diff --git a/README.md b/README.md
index eceeae4..66e2a0f 100644
--- a/README.md
+++ b/README.md
@@ -3,8 +3,8 @@ Circom witness generators
 -------------------------
 
 The original idea behind this small project was to take the "computation graph" 
-files generated by  [`circom-witnesscalc`](https://github.com/iden3/circom-witnesscalc),
-and either interprets or compiles them to various algebra backends.
+files generated by [`circom-witnesscalc`](https://github.com/iden3/circom-witnesscalc),
+and either interpret or compile them to various algebra backends.
 
 While this is quite straightforward in principle, and seems to work on particular
 examples, it turns out that `circom-witnesscalc` itself has some serious limitations.
@@ -30,8 +30,9 @@ Compiler (in Haskell):
 Nim witness generator (to be used with [`nim-groth16`](https://github.com/codex-storage/nim-groth16))
 
 - [x] parsing the graph file
-- [ ] parsing json input
+- [x] parsing json input
 - [ ] generating the witness
+- [ ] exporting the witness
 - [ ] proper error handling
 
 ### Testing & correctness
@@ -50,18 +51,18 @@ by `circom-witnesscalc`.
 
 This has the following format:
 
-- magic header: "wtns.graph.001" (14 bytes)
+- magic header: `"wtns.graph.001"` (14 bytes)
 - number of nodes (8 bytes little endian)
-- list of nodes, in protobuf serialized format. Each prefixed by a `varint` length
-- `GraphMetaData`, in protobuf 
-- 8 bytes offset (yes, _at the very end_...), pointing to the start of `GraphMetaData`
+- list of nodes, in protobuf serialized format. Each one is prefixed by a `varint` length
+- after that, `GraphMetaData`, encoded with protobuf 
+- finally, 8 bytes little-endian  offset (yes, _at the very end_...), pointing to the start of `GraphMetaData`
 
 Node format:
 
 - varint length prefix
-- protobuf tag-byte (lower 3 bits are `0x02`, upper bits are node type 1..5)
+- protobuf tag-byte (lower 3 bits are `0x02`, upper bits are node type `1..5`)
 - varint length 
 - several record fields
-    - protobuf tag-byte (lower 3 bits are 0x00, upper bits are field index 1,2,3,4)
-    - value is varint word32, except for ConstantNode when it's a little-endian bytes
-      (wrapped a few times, because protobuf is being protobuf...)
+    - protobuf tag-byte (lower 3 bits are `0x00`, upper bits are field index `1,2,3,4`)
+    - value is varint word32, except for `ConstantNode` when it's a sequence of little-endian bytes,
+      encoding a field element (wrapped a few times, because protobuf is being protobuf...)
diff --git a/nim/circom_witnessgen/input_json.nim b/nim/circom_witnessgen/input_json.nim
new file mode 100644
index 0000000..97763cf
--- /dev/null
+++ b/nim/circom_witnessgen/input_json.nim
@@ -0,0 +1,39 @@
+
+import std/sequtils
+import std/json
+import std/tables
+
+import ./field
+
+#-------------------------------------------------------------------------------
+
+type 
+  Inputs* = Table[string, seq[F]]
+
+proc printInputs*(inputs: Inputs) = 
+  for key, list in pairs(inputs):
+    echo key
+    for y in list:
+      echo " - " & fToDecimal(y)
+
+proc flattenNode(node: JsonNode): seq[F] = 
+  case node.kind:
+    of JString: return @[decimalToF(node.str)]
+    of JInt:    return @[int64ToF(node.num)]
+    of JArray:  return node.elems.map(flattenNode).concat()
+    else:       assert( false, "parseInputJSON: expecting a number or a list (of numbers)" )
+
+proc parseInputJSON*(json: JsonNode): Inputs = 
+  doAssert json.kind == JObject
+  var stuff: Inputs
+  for key, node in pairs(json.fields):
+    stuff[key] = flattenNode(node)
+  return stuff
+
+proc loadInputJSON*(fpath: string): Inputs = 
+  let text = readFile(fpath)
+  let json = parseJson(text)
+  return parseInputJSON(json)
+
+#-------------------------------------------------------------------------------
+
diff --git a/nim/main.nim b/nim/main.nim
index 23db1a0..7e5aa7f 100644
--- a/nim/main.nim
+++ b/nim/main.nim
@@ -1,9 +1,16 @@
 
 import circom_witnessgen/graph
 import circom_witnessgen/load
+import circom_witnessgen/input_json
+
+const graph_file: string = "../tmp/graph4.bin"
+const input_file: string = "../tmp/input4.json"
 
 when isMainModule:
-  let fn = "/Users/bkomuves/zk/codex/circom-witnessgen-compiler/tmp/graph2.bin"
-  echo "loading in " & fn
-  let g = loadGraph(fn)
-  echo $g
+  echo "\nloading in " & input_file
+  let inp = loadInputJSON(input_file) 
+  printInputs(inp)
+
+  echo "\nloading in " & graph_file
+  let gr = loadGraph(graph_file)
+  # echo $gr