properly fix nim-lang#13196: json serialialization lossless roundtrip

lib/pure/json.nim

@@ -181,6 +181,8 @@ type
     of JArray:
       elems*: seq[JsonNode]
 
+const floatSerializationPrecision = 18
+
 proc newJString*(s: string): JsonNode =
   ## Creates a new `JString JsonNode`.
   result = JsonNode(kind: JString, str: s)
@@ -647,7 +649,7 @@ proc toPretty(result: var string, node: JsonNode, indent = 2, ml = true,
     if lstArr: result.indent(currIndent)
     # Fixme: implement new system.add ops for the JS target
     when defined(js): result.add($node.fnum)
-    else: result.addFloat(node.fnum)
+    else: result.addFloat(node.fnum, precision = floatSerializationPrecision)
   of JBool:
     if lstArr: result.indent(currIndent)
     result.add(if node.bval: "true" else: "false")
@@ -726,7 +728,7 @@ proc toUgly*(result: var string, node: JsonNode) =
     else: result.addInt(node.num)
   of JFloat:
     when defined(js): result.add($node.fnum)
-    else: result.addFloat(node.fnum)
+    else: result.addFloat(node.fnum, precision = floatSerializationPrecision)
   of JBool:
     result.add(if node.bval: "true" else: "false")
   of JNull:
@@ -1486,3 +1488,10 @@ when isMainModule:
   doAssert not isRefSkipDistinct(MyObject)
   doAssert isRefSkipDistinct(MyDistinct)
   doAssert isRefSkipDistinct(MyOtherDistinct)
+
+  # issue #13196
+  # some arbitrary float, not caring about actually significant places here
+  let x = 0.12345678901234567890123456789
+  let y = ($(%* x)).parseJson().getFloat()
+  doAssert x == y
+  doAssert $0.6 == "0.6"

lib/system/formatfloat.nim

@@ -16,7 +16,7 @@ proc writeToBuffer(buf: var array[65, char]; value: cstring) =
     buf[i] = value[i]
     inc i
 
-proc writeFloatToBuffer*(buf: var array[65, char]; value: BiggestFloat): int =
+proc writeFloatToBuffer*(buf: var array[65, char]; value: BiggestFloat, precision: static int = -1): int =
   ## This is the implementation to format floats in the Nim
   ## programming language. The specific format for floating point
   ## numbers is not specified in the Nim programming language and
@@ -29,7 +29,9 @@ proc writeFloatToBuffer*(buf: var array[65, char]; value: BiggestFloat): int =
   ## * `buf` - A buffer to write into. The buffer does not need to be
   ##           initialized and it will be overridden.
   ##
-  var n: int = c_sprintf(addr buf, "%.16g", value)
+  const precision2 = if precision == -1: 16 else: precision
+  const format = "%." & $precision2 & "g"
+  var n: int = c_sprintf(addr buf, format, value)
   var hasDot = false
   for i in 0..n-1:
     if buf[i] == ',':

lib/system/strmantle.nim

@@ -83,8 +83,9 @@ proc addCstringN(result: var string, buf: cstring; buflen: int) =
 
 import formatfloat
 
-proc addFloat*(result: var string; x: float) =
+proc addFloat*(result: var string; x: float, precision: static int = -1) =
   ## Converts float to its string representation and appends it to `result`.
+  ## passing `precision >=0 ` can override the default precision.
   ##
   ## .. code-block:: Nim
   ##   var
@@ -95,7 +96,7 @@ proc addFloat*(result: var string; x: float) =
     result.add $x
   else:
     var buffer: array[65, char]
-    let n = writeFloatToBuffer(buffer, x)
+    let n = writeFloatToBuffer(buffer, x, precision = precision)
     result.addCstringN(cstring(buffer[0].addr), n)
 
 proc add*(result: var string; x: float) {.deprecated: