Update chunk layout documentation

Signed-off-by: beorn7 <beorn@grafana.com>

tsdb/docs/format/chunks.md

@@ -34,22 +34,62 @@ in-file offset (lower 4 bytes) and segment sequence number (upper 4 bytes).
 └───────────────┴───────────────────┴──────────────┴────────────────┘
 ```
 
-## XOR chunk
+Notes:
+* `<uvarint>` has 1 to 10 bytes.
+* `encoding`: Currently either `XOR` or `histogram`.
+* `data`: See below for each encoding.
 
-TODO(beorn7): Add.
-
-## Histogram chunk
-
-TODO(beorn7): This is out of date. Update once settled on the (more or less) final format.
+## XOR chunk data
 
 ```
-┌──────────────┬─────────────────┬──────────────────────────┬──────────────────────────┬──────────────┐
-│ len <uint16> │ schema <varint> │ pos-spans <span-section> │ neg-spans <span-section> │ data <bytes> │
-└──────────────┴─────────────────┴──────────────────────────┴──────────────────────────┴──────────────┘
-
-span-section:
-
-┌──────────────┬──────────────────┬──────────────────┬────────────┐
-│ len <varint> │ length1 <varint> │ offset1 <varint> │ length2... │
-└──────────────┴──────────────────┴──────────────────┴────────────┘
+┌──────────────────────┬───────────────┬───────────────┬──────────────────────┬──────────────────────┬──────────────────────┬──────────────────────┬─────┐
+│ num_samples <uint16> │ ts_0 <varint> │ v_0 <float64> │ ts_1_delta <uvarint> │ v_1_xor <varbit_xor> │ ts_n_dod <varbit_ts> │ v_n_xor <varbit_xor> │ ... │
+└──────────────────────┴───────────────┴───────────────┴──────────────────────┴──────────────────────┴──────────────────────┴──────────────────────┴─────┘
 ```
+
+### Notes:
+
+* `ts` is the timestamp, `v` is the value.
+* `...` means to repeat the previous two fields as needed, with `n` starting at 2 and going up to `num_samples` – 1.
+* `<uint16>` has 2 bytes in big-endian order.
+* `<varint>` and `<uvarint>` have 1 to 10 bytes each.
+* `ts_1_delta` is `ts_1` – `ts_0`.
+* `ts_n_dod` is the “delta of deltas” of timestamps, i.e. (`ts_n` – `ts_n-1`) – (`ts_n-1` – `ts_n-2`).
+* `v_n_xor>` is the result of `v_n` XOR `v_n-1`.
+* `<varbit_xor>` is a specific variable bitwidth encoding of the result of XORing the current and the previous value. It has between 1 bit and 77 bits.
+  See [code for details](https://github.com/prometheus/prometheus/blob/7309c20e7e5774e7838f183ec97c65baa4362edc/tsdb/chunkenc/xor.go#L220-L253).
+* `<varbit_ts>` is a specific variable bitwidth encoding for the “delta of deltas” of timestamps (signed integers that are ideally small).
+  It has between 1 and 68 bits.
+  see [code for details](https://github.com/prometheus/prometheus/blob/7309c20e7e5774e7838f183ec97c65baa4362edc/tsdb/chunkenc/xor.go#L179-L205).
+
+## Histogram chunk data
+
+```
+┌──────────────────────┬───────────────────────────────┬─────────────────────┬──────────────────┬──────────────────┬────────────────┐
+│ num_samples <uint16> │ zero_threshold <1 or 9 bytes> │ schema <varbit_int> │ pos_spans <data> │ neg_spans <data> │ samples <data> │
+└──────────────────────┴───────────────────────────────┴─────────────────────┴──────────────────┴──────────────────┴────────────────┘
+```
+
+### Positive and negative spans data:
+
+```
+┌───────────────────┬────────────────────────┬───────────────────────┬─────┬──────────────────────────┬─────────────────────────┐
+│ num <varbit_uint> │ length_1 <varbit_uint> │ offset_1 <varbit_int> │ ... │ length_num <varbit_uint> │ offset_num <varbit_int> │
+└───────────────────┴────────────────────────┴───────────────────────┴─────┴──────────────────────────┴─────────────────────────┘
+```
+
+### Samples data:
+
+```
+TODO
+```
+
+### Notes:
+
+* `zero_threshold` has a specific encoding:
+  * If 0, it is a single zero byte.
+  * If a power of two between 2^-243 and 2^10, it is a single byte between 1 and 254.
+  * Otherwise, it is a byte with all bits set (255), followed by a float64, resulting in 9 bytes length.
+* `schema` is a specific value defined by the exposition format. Currently valid values are -4 <= n <= 8.
+* `<varbit_int>` is a variable bitwidth encoding for signed integers, optimized for “delta of deltas” of bucket deltas. It has between 1 bit and 9 bytes.
+* `<varbit_uint>` is a variable bitwidth encoding for unsigned integers with the same bit-bucketing as `<varbit_int>`.