lavu/tx: minor code style improvements and additional comments

libavutil/tx.c

@@ -30,7 +30,7 @@ int ff_tx_type_is_mdct(enum AVTXType type)
     }
 }
 
-/* Calculates the modular multiplicative inverse, not fast, replace */
+/* Calculates the modular multiplicative inverse */
 static av_always_inline int mulinv(int n, int m)
 {
     n = n % m;
@@ -91,6 +91,17 @@ int ff_tx_gen_compound_mapping(AVTXContext *s)
     return 0;
 }
 
+static inline int split_radix_permutation(int i, int m, int inverse)
+{
+    m >>= 1;
+    if (m <= 1)
+        return i & 1;
+    if (!(i & m))
+        return (split_radix_permutation(i, m, inverse) << 1);
+    m >>= 1;
+    return (split_radix_permutation(i, m, inverse) << 2) + 1 - 2*(!(i & m) ^ inverse);
+}
+
 int ff_tx_gen_ptwo_revtab(AVTXContext *s, int invert_lookup)
 {
     const int m = s->m, inv = s->inv;
@@ -117,6 +128,7 @@ int ff_tx_gen_ptwo_inplace_revtab_idx(AVTXContext *s)
     if (!(s->inplace_idx = av_malloc(s->m*sizeof(*s->inplace_idx))))
         return AVERROR(ENOMEM);
 
+    /* The first coefficient is always already in-place */
     for (int src = 1; src < s->m; src++) {
         int dst = s->revtab[src];
         int found = 0;
@@ -124,6 +136,9 @@ int ff_tx_gen_ptwo_inplace_revtab_idx(AVTXContext *s)
         if (dst <= src)
             continue;
 
+        /* This just checks if a closed loop has been encountered before,
+         * and if so, skips it, since to fully permute a loop we must only
+         * enter it once. */
         do {
             for (int j = 0; j < nb_inplace_idx; j++) {
                 if (dst == s->inplace_idx[j]) {

libavutil/tx.h

@@ -49,9 +49,11 @@ enum AVTXType {
      * float. Length is the frame size, not the window size (which is 2x frame)
      * For forward transforms, the stride specifies the spacing between each
      * sample in the output array in bytes. The input must be a flat array.
+     *
      * For inverse transforms, the stride specifies the spacing between each
      * sample in the input array in bytes. The output will be a flat array.
      * Stride must be a non-zero multiple of sizeof(float).
+     *
      * NOTE: the inverse transform is half-length, meaning the output will not
      * contain redundant data. This is what most codecs work with.
      */

libavutil/tx_priv.h

@@ -20,9 +20,7 @@
 #define AVUTIL_TX_PRIV_H
 
 #include "tx.h"
-#include <stddef.h>
 #include "thread.h"
-#include "mem.h"
 #include "mem_internal.h"
 #include "avassert.h"
 #include "attributes.h"
@@ -48,12 +46,14 @@ typedef void FFTComplex;
 
 #if defined(TX_FLOAT) || defined(TX_DOUBLE)
 
-#define CMUL(dre, dim, are, aim, bre, bim) do {                                \
+#define CMUL(dre, dim, are, aim, bre, bim)                                     \
+    do {                                                                       \
         (dre) = (are) * (bre) - (aim) * (bim);                                 \
         (dim) = (are) * (bim) + (aim) * (bre);                                 \
     } while (0)
 
-#define SMUL(dre, dim, are, aim, bre, bim) do {                                \
+#define SMUL(dre, dim, are, aim, bre, bim)                                     \
+    do {                                                                       \
         (dre) = (are) * (bre) - (aim) * (bim);                                 \
         (dim) = (are) * (bim) - (aim) * (bre);                                 \
     } while (0)
@@ -66,7 +66,8 @@ typedef void FFTComplex;
 #elif defined(TX_INT32)
 
 /* Properly rounds the result */
-#define CMUL(dre, dim, are, aim, bre, bim) do {                                \
+#define CMUL(dre, dim, are, aim, bre, bim)                                     \
+    do {                                                                       \
         int64_t accu;                                                          \
         (accu)  = (int64_t)(bre) * (are);                                      \
         (accu) -= (int64_t)(bim) * (aim);                                      \
@@ -76,7 +77,8 @@ typedef void FFTComplex;
         (dim)   = (int)(((accu) + 0x40000000) >> 31);                          \
     } while (0)
 
-#define SMUL(dre, dim, are, aim, bre, bim) do {                                \
+#define SMUL(dre, dim, are, aim, bre, bim)                                     \
+    do {                                                                       \
         int64_t accu;                                                          \
         (accu)  = (int64_t)(bre) * (are);                                      \
         (accu) -= (int64_t)(bim) * (aim);                                      \
@@ -93,7 +95,8 @@ typedef void FFTComplex;
 
 #endif
 
-#define BF(x, y, a, b) do {                                                    \
+#define BF(x, y, a, b)                                                         \
+    do {                                                                       \
         x = (a) - (b);                                                         \
         y = (a) + (b);                                                         \
     } while (0)
@@ -120,29 +123,29 @@ struct AVTXContext {
     int   *inplace_idx; /* Required indices to revtab for in-place transforms */
 };
 
-/* Shared functions */
+/* Checks if type is an MDCT */
 int ff_tx_type_is_mdct(enum AVTXType type);
+
+/*
+ * Generates the PFA permutation table into AVTXContext->pfatab. The end table
+ * is appended to the start table.
+ */
 int ff_tx_gen_compound_mapping(AVTXContext *s);
+
+/*
+ * Generates a standard-ish (slightly modified) Split-Radix revtab into
+ * AVTXContext->revtab
+ */
 int ff_tx_gen_ptwo_revtab(AVTXContext *s, int invert_lookup);
+
+/*
+ * Generates an index into AVTXContext->inplace_idx that if followed in the
+ * specific order,  allows the revtab to be done in-place. AVTXContext->revtab
+ * must already exist.
+ */
 int ff_tx_gen_ptwo_inplace_revtab_idx(AVTXContext *s);
 
-/* Also used by SIMD init */
+/* Templated init functions */
-static inline int split_radix_permutation(int i, int n, int inverse)
-{
-    int m;
-    if (n <= 2)
-        return i & 1;
-    m = n >> 1;
-    if (!(i & m))
-        return split_radix_permutation(i, m, inverse)*2;
-    m >>= 1;
-    if (inverse == !(i & m))
-        return split_radix_permutation(i, m, inverse)*4 + 1;
-    else
-        return split_radix_permutation(i, m, inverse)*4 - 1;
-}
-
-/* Templated functions */
 int ff_tx_init_mdct_fft_float(AVTXContext *s, av_tx_fn *tx,
                               enum AVTXType type, int inv, int len,
                               const void *scale, uint64_t flags);