From 6ebb75269009982bab6a8623702470933cee98b8 Mon Sep 17 00:00:00 2001
From: Emery Hemingway <ehmry@posteo.net>
Date: Tue, 15 Jun 2021 12:48:20 +0200
Subject: [PATCH] Store embedded values as pointers

Reduce Preserve from a generic to a concrete type.
---
 README.md              |   1 -
 preserves.nimble       |   2 +-
 src/preserves.nim      | 442 ++++++++++++++++++++++-------------------
 tests/test_rfc8259.nim |  37 ++--
 4 files changed, 260 insertions(+), 222 deletions(-)

diff --git a/README.md b/README.md
index 5def74a..5449276 100644
--- a/README.md
+++ b/README.md
@@ -4,4 +4,3 @@ Missing features:
 * embedded values
 * parsing from human-readable encoding
 * ordering of compound values
-* conversion of Nim tuples and objects
diff --git a/preserves.nimble b/preserves.nimble
index aadf4bf..e148139 100644
--- a/preserves.nimble
+++ b/preserves.nimble
@@ -9,4 +9,4 @@ srcDir        = "src"
 
 # Dependencies
 
-requires "nim >= 1.4.6", "bigints"
+requires "nim >= 1.4.8", "bigints"
diff --git a/src/preserves.nim b/src/preserves.nim
index c0215f5..0627724 100644
--- a/src/preserves.nim
+++ b/src/preserves.nim
@@ -5,12 +5,11 @@ import bigints
 
 type
   PreserveKind* = enum
-    pkBoolean, pkFloat, pkDouble, pkSignedInteger, pkBigInteger, pkString,
-        pkByteString, pkSymbol, pkRecord, pkSequence, pkSet, pkDictionary, pkEmbedded
+    pkBoolean, pkFloat, pkDouble, pkSignedInteger, pkBigInteger, pkString, pkByteString,
+        pkSymbol, pkRecord, pkSequence, pkSet, pkDictionary, pkEmbedded
 
-  Preserve*[T] {.acyclic.} = object
+  Preserve* {.acyclic.} = object
     ## Type that stores a Preserves value.
-    ## ``T`` is the domain-specific type of "embedded" values, otherwise ``void``.
     case kind*: PreserveKind
     of pkBoolean:
       bool*: bool
@@ -29,17 +28,124 @@ type
     of pkSymbol:
       symbol*: string
     of pkRecord:
-      record*: seq[Preserve[T]] # record[0] is the label
+      record*: seq[Preserve] # record[0] is the label
     of pkSequence:
-      seq*: seq[Preserve[T]]
+      sequence*: seq[Preserve]
     of pkSet:
-      set*: HashSet[Preserve[T]]
+      set*: HashSet[Preserve]
     of pkDictionary:
-      dict*: Table[Preserve[T], Preserve[T]]
+      dict*: Table[Preserve, Preserve]
     of pkEmbedded:
-      embedded*: T
+      embedded*: pointer
 
-proc `$`*[T](prs: Preserve[T]): string =
+proc `<`(x, y: string | seq[byte]): bool =
+  for i in 0 .. min(x.high, y.high):
+    if x[i] < y[i]:
+      return true
+  x.len < y.len
+
+proc `<`*(x, y: Preserve): bool =
+  if x.kind != y.kind:
+    if x.kind == pkSignedInteger and y.kind == pkBigInteger:
+      result = x.int.initBigInt < y.bigint
+    elif x.kind == pkBigInteger and y.kind == pkSignedInteger:
+      result = x.bigint < y.int.initBigInt
+    else:
+      result = x.kind < y.kind
+  else:
+    case x.kind
+    of pkBoolean:
+      result = (not x.bool) and y.bool
+    of pkSignedInteger:
+      result = x.int < y.int
+    of pkBigInteger:
+      result = x.bigint < y.bigint
+    of pkString:
+      result = x.string < y.string
+    of pkByteString:
+      result = x.bytes < y.bytes
+    of pkSymbol:
+      result = x.symbol < y.symbol
+    else:
+      discard
+
+proc hash*(prs: Preserve): Hash =
+  var h = hash(prs.kind.int)
+  case prs.kind
+  of pkBoolean:
+    h = h !& hash(prs.bool)
+  of pkFloat:
+    h = h !& hash(prs.float)
+  of pkDouble:
+    h = h !& hash(prs.double)
+  of pkSignedInteger:
+    h = h !& hash(prs.int)
+  of pkBigInteger:
+    h = h !& hash(prs.bigint.flags)
+    h = h !& hash(prs.bigint)
+  of pkString:
+    h = h !& hash(prs.string)
+  of pkByteString:
+    h = h !& hash(prs.bytes)
+  of pkSymbol:
+    h = h !& hash(prs.symbol)
+  of pkRecord:
+    for val in prs.record:
+      h = h !& hash(val)
+  of pkSequence:
+    for val in prs.sequence:
+      h = h !& hash(val)
+  of pkSet:
+    for val in prs.set.items:
+      h = h !& hash(val)
+  of pkDictionary:
+    for (key, val) in prs.dict.pairs:
+      h = h !& hash(val)
+  of pkEmbedded:
+    h = h !& hash(prs.embedded)
+  !$h
+
+proc `==`*(x, y: Preserve): bool =
+  if x.kind == y.kind:
+    case x.kind
+    of pkBoolean:
+      result = x.bool == y.bool
+    of pkFloat:
+      result = x.float == y.float
+    of pkDouble:
+      result = x.double == y.double
+    of pkSignedInteger:
+      result = x.int == y.int
+    of pkBigInteger:
+      result = x.bigint == y.bigint
+    of pkString:
+      result = x.string == y.string
+    of pkByteString:
+      result = x.bytes == y.bytes
+    of pkSymbol:
+      result = x.symbol == y.symbol
+    of pkRecord:
+      for i, val in x.record:
+        if y.record[i] != val: return false
+      result = true
+    of pkSequence:
+      for i, val in x.sequence:
+        if y.sequence[i] != val: return false
+      result = true
+    of pkSet:
+      for val in x.set.items:
+        if not y.set.contains(val): return false
+      for val in y.set.items:
+        if not x.set.contains(val): return false
+      result = true
+    of pkDictionary:
+      for (key, val) in x.dict.pairs:
+        if y.dict[key] != val: return false
+      result = true
+    of pkEmbedded:
+      result = x.embedded == y.embedded
+
+proc `$`*(prs: Preserve): string =
   case prs.kind:
   of pkBoolean:
     case prs.bool
@@ -69,14 +175,14 @@ proc `$`*[T](prs: Preserve[T]): string =
     result.add('>')
   of pkSequence:
     result.add('[')
-    for i, val in prs.seq:
+    for i, val in prs.sequence:
       if i > 0:
         result.add(' ')
       result.add($val)
     result.add(']')
   of pkSet:
     result.add("#{")
-    for val in prs.set:
+    for val in prs.set.items:
       result.add($val)
       result.add(' ')
     if result.len > 1:
@@ -86,178 +192,50 @@ proc `$`*[T](prs: Preserve[T]): string =
     result.add('{')
     for (key, value) in prs.dict.pairs:
       result.add($key)
-      result.add(" :")
+      result.add(": ")
       result.add($value)
       result.add(' ')
     if result.len > 1:
       result.setLen(result.high)
     result.add('}')
   of pkEmbedded:
-    when not T is void:
-      $prs.embedded
+    result.add(prs.embedded.repr)
 
-proc toPreserve*(b: bool): Preserve[void] =
-  Preserve[void](kind: pkBoolean, bool: b)
+proc isNamedTuple(T: typedesc): bool {.magic: "TypeTrait".}
+proc distinctBase(T: typedesc): typedesc {.magic: "TypeTrait".}
+template distinctBase[T](a: T): untyped = distinctBase(type(a))(a)
 
-proc toPreserve*(n: SomeInteger): Preserve[void] =
-  Preserve[void](kind: pkSignedInteger, int: n.BiggestInt)
-
-proc toPreserve*(n: BigInt): Preserve[void] =
-  if initBigInt(low(BiggestInt)) < n and n < initBigInt(high(BiggestInt)):
-    var tmp: BiggestUint
-    for limb in n.limbs:
-      tmp = (tmp shl 32) or limb
-    if Negative in n.flags:
-      tmp = (not tmp) + 1
-    result = Preserve[void](kind: pkSignedInteger, int: cast[BiggestInt](tmp))
-  else:
-    result = Preserve[void](kind: pkBigInteger, bigint: n)
-
-proc toPreserve*(s: string): Preserve[void] =
-  Preserve[void](kind: pkString, string: s)
-
-proc symbol*[T](s: string): Preserve[void] {.inline.} =
+proc symbol*(s: string): Preserve {.inline.} =
   ## Symbol constructor.
-  Preserve[T](kind: pkSymbol, symbol: s)
+  Preserve(kind: pkSymbol, symbol: s)
 
-proc record*[T](label: Preserve[T], args: varargs[Preserve[T]]): Preserve[T] =
+proc initRecord*(label: Preserve, args: varargs[Preserve]): Preserve =
   ## Record constructor.
-  result = Preserve[T](kind: pkRecord, record: newSeqOfCap(1+args.len))
+  result = Preserve(kind: pkRecord,
+      record: newSeqOfCap[Preserve](1+args.len))
   result.record.add(label)
   for arg in args: result.record.add(arg)
 
-proc record*[T](label: string, args: varargs[Preserve[T]]): Preserve[T] {.inline.} =
+proc initRecord*(label: string, args: varargs[Preserve]): Preserve {.inline.} =
   ## Record constructor that converts ``label`` to a symbol.
-  record(symbol[T](label), args)
+  initRecord(symbol(label), args)
 
-proc label*[T](prs: Preserve[T]): Preserve[T] {.inline.} =
+proc label*(prs: Preserve): Preserve {.inline.} =
   ## Return the label of a record value.
   prs.record[0]
 
-proc arity*[T](prs: Preserve[T]): int {.inline.} =
+proc arity*(prs: Preserve): int {.inline.} =
   ## Return the number of fields in a record value.
   pred(prs.record.len)
 
-proc fields*[T](prs: Preserve[T]): seq[Preserve[T]] {.inline.} =
+proc fields*(prs: Preserve): seq[Preserve] {.inline.} =
   ## Return the fields of a record value.
   prs.record[1..prs.record.high]
 
-iterator fields*[T](prs: Preserve[T]): Preserve[T] =
+iterator fields*(prs: Preserve): Preserve =
   ## Iterate the fields of a record value.
   for i in 1..prs.record.high: yield prs.record[i]
 
-iterator setItems*[T](prs: Preserve[T]): Preserve[T] =
-  for v in prs.set.keys: yield v
-
-proc `<`(x, y: string | seq[byte]): bool =
-  for i in 0 .. min(x.high, y.high):
-    if x[i] < y[i]:
-      return true
-  x.len < y.len
-
-proc `<`*[T](x, y: Preserve[T]): bool =
-  if x.kind != y.kind:
-    if x.kind == pkSignedInteger and y.kind == pkBigInteger:
-      result = x.int < y.bigint
-    elif x.kind == pkBigInteger and y.kind == pkSignedInteger:
-      result = x.bigint < y.int
-    else:
-      result = x.kind < y.kind
-  else:
-    case x.kind
-    of pkBoolean:
-      result = (not x.bool) and y.bool
-    of pkSignedInteger:
-      result = x.int < y.int
-    of pkBigInteger:
-      result = x.bigint < y.bigint
-    of pkString:
-      result = x.string < y.string
-    of pkByteString:
-      result = x.bytes < y.bytes
-    of pkSymbol:
-      result = x.symbol < y.symbol
-    else:
-      discard
-
-proc `==`*[T](x, y: Preserve[T]): bool =
-  if x.kind == y.kind:
-    case x.kind
-    of pkBoolean:
-      result = x.bool == y.bool
-    of pkFloat:
-      result = x.float == y.float
-    of pkDouble:
-      result = x.double == y.double
-    of pkSignedInteger:
-      result = x.int == y.int
-    of pkBigInteger:
-      result = x.bigint == y.bigint
-    of pkString:
-      result = x.string == y.string
-    of pkByteString:
-      result = x.bytes == y.bytes
-    of pkSymbol:
-      result = x.symbol == y.symbol
-    of pkRecord:
-      for i, val in x.record:
-        if y.record[i] != val: return false
-      result = true
-    of pkSequence:
-      for i, val in x.seq:
-        if y.seq[i] != val: return false
-      result = true
-    of pkSet:
-      for val in x.set:
-        if not y.set.contains(val): return false
-      for val in y.set:
-        if not x.set.contains(val): return false
-      result = true
-    of pkDictionary:
-      for (key, val) in x.dict.pairs:
-        if y.dict[key] != val: return false
-      result = true
-    of pkEmbedded:
-      when not T is void:
-        result = x.embedded == y.embedded
-
-proc hash*[T](prs: Preserve[T]): Hash =
-  var h = hash(prs.kind.int)
-  case prs.kind
-  of pkBoolean:
-    h = h !& hash(prs.bool)
-  of pkFloat:
-    h = h !& hash(prs.float)
-  of pkDouble:
-    h = h !& hash(prs.double)
-  of pkSignedInteger:
-    h = h !& hash(prs.int)
-  of pkBigInteger:
-    h = h !& hash(prs.bigint.flags)
-    h = h !& hash(prs.bigint)
-  of pkString:
-    h = h !& hash(prs.string)
-  of pkByteString:
-    h = h !& hash(prs.bytes)
-  of pkSymbol:
-    h = h !& hash(prs.symbol)
-  of pkRecord:
-    for val in prs.record:
-      h = h !& hash(val)
-  of pkSequence:
-    for val in prs.seq:
-      h = h !& hash(val)
-  of pkSet:
-    for val in prs.set:
-      h = h !& hash(val)
-  of pkDictionary:
-    for (key, val) in prs.dict.pairs:
-      h = h !& hash(val)
-  of pkEmbedded:
-    when not T is void:
-      h = h !& hash(prs.embedded)
-  !$h
-
 proc writeVarint(s: Stream; n: int) =
   var n = n
   while true:
@@ -278,7 +256,7 @@ proc readVarint(s: Stream): int =
       break
     shift.inc 7
 
-proc write*[T](str: Stream; prs: Preserve[T]) =
+proc write*(str: Stream; prs: Preserve) =
   case prs.kind:
   of pkBoolean:
     case prs.bool
@@ -354,7 +332,7 @@ proc write*[T](str: Stream; prs: Preserve[T]) =
     str.write(0x84'u8)
   of pkSequence:
     str.write(0xb5'u8)
-    for e in prs.seq:
+    for e in prs.sequence:
       str.write(e)
     str.write(0x84'u8)
   of pkSet:
@@ -369,65 +347,63 @@ proc write*[T](str: Stream; prs: Preserve[T]) =
       str.write(value)
     str.write(0x84'u8)
   of pkEmbedded:
-    when not T is void:
-      str.write(0x86'u8)
-      str.write(prs.embedded)
+    str.write(0x86'u8)
+    raiseAssert("binary representation of embedded values is undefined")
 
-proc parsePreserve*(s: Stream): Preserve[void] =
+proc parsePreserve*(s: Stream): Preserve =
   proc assertStream(check: bool) =
     if not check:
       raise newException(ValueError, "invalid Preserves stream")
   const endMarker = 0x84
   let tag = s.readUint8()
   case tag
-  of 0x80: result = Preserve[void](kind: pkBoolean, bool: false)
-  of 0x81: result = Preserve[void](kind: pkBoolean, bool: true)
+  of 0x80: result = Preserve(kind: pkBoolean, bool: false)
+  of 0x81: result = Preserve(kind: pkBoolean, bool: true)
   of 0x82:
     when system.cpuEndian == bigEndian:
-      result = Preserve[void](kind: pkFloat, float: s.readFloat32())
+      result = Preserve(kind: pkFloat, float: s.readFloat32())
     else:
-      result = Preserve[void](kind: pkFloat)
+      result = Preserve(kind: pkFloat)
       var be = s.readFloat32()
       swapEndian32(result.float.addr, be.addr)
   of 0x83:
     when system.cpuEndian == bigEndian:
-      result = Preserve[void](kind: pkDouble, double: s.readFloat64())
+      result = Preserve(kind: pkDouble, double: s.readFloat64())
     else:
-      result = Preserve[void](kind: pkDouble)
+      result = Preserve(kind: pkDouble)
       var be = s.readFloat64()
       swapEndian64(result.double.addr, be.addr)
   of 0x84:
     assertStream(false)
   of 0xb1:
-    result = Preserve[void](kind: pkString)
+    result = Preserve(kind: pkString)
     let len = s.readVarint()
     result.string = s.readStr(len)
   of 0xb2:
-    result = Preserve[void](kind: pkByteString)
+    result = Preserve(kind: pkByteString)
     let len = s.readVarint()
     result.bytes = cast[seq[byte]](s.readStr(len))
   of 0xb3:
     let len = s.readVarint()
-    result = symbol[void](s.readStr(len))
+    result = symbol(s.readStr(len))
   of 0xb4:
-    result = Preserve[void](kind: pkRecord)
+    result = Preserve(kind: pkRecord)
     while s.peekUint8() != endMarker:
       result.record.add(s.parsePreserve())
     discard s.readUint8()
     assertStream(result.record.len > 0)
   of 0xb5:
-    result = Preserve[void](kind: pkSequence)
+    result = Preserve(kind: pkSequence)
     while s.peekUint8() != endMarker:
-      result.seq.add(s.parsePreserve())
+      result.sequence.add(s.parsePreserve())
     discard s.readUint8()
   of 0xb6:
-    result = Preserve[void](kind: pkSet)
+    result = Preserve(kind: pkSet)
     while s.peekUint8() != endMarker:
-      let val = s.parsePreserve()
-      result.set.incl(val)
+      result.set.incl(s.parsePreserve())
     discard s.readUint8()
   of 0xb7:
-    result = Preserve[void](kind: pkDictionary)
+    result = Preserve(kind: pkDictionary)
     while s.peekUint8() != endMarker:
       let key = s.parsePreserve()
       let val = s.parsePreserve()
@@ -435,54 +411,100 @@ proc parsePreserve*(s: Stream): Preserve[void] =
     discard s.readUint8()
   of 0xb0:
     let len = s.readVarint()
-    result = Preserve[void](kind: pkBigInteger)
+    result = Preserve(kind: pkBigInteger)
     for _ in 1..len:
       result.bigint = (result.bigint shl 8) + s.readUint8().int32
   else:
     case 0xf0 and tag
     of 0x90:
       var n = tag.BiggestInt
-      result = Preserve[void](kind: pkSignedInteger,
+      result = Preserve(kind: pkSignedInteger,
           int: n - (if n > 0x9c: 0xa0 else: 0x90))
     of 0xa0:
       let len = (tag.int and 0x0f) + 1
       if len <= 8:
-        result = Preserve[void](kind: pkSignedInteger, int: s.readUint8().BiggestInt)
+        result = Preserve(kind: pkSignedInteger, int: s.readUint8().BiggestInt)
         if (result.int and 0x80) != 0: result.int.dec(0x100)
         for i in 1..<len:
           result.int = (result.int shl 8) or s.readUint8().BiggestInt
       else:
-        result = Preserve[void](kind: pkBigInteger)
+        result = Preserve(kind: pkBigInteger)
         for i in 0..<len:
           result.bigint = (result.bigint shl 8) + s.readUint8().int32
     else:
       assertStream(false)
 
-proc toPreserve*(js: JsonNode): Preserve[void] =
+proc toPreserveHook*(n: BigInt): Preserve =
+  if initBigInt(low(BiggestInt)) < n and n < initBigInt(high(BiggestInt)):
+    var tmp: BiggestUint
+    for limb in n.limbs:
+      tmp = (tmp shl 32) or limb
+    if Negative in n.flags:
+      tmp = (not tmp) + 1
+    result = Preserve(kind: pkSignedInteger, int: cast[BiggestInt](tmp))
+  else:
+    result = Preserve(kind: pkBigInteger, bigint: n)
+
+proc toPreserveHook*(js: JsonNode): Preserve =
   case js.kind
   of JString:
-    result = js.str.toPreserve
+    result = Preserve(kind: pkString, string: js.str)
   of JInt:
-    result = Preserve[void](kind: pkSignedInteger, int: js.num)
+    result = Preserve(kind: pkSignedInteger, int: js.num)
   of JFloat:
-    result = Preserve[void](kind: pkDouble, double: js.fnum)
+    result = Preserve(kind: pkDouble, double: js.fnum)
   of JBool:
     result = case js.bval
-      of false: symbol[void]"false"
-      of true: symbol[void]"true"
+      of false: symbol"false"
+      of true: symbol"true"
   of JNull:
-    result = symbol[void]"null"
+    result = symbol"null"
   of JObject:
-    result = Preserve[void](kind: pkDictionary)
+    result = Preserve(kind: pkDictionary)
     for key, val in js.fields.pairs:
-      result.dict[key.toPreserve] = val.toPreserve
+      result.dict[Preserve(kind: pkString, string: key)] = toPreserveHook(val)
   of JArray:
-    result = Preserve[void](kind: pkSequence, seq: newSeq[Preserve[void]](
-        js.elems.len))
+    result = Preserve(kind: pkSequence,
+        sequence: newSeq[Preserve](js.elems.len))
     for i, e in js.elems:
-      result.seq[i] = e.toPreserve
+      result.sequence[i] = toPreserveHook(e)
 
-proc toJson*[T](prs: Preserve[T]): JsonNode =
+proc toPreserve*[T](x: T): Preserve =
+  ## Serializes `x` to Preserves; uses `toPreserveHook(x: A)` if it's in scope to
+  ## customize serialization.
+  when T is Preserve: result = x
+  elif compiles(toPreserveHook(x)):
+    result = toPreserveHook(x)
+  elif T is array | seq:
+    result = Preserve(kind: pkSequence)
+    for v in x.items: result.sequence.add(toPreserve(v))
+  elif T is bool:
+    result = Preserve(kind: pkBoolean, bool: x)
+  elif T is distinct:
+    result = toPreserve(x.distinctBase)
+  elif T is float:
+    result = Preserve(kind: pkFloat, float: x)
+  elif T is float64:
+    result = Preserve(kind: pkDouble, double: x)
+  elif T is object:
+    result = Preserve(kind: pkDictionary)
+    for k, v in x.fieldPairs: result.dict[symbol(k)] = toPreserve(v)
+  elif T is Ordinal:
+    result = Preserve(kind: pkSignedInteger, int: x.ord.BiggestInt)
+  elif T is ptr | ref:
+    if system.`==`(x, nil): result = symbol"null"
+    else: result = toPreserve(x[])
+  elif T is string:
+    result = Preserve(kind: pkString, string: x)
+  elif T is SomeInteger:
+    result = Preserve(kind: pkSignedInteger, int: x.BiggestInt)
+  elif T is tuple:
+    result = Preserve(kind: pkRecord, record: @[symbol("")])
+    for v in x.fields: result.record.add(toPreserve(v))
+  else:
+    raiseAssert("cannot convert to Preserves: " & $T)
+
+proc toJsonHook*(prs: Preserve): JsonNode =
   case prs.kind:
   of pkBoolean:
     result = newJBool(prs.bool)
@@ -512,8 +534,8 @@ proc toJson*[T](prs: Preserve[T]): JsonNode =
     raise newException(ValueError, "cannot convert record to JSON")
   of pkSequence:
     result = newJArray()
-    for val in prs.seq:
-      result.add(val.toJSON)
+    for val in prs.sequence:
+      result.add(toJsonHook(val))
   of pkSet:
     raise newException(ValueError, "cannot convert set to JSON")
   of pkDictionary:
@@ -521,7 +543,7 @@ proc toJson*[T](prs: Preserve[T]): JsonNode =
     for (key, val) in prs.dict.pairs:
       if key.kind != pkString:
         raise newException(ValueError, "cannot convert non-string dictionary key to JSON")
-      result[key.string] = val.toJson
+      result[key.string] = toJsonHook(val)
   of pkEmbedded:
     raise newException(ValueError, "cannot convert embedded value to JSON")
 
@@ -530,20 +552,36 @@ type Record* = object
   label*: string
   arity*: Natural
 
-proc init*[T](rec: Record, fields: varargs[Preserve[T]]): Preserve[T] =
+proc init*(rec: Record, fields: varargs[Preserve]): Preserve =
   ## Initialize a new record value.
   assert(fields.len == rec.arity)
-  record(rec.label, fields)
+  initRecord(rec.label, fields)
 
-proc isClassOf*[T](rec: Record, val: Preserve[T]): bool =
+proc isClassOf*(rec: Record, val: Preserve): bool =
   ## Compare the label and arity of ``val`` to the record type ``rec``.
   if val.kind == pkRecord:
     let label = val.label
     if label.kind == pkSymbol:
       result = label.symbol == rec.label and rec.arity == val.arity
 
-proc classOf*[T](val: Preserve[T]): Record =
+proc classOf*(val: Preserve): Record =
   ## Derive the ``Record`` type of ``val``.
   if val.kind != pkRecord or val.label.kind == pkSymbol:
     raise newException(ValueError, "cannot derive class of non-record value")
   Record(label: val.label.symbol, arity: val.arity)
+
+proc len*(prs: Preserve): int =
+  ## Return the number of values one level below ``prs``.
+  case prs.kind
+  of pkRecord: prs.record.len.pred
+  of pkSequence: prs.sequence.len
+  of pkSet: prs.set.len
+  of pkDictionary: prs.dict.len
+  else: 0
+
+proc `[]`*(prs: Preserve; i: int): Preserve =
+  case prs.kind
+  of pkRecord: prs.record[succ(i)]
+  of pkSequence: prs.sequence[i]
+  else:
+    raise newException(ValueError, "`[]` is not valid for " & $prs.kind)
diff --git a/tests/test_rfc8259.nim b/tests/test_rfc8259.nim
index 84d4581..e40ae68 100644
--- a/tests/test_rfc8259.nim
+++ b/tests/test_rfc8259.nim
@@ -1,8 +1,7 @@
 # SPDX-License-Identifier: ISC
 
-import json, streams, unittest
-
 import preserves
+import std/[json,jsonutils,streams, unittest]
 
 let testVectors = [
 """
@@ -29,24 +28,24 @@ let testVectors = [
 """
 [
   {
-    "precision": "zip",
-    "Latitude": 37.7668,
-    "Longitude": -122.3959,
-    "Address": "",
-    "City": "SAN FRANCISCO",
-    "State": "CA",
-    "Zip": "94107",
-    "Country": "US"
+    "space": "C3D2",
+    "logo": "https://www.c3d2.de/images/ck.png",
+    "url": "https://www.c3d2.de/",
+    "location": {
+      "address": "Raum 1.04.01, Haus B, Zentralwerk, Riesaer Straße 32, 01127 Dresden, Germany",
+      "lat": 51.0810791,
+      "lon": 13.7286123
+    }
   },
   {
-    "precision": "zip",
-    "Latitude": 37.371991,
-    "Longitude": -122.02602,
-    "Address": "",
-    "City": "SUNNYVALE",
-    "State": "CA",
-    "Zip": "94085",
-    "Country": "US"
+    "space": "LAG",
+    "logo": "http://laglab.org/logo.png",
+    "url": "http://laglab.org",
+    "location": {
+      "address": "Eerste Schinkelstraat 16, 1075 TX Amsterdam, The Netherlands",
+      "lat": 52.35406,
+      "lon": 4.85423
+    }
   }
 ]
 """
@@ -54,9 +53,11 @@ let testVectors = [
 
 for i, jsText in testVectors:
   test $i:
+    checkpoint(jsText)
     let
       control = parseJson jsText
       x = control.toPreserve
+    checkpoint($x)
     var stream = newStringStream()
     stream.write(x)
     stream.setPosition(0)