Add function to chain (simple) slice expressions

Author: funkey

funlib/persistence/arrays/slices.py

@@ -0,0 +1,94 @@
+import numpy as np
+
+
+def chain_slices(slices_a, slices_b):
+
+    # make sure both slice expressions are tuples
+    if not isinstance(slices_a, tuple):
+        slices_a = (slices_a,)
+    if not isinstance(slices_b, tuple):
+        slices_b = (slices_b,)
+
+    # dimension of a is number of non-int expressions
+    dim_a = sum([not isinstance(x, int) for x in slices_a])
+
+    # slices_b can't slice more dimensions than a has
+    assert (
+        len(slices_b) <= dim_a
+    ), f"Slice expression {slices_b} has too many dimensions to chain with {slices_a}"
+
+    chained = []
+
+    j = 0
+    for slice_a in slices_a:
+
+        # if slice_a is int that dimension does not exist any longer, skip
+        # also skip if b has no more elements
+        if j == len(slices_b) or isinstance(slice_a, int):
+            chained.append(slice_a)
+        else:
+            slice_b = slices_b[j]
+            chained.append(_chain_slice(slice_a, slice_b))
+            j += 1
+
+    return tuple(chained)
+
+
+def _chain_slice(a, b):
+
+    # a is a slice(start, stop, step) expression
+    if isinstance(a, slice):
+
+        start_a = a.start if a.start else 0
+        step_a = a.step if a.step else 1
+
+        if isinstance(b, int):
+
+            idx = start_a + step_a * b
+            assert not a.stop or idx < a.stop, f"Slice {b} out of range for {b}"
+            return idx
+
+        elif isinstance(b, slice):
+
+            start_b = b.start if b.start else 0
+            step_b = b.step if b.step else 1
+
+            start = start_a + step_a * start_b if a.start or b.start else None
+            stop = step_a * b.stop if b.stop else a.stop
+            step = step_a * step_b if a.step or b.step else None
+
+            return slice(start, stop, step)
+
+        elif isinstance(b, list):
+
+            return list(_chain_slice(a, x) for x in b)
+
+        elif isinstance(b, np.ndarray):
+
+            # is b a mask array?
+            if b.dtype == bool:
+                raise RuntimeError("Not yet implemented")
+
+            return np.array([_chain_slice(a, x) for x in b])
+
+        else:
+
+            raise RuntimeError(
+                f"Don't know how to deal with slice {b} of type {type(b)}"
+            )
+
+    # is an index array
+    elif isinstance(a, list):
+
+        return list(np.array(a)[(b,)])
+
+    elif isinstance(a, np.ndarray):
+
+        if a.dtype == bool:
+            raise RuntimeError("Not yet implemented")
+
+        return a[(b,)]
+
+    else:
+
+        raise RuntimeError(f"Don't know how to deal with slice {a} of type {type(a)}")

tests/test_slices.py

@@ -0,0 +1,33 @@
+import numpy as np
+from funlib.persistence.arrays.slices import chain_slices
+
+
+def test_slice_chaining():
+
+    base = np.s_[::2, 0, :4]
+
+    # chain with index expressions
+
+    s1 = chain_slices(base, np.s_[0])
+    assert s1 == np.s_[0, 0, :4]
+
+    s2 = chain_slices(s1, np.s_[1])
+    assert s2 == np.s_[0, 0, 1]
+
+    # chain with index arrays
+
+    s1 = chain_slices(base, np.s_[[0, 1, 1, 2, 3, 5], :])
+    assert s1 == np.s_[[0, 2, 2, 4, 6, 10], 0, :4]
+
+    # ...and another index array
+    s21 = chain_slices(s1, np.s_[[0, 3], :])
+    assert s21 == np.s_[[0, 4], 0, :4]
+
+    # ...and a slice() expression
+    s22 = chain_slices(s1, np.s_[1:4])
+    assert s22 == np.s_[[2, 2, 4], 0, :4]
+
+    # chain with slice expressions
+
+    s1 = chain_slices(base, np.s_[10:20, ::2])
+    assert s1 == np.s_[20:40:2, 0, :4:2]