feat(statevector): add the host code that GPU backends share

CHANGELOG.md

@@ -42,6 +42,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   in `CS 0 0`, raise an error naming the gate instead of its decomposition.
 
 ### Changed
+- `strategy="active_statevector"` may run a component's measurements later
+  where that frees no axis that a later rotation reuses and leaves more of the
+  tape before its first shot-dependent action, which a GPU backend runs once for
+  all shots. The samples follow the same distribution, but some circuits' seeds
+  give other samples than in 1.0.
 - `ChannelSampler.signature_matrix` is now a `tsim.utils.linalg.SparseGF2Matrix`
   instead of a dense uint8 array; `np.asarray` gives the dense matrix.
 

src/tsim/statevector/gpu/__init__.py

@@ -0,0 +1,14 @@
+"""What the statevector executor's GPU backends share.
+
+A GPU backend runs the noise sampler and every component's tape as kernels of
+its own language. The host side is common to them: the noise tables that the
+noise kernels read (``noise``), the tape generator, which writes a component's
+tape in any of their languages (``tape``), and the selection of outputs from
+the rows that the kernels return (``rows``).
+"""
+
+FORKED = (
+    "This process was forked after its parent used the GPU, so it samples on the "
+    "CPU; start it with the 'spawn' or 'forkserver' method to sample on the GPU."
+)
+"""The warning of a GPU sampler in a process forked after its parent used the GPU."""

src/tsim/statevector/gpu/rows.py

@@ -0,0 +1,26 @@
+"""The rows of outputs that the GPU backends return."""
+
+from __future__ import annotations
+
+import numpy as np
+
+
+def select(
+    rows: np.ndarray, columns: np.ndarray, num_outputs: int, bit_packed: bool
+) -> np.ndarray:
+    """Return the outputs ``columns`` of rows of every output, as ``read_outputs`` does.
+
+    The rows are bit-packed as ``np.packbits`` does, with zero bits past the
+    outputs; leading outputs in order are a view of them, or of whole bytes.
+    """
+    count = len(columns)
+    if bit_packed and np.array_equal(columns, np.arange(count)):
+        packed = rows[:, : -(-count // 8)]
+        if count == num_outputs or count % 8 == 0:
+            return packed
+        packed = packed.copy()
+        packed[:, -1] &= np.uint8((1 << (count % 8)) - 1)
+        return packed
+    bits = np.unpackbits(rows, axis=1, count=num_outputs, bitorder="little")
+    bits = bits.view(np.bool_)[:, columns]
+    return np.packbits(bits, axis=1, bitorder="little") if bit_packed else bits