Add reshape codec

Author: jbms

codecs/reshape/README.md

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+# reshape codec
+
+Defines an `array -> array` codec that performs a reshaping operation.
+
+Note that `reshape` always preserves the (logical) lexicographical order (i.e. C
+order traversal) of elements within an array, but may be combined with the
+`transpose` codec to both reorder and reshape an array.
+
+## Codec name
+
+The value of the `name` member in the codec object MUST be `reshape`.
+
+## Configuration parameters
+
+### `shape`
+An array specifying the size `B_shape[i]` of each dimension `i` of
+the *output* array `B` as a function of the shape `A_shape` of the input array
+`A`.  Each element `shape[i]` must be one of:
+
+- a positive integer `size`, specifying that `B_shape[i] := size`.
+
+- an array of integers `input_dims`, specifying that:
+
+  ```
+  B_shape[i] := prod(A_shape[input_dims]]
+  ```.
+
+  Specifying the corresponding `input_dims` rather than an explicit `size` is
+  paticularly useful when using variable-size chunking.
+
+- the special value `-1`, which must occur at most once, specifying that
+  `B_shape[i]` is determined automatically in order to satisfy the
+  invariant that `prod(B_shape) == prod(A_shape)`.
+
+Implementations MUST return an error if the invariant
+
+```
+prod(B_shape) == prod(A_shape)
+```
+
+cannot be satisfied.
+
+Additionally, when `shape[i]` is specified as an array of integers `input_dims`,
+implementations MUST return an error if the following constraints are not
+satisfied:
+
+- the flattened list of input dimensions, over all elements of `shape`, must be
+  in monotonically increasing order, i.e. `"shape": [[0, 1], 10, [3, 4]]` is
+  allowed but the following are NOT allowed:
+
+  - `"shape": [[1], [0]]`
+  - `"shape": [[1, 0], 10, [3, 4]]`
+  - `"shape": [[3, 4], 10, [0, 1]]` are not allowed.
+
+  This constraint serves to avoid confusing `shape` configurations that may
+  (incorrectly) suggest a transpose, when in fact the `reshape` codec never
+  performs a transpose.
+
+- If `input_dims` specifies `k > 0` input dimensions:
+
+  `prod(B_shape[:i]) == prod(A_shape[input_dims[0]])` and
+  `prod(B_shape[i+1:]) == prod(A_shape[input_dims[k-1]+1:])`.
+
+This two constraints ensure that if the size of output dimension `i` is
+specified by `input_dims`, the coordinates in the input array along `input_dims`
+actually correspond to the raveled index along output dimension `i`.
+
+## Example
+
+For example, the array metadata below specifies that the compressor is the Zstd
+codec configured with a compression level of 1 and with the checksum stored:
+
+```json
+{
+    "chunk_grid": {
+      "name": "regular",
+      "configuration": {
+        "chunk_shape": [100, 50, 64, 3]
+      }
+    },
+    "codecs": [
+      {
+        "name": "reshape",
+        "configuration": {
+            "shape": [[0, 1], [2], 3]
+        }
+      },
+      {
+        "name": "bytes"
+        "configuration": {"endian": "little"}
+      }
+    ]
+}
+```
+
+## Format and algorithm
+
+This is an `array -> array` codec.
+
+The dimensionality of the output array `B` is equal to the length of the `shape`
+configuration parameter, and the output shape `B_shape` is determined as
+specified above.
+
+As this codec does NOT alter the lexicographical order of elements, the contents
+of the output array `B` is related to the contents of the input array `A` by:
+`ravel(B) == ravel(A)`.
+
+Implementations should, when possible, construct a virtual view rather than copy
+the array.
+
+## Change log
+
+No changes yet.
+
+## Current maintainers
+
+* Jeremy Maitin-Shepard ([@jbms](https://github.com/jbms)), Google

codecs/reshape/schema.json

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+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "object",
+  "properties": {
+    "name": {
+      "const": "reshape"
+    },
+    "configuration": {
+      "type": "object",
+      "properties": {
+        "shape": {
+          "type": "array",
+          "items": {
+	    "oneOf": [
+	      {"type": "integer", "minimum": -1},
+	      {"type": "array", "items": {"type": "integer", "minimum": 0}}
+	    ]
+	  }
+        }
+      },
+      "required": ["shape"],
+      "additionalProperties": false
+    }
+  },
+  "required": ["name", "configuration"],
+  "additionalProperties": false
+}