core: add bit_transpose function

.. this version can only handle exactly 8 bits "across".  The restriction
may be relaxed in a future revision.

2 files changed, 110 insertions, 0 deletions

diff --git a/shared-module/_bit_transpose/__init__.c b/shared-module/_bit_transpose/__init__.c
new file mode 100644
index 000000000..e30f989b9
--- /dev/null
+++ b/shared-module/_bit_transpose/__init__.c
@@ -0,0 +1,83 @@
+/*
+ * This file is part of the Circuit Python project, https://github.com/adafruit/circuitpython
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2021 Jeff Epler
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include "shared-bindings/_bit_transpose/__init__.h"
+
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+
+// adapted from "Hacker's Delight" - Figure 7-2 Transposing an 8x8-bit matrix
+// basic idea is:
+// > First, treat the 8x8-bit matrix as 16 2x2-bit matrices, and transpose each
+// > of the 16 2x2-bit matrices. Second, treat the matrix as four 2x2 submatrices
+// > whose elements are 2x2-bit matrices and transpose each of the four 2x2
+// > submatrices. Finally, treat the matrix as a 2x2 matrix whose elements are
+// > 4x4-bit matrices, and transpose the 2x2 matrix. These transformations are
+// > illustrated below.
+// We want a different definition of bit/byte order, deal with strides differently, etc.
+// so the code is heavily re-worked compared to the original.
+static void transpose8(uint32_t *result, const uint8_t *src, int src_stride) {
+    uint32_t x, y, t;
+
+    y = *src; src += src_stride;
+    y |= (*src << 8); src += src_stride;
+    y |= (*src << 16); src += src_stride;
+    y |= (*src << 24); src += src_stride;
+    x = *src; src += src_stride;
+    x |= (*src << 8); src += src_stride;
+    x |= (*src << 16); src += src_stride;
+    x |= (*src << 24); src += src_stride;
+
+    t = (x ^ (x >> 7)) & 0x00AA00AA;  x = x ^ t ^ (t << 7);
+    t = (y ^ (y >> 7)) & 0x00AA00AA;  y = y ^ t ^ (t << 7);
+
+    t = (x ^ (x >>14)) & 0x0000CCCC;  x = x ^ t ^ (t <<14);
+    t = (y ^ (y >>14)) & 0x0000CCCC;  y = y ^ t ^ (t <<14);
+
+    t = (x & 0xF0F0F0F0) | ((y >> 4) & 0x0F0F0F0F);
+    y = ((x << 4) & 0xF0F0F0F0) | (y & 0x0F0F0F0F);
+    x = t;
+
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+    x = __builtin_bswap32(x);
+    y = __builtin_bswap32(y);
+#endif
+    result[0] = x;
+    result[1] = y;
+}
+
+static void bit_transpose(uint32_t *result, const uint8_t *src, size_t src_stride, size_t n) {
+    for(size_t i=0; i<n; i++) {
+        transpose8(result, src, src_stride);
+        result += 2;
+        src += 1;
+    }
+}
+
+void common_hal_bit_transpose_bit_transpose(uint8_t *result, const uint8_t *src, size_t n) {
+    bit_transpose((uint32_t*)(void*)result, src, n/8, n/8);
+}
diff --git a/shared-module/_bit_transpose/__init__.h b/shared-module/_bit_transpose/__init__.h
new file mode 100644
index 000000000..945e48b72
--- /dev/null
+++ b/shared-module/_bit_transpose/__init__.h
@@ -0,0 +1,27 @@
+/*
+ * This file is part of the Circuit Python project, https://github.com/adafruit/circuitpython
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2021 Jeff Epler
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#pragma once