properly fix #13196: json serialization with option for lossless roundtrip

lib/pure/json.nim

@@ -181,6 +181,21 @@ type
     of JArray:
       elems*: seq[JsonNode]
 
+const precisionDefault* = -1
+  ## default precision for stringification of float; semantics are subject to
+  ## future implementation improvements, see https://github.com/nim-lang/Nim/issues/13365
+  # -1 is tied to strmantle.nim and formatfloat.nim (we could also expose that symbol
+  # to avoid DRY issues)
+
+const precisionRoundtrip* = 17
+  ## precision for stringification of float that is high enough to be roundtrip safe
+  ## to avoid issue #13196.
+  # this works with nextafter(1.0, Inf).
+  # note that 17 seems enough, unlike what is mentioned here for D which recommended 18, quoting:
+  # > ceil(log(pow(2.0, double.mant_dig - 1)) / log(10.0) + 1) == (double.dig + 2)
+  # see:
+  # https://github.com/dlang/phobos/blob/b885f607e26750673aba694c46899583779d2361/std/json.d#L1644
+
 proc newJString*(s: string): JsonNode =
   ## Creates a new `JString JsonNode`.
   result = JsonNode(kind: JString, str: s)
@@ -612,7 +627,7 @@ proc escapeJson*(s: string): string =
   escapeJson(s, result)
 
 proc toPretty(result: var string, node: JsonNode, indent = 2, ml = true,
-              lstArr = false, currIndent = 0) =
+              lstArr = false, currIndent = 0, precision = precisionDefault) =
   case node.kind
   of JObject:
     if lstArr: result.indent(currIndent) # Indentation
@@ -647,7 +662,7 @@ 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 = precision)
   of JBool:
     if lstArr: result.indent(currIndent)
     result.add(if node.bval: "true" else: "false")
@@ -670,14 +685,14 @@ proc toPretty(result: var string, node: JsonNode, indent = 2, ml = true,
     if lstArr: result.indent(currIndent)
     result.add("null")
 
-proc pretty*(node: JsonNode, indent = 2): string =
+proc pretty*(node: JsonNode, indent = 2, precision = precisionDefault): string =
   ## Returns a JSON Representation of `node`, with indentation and
   ## on multiple lines.
   ##
   ## Similar to prettyprint in Python.
   runnableExamples:
     let j = %* {"name": "Isaac", "books": ["Robot Dreams"],
-                "details": {"age": 35, "pi": 3.1415}}
+                "details": {"age": 35, "number": 3.125}}
     doAssert pretty(j) == """
 {
   "name": "Isaac",
@@ -686,13 +701,14 @@ proc pretty*(node: JsonNode, indent = 2): string =
   ],
   "details": {
     "age": 35,
-    "pi": 3.1415
+    "number": 3.125
   }
 }"""
+    doAssert pretty(j["details"]["number"], precision = 2) == "3.1"
   result = ""
-  toPretty(result, node, indent)
+  toPretty(result, node, indent, precision = precision)
 
-proc toUgly*(result: var string, node: JsonNode) =
+proc toUgly*(result: var string, node: JsonNode, precision = precisionDefault) =
   ## Converts `node` to its JSON Representation, without
   ## regard for human readability. Meant to improve ``$`` string
   ## conversion performance.
@@ -726,16 +742,16 @@ proc toUgly*(result: var string, node: JsonNode) =
     else: result.addInt(node.num)
   of JFloat:
     when defined(js): result.add($node.fnum)
-    else: result.addFloat(node.fnum)
+    else: result.addFloat(node.fnum, precision = precision)
   of JBool:
     result.add(if node.bval: "true" else: "false")
   of JNull:
     result.add "null"
 
-proc `$`*(node: JsonNode): string =
+proc `$`*(node: JsonNode, precision = precisionDefault): string =
   ## Converts `node` to its JSON Representation on one line.
   result = newStringOfCap(node.len shl 1)
-  toUgly(result, node)
+  toUgly(result, node, precision = precision)
 
 iterator items*(node: JsonNode): JsonNode =
   ## Iterator for the items of `node`. `node` has to be a JArray.
@@ -1486,3 +1502,11 @@ when isMainModule:
   doAssert not isRefSkipDistinct(MyObject)
   doAssert isRefSkipDistinct(MyDistinct)
   doAssert isRefSkipDistinct(MyOtherDistinct)
+
+  block: # issue #13196
+    # some arbitrary float, not caring about actually significant places here
+    let x = 0.12345678901234567890123456789
+    let j = %* x
+    let y = (`$`(j, precision = precisionRoundtrip)).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 = -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 +29,8 @@ 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)
+  let precision2 = if precision == -1: 16 else: precision
+  var n: int = c_sprintf(addr buf, "%.*g", precision2.cint, 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 = -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: