (ptr < end && val[ptr] >= '0' && val[ptr] <= '9') {
+                if (!ProcessMantissaDigit(val[ptr], mantissa, mantissa_tzeros))
+                    return false; /* overflow */
+                ++ptr;
+            }
+        } else return false; /* missing expected digit */
+    } else return false; /* empty string or loose '-' */
+    if (ptr < end && val[ptr] == '.')
+    {
+        ++ptr;
+        if (ptr < end && val[ptr] >= '0' && val[ptr] <= '9')
+        {
+            while (ptr < end && val[ptr] >= '0' && val[ptr] <= '9') {
+                if (!ProcessMantissaDigit(val[ptr], mantissa, mantissa_tzeros))
+                    return false; /* overflow */
+                ++ptr;
+                ++point_ofs;
+            }
+        } else return false; /* missing expected digit */
+    }
+    if (ptr < end && (val[ptr] == 'e' || val[ptr] == 'E'))
+    {
+        ++ptr;
+        if (ptr < end && val[ptr] == '+')
+            ++ptr;
+        else if (ptr < end && val[ptr] == '-') {
+            exponent_sign = true;
+            ++ptr;
+        }
+        if (ptr < end && val[ptr] >= '0' && val[ptr] <= '9') {
+            while (ptr < end && val[ptr] >= '0' && val[ptr] <= '9') {
+                if (exponent > (UPPER_BOUND / 10LL))
+                    return false; /* overflow */
+                exponent = exponent * 10 + val[ptr] - '0';
+                ++ptr;
+            }
+        } else return false; /* missing expected digit */
+    }
+    if (ptr != end)
+        return false; /* trailing garbage */
+
+    /* finalize exponent */
+    if (exponent_sign)
+        exponent = -exponent;
+    exponent = exponent - point_ofs + mantissa_tzeros;
+
+    /* finalize mantissa */
+    if (mantissa_sign)
+        mantissa = -mantissa;
+
+    /* convert to one 64-bit fixed-point value */
+    exponent += decimals;
+    if (exponent < 0)
+        return false; /* cannot represent values smaller than 10^-decimals */
+    if (exponent >= 18)
+        return false; /* cannot represent values larger than or equal to 10^(18-decimals) */
+
+    for (int i=0; i < exponent; ++i) {
+        if (mantissa > (UPPER_BOUND / 10LL) || mantissa < -(UPPER_BOUND / 10LL))
+            return false; /* overflow */
+        mantissa *= 10;
+    }
+    if (mantissa > UPPER_BOUND || mantissa < -UPPER_BOUND)
+        return false; /* overflow */
+
+    if (amount_out)
+        *amount_out = mantissa;
+
+    return true;
+}
+