initial nson nip draft.

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+NIP-93
+======
+
+NSON
+----
+
+`draft` `optional` `author:fiatjaf`
+
+### Preamble
+
+Some [benchmarks](https://github.com/fiatjaf/nostr-json-benchmarks/tree/2f254fff91b3ad063ef9726bb4a3d25316cf12d8) made using all libraries available on Golang show that JSON decoding is very slow. And even when people do assembly-level optimizations things only improve up to a point (e.g. for decoding a Nostr event, the decoding time is 50% smaller).
+
+Meanwhile, doing a simple TLV encoding reduces the decoding time to 35% and a simpler static binary format for Nostr events reduces makes that number drop to 4%. However, it would be bad for Nostr if a binary encoding was introduced, as it would be likely to cause compatibility issues, centralize the protocol and/or increase the work for everybody, more about this at [this comment](https://github.com/nostr-protocol/nips/pull/512#issuecomment-1542368664).
+
+### The actual NIP
+
+NSON is a crazy idea that, according to the benchmarks above, reduces the decoding time to 14% of that of the standard library. It works by having the JSON sender encode the event _as JSON_, but in a specific, very strict, order of fields (taking advantage of the fact that Nostr events have static fields, static lengths for some fields, and an overall rigid structure) and include in the JSON object a new field called `"nson"` that contains metadata the JSON receiver can read to help in the decoding process.
+
+Here's an example of a NSON-encoded Nostr event:
+
+`{"id":"57ff66490a6a2af3992accc26ae95f3f60c6e5f84ed0ddf6f59c534d3920d3d2","pubkey":"79be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798","sig":"504d142aed7fa7e0f6dab5bcd7eed63963b0277a8e11bbcb03b94531beb4b95a12f1438668b02746bd5362161bc782068e6b71494060975414e793f9e19f57ea","created_at":1683762317,"nson":"2801000b0203000100400005040001004000000014","kind":1,"content":"hello world","tags":[["e","b6de44a9dd47d1c000f795ea0453046914f44ba7d5e369608b04867a575ea83e","reply"],["p","c26f7b252cea77a5b94f42b1a4771021be07d4df766407e47738605f7e3ab774","","wss://relay.damus.io"]]}`
+
+The idea is that `"id"` comes first, so it can be accessed by reading a slice of the string from character `7` to character `71`, `pubkey` from character `83` to `147` and so on. `"content"`, `"kind"` and `"tags"` have dynamic sizes, so these are given by the values inside the `"nson"` field (which is also dynamic, its size by its first byte).
+
+### Anatomy of the `"nson"` field
+
+It is hex-encoded. Some fields are a single byte, others are two bytes (4 characters).
+
+Each explanation starts at the same line as the field it is referring to.
+
+                    number of tags (let's say it's two)
+                      number of items on the first tag (let's say it's three)
+                        number of chars on the first item
+                            number of chars on the second item
+                                number of chars on the third item
+                                    number of items on the second tag (let's say it's two)
+                                      number of chars on the first item
+                                          number of chars on the second item
+    "nson":"xxkkccccttnn111122223333nn11112222"
+            nson size
+              kind chars
+                content chars
+
+### Reference implementation
+
+Beware, all Rust maniacs, the following reference implementation is written in Go:
+
+```go
+func decodeNson(data string) *Event {
+        evt := &Event{}
+
+        // static fields
+        evt.ID = data[7 : 7+64]
+        evt.PubKey = data[83 : 83+64]
+        evt.Sig = data[156 : 156+128]
+        ts, _ := strconv.ParseInt(data[299:299+10], 10, 64)
+        evt.CreatedAt = Timestamp(ts)
+
+        // nson values
+        nsonSizeBytes, _ := hex.DecodeString(data[318 : 318+2])
+        nsonSize := int(nsonSizeBytes[0])
+        nsonDescriptors, _ := hex.DecodeString(data[320 : 320+nsonSize])
+
+        // dynamic fields
+        // kind
+        kindChars := int(nsonDescriptors[0])
+        kindStart := 320 + nsonSize + 9 // len(`","kind":`)
+        evt.Kind, _ = strconv.Atoi(data[kindStart : kindStart+kindChars])
+
+        // content
+        contentChars := int(binary.BigEndian.Uint16(nsonDescriptors[1:3]))
+        contentStart := kindStart + kindChars + 12 // len(`,"content":"`)
+        evt.Content, _ = strconv.Unquote(`"` + data[contentStart:contentStart+contentChars] + `"`)
+
+        // tags
+        nTags := int(nsonDescriptors[3])
+        evt.Tags = make(Tags, nTags)
+        tagsStart := contentStart + contentChars + 9 // len(`","tags":`)
+
+        nsonIndex := 3
+        tagsIndex := tagsStart
+        for t := 0; t < nTags; t++ {
+                nsonIndex++
+                tagsIndex += 1 // len(`[`) or len(`,`)
+                nItems := int(nsonDescriptors[nsonIndex])
+                tag := make(Tag, nItems)
+                for n := 0; n < nItems; n++ {
+                        nsonIndex++
+                        itemStart := tagsIndex + 2 // len(`["`) or len(`,"`)
+                        itemChars := int(binary.BigEndian.Uint16(nsonDescriptors[nsonIndex:]))
+                        nsonIndex++
+                        tag[n], _ = strconv.Unquote(`"` + data[itemStart:itemStart+itemChars] + `"`)
+                        tagsIndex = itemStart + itemChars + 1 // len(`"`)
+                }
+                tagsIndex += 1 // len(`]`)
+                evt.Tags[t] = tag
+        }
+
+        return evt
+}
+
+func encodeNson(evt *Event) string {
+        // start building the nson descriptors (without the first byte that represents the nson size)
+        nsonBuf := make([]byte, 256)
+
+        // build the tags
+        nTags := len(evt.Tags)
+        nsonBuf[3] = uint8(nTags)
+        nsonIndex := 3 // start here
+
+        tagBuilder := strings.Builder{}
+        tagBuilder.Grow(1000) // a guess
+        tagBuilder.WriteString(`[`)
+        for t, tag := range evt.Tags {
+                nItems := len(tag)
+                nsonIndex++
+                nsonBuf[nsonIndex] = uint8(nItems)
+
+                tagBuilder.WriteString(`[`)
+                for i, item := range tag {
+                        v := strconv.Quote(item)
+                        nsonIndex++
+                        binary.BigEndian.PutUint16(nsonBuf[nsonIndex:], uint16(len(v)-2))
+                        nsonIndex++
+                        tagBuilder.WriteString(v)
+                        if nItems > i+1 {
+                                tagBuilder.WriteString(`,`)
+                        }
+                }
+                tagBuilder.WriteString(`]`)
+                if nTags > t+1 {
+                        tagBuilder.WriteString(`,`)
+                }
+        }
+        tagBuilder.WriteString(`]}`)
+        nsonBuf = nsonBuf[0 : nsonIndex+1]
+
+        kind := strconv.Itoa(evt.Kind)
+        kindChars := len(kind)
+        nsonBuf[0] = uint8(kindChars)
+
+        content := strconv.Quote(evt.Content)
+        contentChars := len(content) - 2
+        binary.BigEndian.PutUint16(nsonBuf[1:3], uint16(contentChars))
+
+        // actually build the json
+        base := strings.Builder{}
+        base.Grow(320 + // everything up to "nson":
+                2 + len(nsonBuf)*2 + // nson
+                9 + kindChars + // kind and its label
+                12 + contentChars + // content and its label
+                9 + tagBuilder.Len() + // tags and its label
+                2, // the end
+        )
+        base.WriteString(`{"id":"` + evt.ID + `","pubkey":"` + evt.PubKey + `","sig":"` + evt.Sig + `","created_at":` + strconv.FormatInt(int64(evt.CreatedAt), 10) + `,"nson":"`)
+        base.WriteString(hex.EncodeToString([]byte{uint8(len(nsonBuf) * 2)})) // nson size
+        base.WriteString(hex.EncodeToString(nsonBuf))                         // nson descriptors
+        base.WriteString(`","kind":` + kind + `,"content":` + content + `,"tags":`)
+        base.WriteString(tagBuilder.String() /* includes the end */)
+
+        return base.String()
+}
+```
+
+### Other restrictions
+
+Besides the field ordering and the presence of the `"nson"` field, other restrictions must be applied:
+
+- the `"created_at"` field must have 10, characters, which gives us a range of dates from about 20 years ago up to 250 years in the future.
+- to simplify decoding of `"content"` and `"tags"` strings, escape codes like `\uXXXX` are forbidden in NSON, UTF-8 must be used instead. Only `\n`, `\\` and `\"` are the only valid escaped sequences.
+
+### Backwards-compatibility
+
+Any reader who is not aware of the NSON-encoding can receive these events and decode them using whatever other JSON decoder they happen to have in hand. The `"nson"` field will just be ignored and life will continue as normal.
+
+Also, other event fields that may be present (for example, the NIP-03 `"ots"` field) can be added at the end, after `"tags"`, with no loss.
+
+### Other points worth mentioning
+
+- Relays can receive non-nsonified events from clients, then reformat and store them nsonified so they can serve future clients better by sending them NSON always.
+
+### Open questions to be edited out of the NIP
+
+- How to signal NSON support? I thought it would work to have an initial field `"n":1` (before `"id"`) on the JSON, which could be read very fast, but I don't know.