spec/ directory with v2 and v3 specs [#62,#41,#4]

spec/v2/spec.md

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+# PMTiles version 2
+
+*Note: this is deprecated in favor of spec version 3.*
+
+PMTiles is a binary serialization format designed for two main access patterns: over the network, via HTTP 1.1 Byte Serving (`Range:` requests), or via memory-mapped files on disk. **All integer values are little-endian.**
+
+A PMTiles archive is composed of:
+* a fixed-size 512,000 byte header section
+* Followed by any number of tiles in arbitrary format
+* Optionally followed by any number of *leaf directories*
+
+### Header
+* The header begins with a 2-byte magic number, "PM"
+* Followed by 2 bytes, the PMTiles specification version (currently 2).
+* Followed by 4 bytes, the length of metadata (M bytes)
+* Followed by 2 bytes, the number of entries in the *root directory* (N entries)
+* Followed by M bytes of metadata, which **must be a JSON string with bounds, minzoom and maxzoom properties (new in v2)**
+* Followed by N * 17 bytes, the root directory.
+
+### Directory structure
+A directory is a contiguous sequence of 17 byte entries. A directory can have at most 21,845 entries. **A directory must be sorted by Z, X and then Y order (new in v2).**
+
+An entry consists of:
+* 1 byte: the zoom level (Z) of the entry, with the top bit set to 1 instead of 0 to indicate the offset/length points to a leaf directory and not a tile.
+* 3 bytes: the X (column) of the entry.
+* 3 bytes: the Y (row) of the entry.
+* 6 bytes: the offset of where the tile begins in the archive.
+* 4 bytes: the length of the tile, in bytes.
+
+**All leaf directory entries follow non-leaf entries. All leaf directories in a single directory must have the same Z value. (new in v2).**
+
+### Notes
+* A full directory of 21,845 entries holds exactly a complete pyramid with 8 levels, or 1+4+16+64+256+1024+4096+16384.
+* A PMTiles archive with less than 21,845 tiles should have a root directory and no leaf directories.
+* Multiple tile entries can point to the same offset; this is useful for de-duplicating certain tiles, such as an empty "ocean" tile.
+* Analogously, multiple leaf directory entries can point to the same offset; this can avoid inefficiently-packed small leaf directories.
+* The tentative media type for PMTiles archives is `application/vnd.pmtiles`.
+
+### Implementation suggestions
+* PMTiles is designed to make implementing a writer simple. Reserve 512KB, then write all tiles, recording their entry information; then write all leaf directories; finally, rewind to 0 and write the header.
+* The order of tile data in the archive is unspecified; an optimized implementation should arrange tiles on a 2D space-filling curve.
+* PMTiles readers should cache directory entries by byte offset, not by Z/X/Y. This means that deduplicated leaf directories result in cache hits.

spec/v3/spec.md

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+# PMTiles version 3
+
+## File structure
+
+A PMTiles archive is a single-file archive of square tiles with five main sections:
+
+1. A fixed-size, 127-byte **Header** starting with `PMTiles` and then the spec version - currently `3` - that contains offsets to the next sections.
+2. A root **Directory**, described below. The Header and Root combined must be less than 16,384 bytes.
+3. JSON metadata.
+4. Optionally, a section of **Leaf Directories**, encoded the same way as the root.
+5. The tile data.
+
+## Entries
+
+A Directory is a list of `Entries`, in ascending order by `TileId`:
+
+    Entry = (TileId uint64, Offset uint64, Length uint32, RunLength uint32)
+
+* `TileId` starts at 0 and corresponds to a cumulative position on the series of square Hilbert curves starting at z=0.
+* `Offset` is the position of the tile in the file relative to the start of the data section.
+*	`Length` is the size of the tile in bytes. 
+* `RunLength` is how many times this tile is repeated: the `TileId=5,RunLength=2` means that tile is present at IDs 5 and 6.
+* If `RunLength=0`, the offset/length points to a Leaf Directory where `TileId` is the first entry.
+
+# Directory Serialization
+
+Entries are stored in memory as integers, but serialized to disk using these compression steps:
+	1. A little-endian varint indicating the # of entries.
+	2. Delta encoding of `TileId`
+	3. Zeroing of `Offset`:
+		* `0` if it is equal to the `Offset` + `Length` of the previous entry
+		* `Offset+1` otherwise
+	4. Varint encoding of ll numbers
+	5. Columnar ordering: all `TileId`s, all `RunLength`s, all `Length`s, then all `Offset`s
+	6. Finally, general purpose compression as described by the `Header`'s `InternalCompression` field.
+
+# Directory Hierarchy
+* The number of entries in the root directory and leaf directories is up to the implementation.
+* However, the compressed size of the header plus root directory is required in v3 to be under **16,384 bytes**. This is to allow latency-optimized clients to prefetch the root directory and guarantee it is complete. A sophisticated writer might need several attempts to optimize this. 
+* Root size, leaf sizes and depth should be configurable by the user to adjust for optimize for different trade-offs: cost, bandwidth, latency.
+
+# Header Design
+
+*Certain fields belonging to metadata in v2 are promoted to fixed-size header fields. This allows a map container to be initialized to the desired extent or center without blocking on the JSON metadata.*
+
+The `Header` is 127 bytes, with little-endian integer values:
+
+| offset | description | width |
+| --- | --- | --- |
+| 0 | magic number `PMTiles` | 7 |
+| 7 | spec version, currently `3` | 1 |
+| 8 | offset of root directory | 8 |
+| 16 | length of root directory | 8 |
+| 24 | offset of JSON metadata, possibly compressed by `InternalCompression` | 8 |
+| 32 | length of JSON metadata | 8 |
+| 40 | offset of leaf directories | 8 |
+| 48 | length of leaf directories | 8 |
+| 56 | offset of tile data | 8 |
+| 64 | length of tile data | 8 |
+| 72 | # of addressed tiles, 0 if unknown | 8 |
+| 80 | # of tile entries, 0 if unknown | 8 |
+| 88 | # of tile contents, 0 if unknown | 8 |
+| 96 | boolean clustered flag | 1 |
+| 97 | internal compression enum (0 = Unknown, 1 = None, 2 = Gzip, 3 = Brotli, 4 = Zstd) | 1 |
+| 98 | tile compression enum | 1 |
+| 99 | tile type enum (0 = Unknown/Other, 1 = MVT (PBF Vector Tile), 2 = PNG, 3 = JPEG, 4 = WEBP | 1 |
+| 100 | min zoom | 1 |
+| 101 | max zoom | 1 |
+| 102 | min longitude (IEEE 754 float) | 4 |
+| 106 | min latitude | 4 |
+| 110 | max longitude | 4 |
+| 114 | max latitude | 4 |
+| 118 | center zoom | 1 |
+| 119 | center longitude | 4 |
+| 123 | center latitude | 4 |
+
+### Notes
+
+* **# of addressed tiles**: the total number of tiles before run-length encoding, i.e. `Sum(RunLlength)` over all entries.
+* **# of tile entries**: the total number of entries across all directories where `RunLength > 0`.
+* **# # of tile contents**: the number of referenced blobs in the tile section, or the unique # of offsets. If the archive is completely deduplicated, this is equal to the # of unique tile contents. If there is no deduplication, this is equal to the number of tile entries above.
+* **boolean clustered flag**: if `True`, blobs in the data section are generally ordered by Hilbert TileID. More concretely, this means that: when traversing all entries in TileID order, the offsets are either contiguous with the immediately previous entry, or refer to a lesser offset - a deduplicated tile.
+* **compression enum**: Mandatory, tells the client how to decompress contents as well as provide correct `Content-Encoding` headers to browsers.
+* **tile type**: A hint as to the tile contents. Clients and proxies may use this to:
+ 	* Automatically determine a visualization method
+	* provide a conventional MIME type HTTP `Content-Type` header
+	* Enforce a canonical file path extension e.g. `.mvt`, `png`, `jpeg`, `.webp`
+
+### Organization
+
+In most cases, the archive should be in the order `Header`, Root Directory, JSON Metadata, Leaf Directories, Tile Data. It is possible to relocate sections other than `Header` arbitrarily, but no current writers/readers take advantage of this.