ndb: start implementing existing NostrEvent functionality

We eventually want to switch over to NdbNote instead of NostrEvent. To
facilitate this, the plan is to eventually make NostrEvent an alias of
NdbNote. For this to work, let's make sure the NostrEvent extensions are
implemented on NdbNote.

We will likely switch away from string properties as well, but for now
we will try to emulate as much as possible to make sure everything is
working first.

damus/ContentParsing.swift

@@ -0,0 +1,75 @@
+//
+//  ContentParsing.swift
+//  damus
+//
+//  Created by William Casarin on 2023-07-22.
+//
+
+import Foundation
+
+func tag_to_refid_ndb(_ tag: TagSequence) -> ReferencedId? {
+    guard let ref_id = tag[1]?.string(),
+          let key = tag[0]?.string() else { return nil }
+
+    let relay_id = tag[2]?.string()
+
+    return ReferencedId(ref_id: ref_id, relay_id: relay_id, key: key)
+}
+
+func convert_mention_index_block_ndb(ind: Int, tags: TagsSequence) -> Block? {
+    if ind < 0 || (ind + 1 > tags.count) || tags[ind]!.count < 2 {
+        return .text("#[\(ind)]")
+    }
+        
+    guard let tag = tags[ind], let fst = tag.first(where: { _ in true }) else {
+        return nil
+    }
+
+    guard let mention_type = parse_mention_type_ndb(fst) else {
+        return .text("#[\(ind)]")
+    }
+    
+    guard let ref = tag_to_refid_ndb(tag) else {
+        return .text("#[\(ind)]")
+    }
+    
+    return .mention(Mention(index: ind, type: mention_type, ref: ref))
+}
+
+
+func convert_block_ndb(_ b: block_t, tags: TagsSequence) -> Block? {
+    if b.type == BLOCK_MENTION_INDEX {
+        return convert_mention_index_block_ndb(ind: Int(b.block.mention_index), tags: tags)
+    }
+
+    return convert_block(b, tags: [])
+}
+
+
+func parse_note_content_ndb(note: NdbNote) -> Blocks {
+    var out: [Block] = []
+    
+    var bs = note_blocks()
+    bs.num_blocks = 0;
+    
+    blocks_init(&bs)
+    
+    damus_parse_content(&bs, note.content_raw)
+
+    var i = 0
+    while (i < bs.num_blocks) {
+        let block = bs.blocks[i]
+        
+        if let converted = convert_block_ndb(block, tags: note.tags()) {
+            out.append(converted)
+        }
+        
+        i += 1
+    }
+    
+    let words = Int(bs.words)
+    blocks_free(&bs)
+    
+    return Blocks(words: words, blocks: out)
+}
+