bit_transpose: Support from 2 to 7 strands, not just 8

1 files changed, 73 insertions, 5 deletions

diff --git a/shared-module/_bit_transpose/__init__.c b/shared-module/_bit_transpose/__init__.c
index e30f989b9..517d2d840 100644
--- a/shared-module/_bit_transpose/__init__.c
+++ b/shared-module/_bit_transpose/__init__.c
@@ -30,6 +30,12 @@
 #include <stdlib.h>
 #include <string.h>
 
+#ifdef __GNUC__
+#define FALLTHROUGH __attribute__((fallthrough))
+#else
+#define FALLTHROUGH ((void)0) /* FALLTHROUGH */
+#endif
+
 // 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
@@ -40,7 +46,57 @@
 // > 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) {
+static void transpose_var(uint32_t *result, const uint8_t *src, int src_stride, int num_strands) {
+    uint32_t x = 0, y = 0, t;
+
+    src += (num_strands-1) * src_stride;
+
+    switch(num_strands) {
+    case 7:
+        x |= *src << 16;
+        src -= src_stride;
+        FALLTHROUGH;
+    case 6:
+        x |= *src << 8;
+        src -= src_stride;
+        FALLTHROUGH;
+    case 5:
+        x |= *src;
+        src -= src_stride;
+        FALLTHROUGH;
+    case 4:
+        y |= *src << 24;
+        src -= src_stride;
+        FALLTHROUGH;
+    case 3:
+        y |= *src << 16;
+        src -= src_stride;
+        FALLTHROUGH;
+    case 2:
+        y |= *src << 8;
+        src -= src_stride;
+        y |= *src;
+    }
+
+    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 transpose_8(uint32_t *result, const uint8_t *src, int src_stride) {
     uint32_t x, y, t;
 
     y = *src; src += src_stride;
@@ -70,14 +126,26 @@ static void transpose8(uint32_t *result, const uint8_t *src, int src_stride) {
     result[1] = y;
 }
 
-static void bit_transpose(uint32_t *result, const uint8_t *src, size_t src_stride, size_t n) {
+static void bit_transpose_8(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);
+        transpose_8(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);
+static void bit_transpose_var(uint32_t *result, const uint8_t *src, size_t src_stride, size_t n, int num_strands) {
+    for(size_t i=0; i<n; i++) {
+        transpose_var(result, src, src_stride, num_strands);
+        result += 2;
+        src += 1;
+    }
+}
+
+void common_hal_bit_transpose_bit_transpose(uint8_t *result, const uint8_t *src, size_t inlen, size_t num_strands) {
+    if(num_strands == 8) {
+        bit_transpose_8((uint32_t*)(void*)result, src, inlen/8, inlen/8);
+    } else  {
+        bit_transpose_var((uint32_t*)(void*)result, src, inlen/num_strands, inlen/num_strands, num_strands);
+    }
 }