Add fuzz/c/std/pixel_swizzler_fuzzer.c
diff --git a/fuzz/c/std/pixel_swizzler_fuzzer.c b/fuzz/c/std/pixel_swizzler_fuzzer.c
new file mode 100644
index 0000000..2ce1e87
--- /dev/null
+++ b/fuzz/c/std/pixel_swizzler_fuzzer.c
@@ -0,0 +1,159 @@
+// Copyright 2021 The Wuffs Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// ----------------
+
+// Silence the nested slash-star warning for the next comment's command line.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wcomment"
+
+/*
+This fuzzer (the fuzz function) is typically run indirectly, by a framework
+such as https://github.com/google/oss-fuzz calling LLVMFuzzerTestOneInput.
+
+When working on the fuzz implementation, or as a coherence check, defining
+WUFFS_CONFIG__FUZZLIB_MAIN will let you manually run fuzz over a set of files:
+
+gcc -DWUFFS_CONFIG__FUZZLIB_MAIN pixel_swizzler_fuzzer.c
+./a.out ../../../test/data
+rm -f ./a.out
+
+It should print "PASS", amongst other information, and exit(0).
+*/
+
+#pragma clang diagnostic pop
+
+// Wuffs ships as a "single file C library" or "header file library" as per
+// https://github.com/nothings/stb/blob/master/docs/stb_howto.txt
+//
+// To use that single file as a "foo.c"-like implementation, instead of a
+// "foo.h"-like header, #define WUFFS_IMPLEMENTATION before #include'ing or
+// compiling it.
+#define WUFFS_IMPLEMENTATION
+
+// Defining the WUFFS_CONFIG__MODULE* macros are optional, but it lets users of
+// release/c/etc.c choose which parts of Wuffs to build. That file contains the
+// entire Wuffs standard library, implementing a variety of codecs and file
+// formats. Without this macro definition, an optimizing compiler or linker may
+// very well discard Wuffs code for unused codecs, but listing the Wuffs
+// modules we use makes that process explicit. Preprocessing means that such
+// code simply isn't compiled.
+#define WUFFS_CONFIG__MODULES
+#define WUFFS_CONFIG__MODULE__BASE
+
+// If building this program in an environment that doesn't easily accommodate
+// relative includes, you can use the script/inline-c-relative-includes.go
+// program to generate a stand-alone C file.
+#include "../../../release/c/wuffs-unsupported-snapshot.c"
+#include "../fuzzlib/fuzzlib.c"
+
+const uint32_t pixfmts[] = {
+    WUFFS_BASE__PIXEL_FORMAT__Y,
+    WUFFS_BASE__PIXEL_FORMAT__Y_16BE,
+    WUFFS_BASE__PIXEL_FORMAT__INDEXED__BGRA_NONPREMUL,
+    WUFFS_BASE__PIXEL_FORMAT__INDEXED__BGRA_BINARY,
+    WUFFS_BASE__PIXEL_FORMAT__BGR_565,
+    WUFFS_BASE__PIXEL_FORMAT__BGR,
+    WUFFS_BASE__PIXEL_FORMAT__BGRA_NONPREMUL,
+    WUFFS_BASE__PIXEL_FORMAT__BGRA_NONPREMUL_4X16LE,
+    WUFFS_BASE__PIXEL_FORMAT__BGRA_PREMUL,
+    WUFFS_BASE__PIXEL_FORMAT__BGRX,
+    WUFFS_BASE__PIXEL_FORMAT__RGB,
+    WUFFS_BASE__PIXEL_FORMAT__RGBA_NONPREMUL,
+    WUFFS_BASE__PIXEL_FORMAT__RGBA_PREMUL,
+};
+
+const wuffs_base__pixel_blend blends[] = {
+    WUFFS_BASE__PIXEL_BLEND__SRC,
+    WUFFS_BASE__PIXEL_BLEND__SRC_OVER,
+};
+
+// fuzz tests that, regardless of the randomized inputs, calling
+// wuffs_base__pixel_swizzler__swizzle_interleaved_from_slice will not crash
+// the fuzzer (e.g. due to reads or write past buffer bounds).
+const char*  //
+fuzz(wuffs_base__io_buffer* src, uint64_t hash) {
+  uint8_t dst_palette_array[1024];
+  uint8_t src_palette_array[1024];
+  if ((src->meta.wi - src->meta.ri) < 2048) {
+    return "not enough data";
+  }
+  memcpy(dst_palette_array, src->data.ptr + src->meta.ri, 1024);
+  src->meta.ri += 1024;
+  memcpy(src_palette_array, src->data.ptr + src->meta.ri, 1024);
+  src->meta.ri += 1024;
+  wuffs_base__slice_u8 dst_palette =
+      wuffs_base__make_slice_u8(&dst_palette_array[0], 1024);
+  wuffs_base__slice_u8 src_palette =
+      wuffs_base__make_slice_u8(&src_palette_array[0], 1024);
+
+  size_t num_pixfmts = sizeof(pixfmts) / sizeof(pixfmts[0]);
+  wuffs_base__pixel_format dst_pixfmt = wuffs_base__make_pixel_format(
+      pixfmts[(0xFF & (hash >> 0)) % num_pixfmts]);
+  wuffs_base__pixel_format src_pixfmt = wuffs_base__make_pixel_format(
+      pixfmts[(0xFF & (hash >> 8)) % num_pixfmts]);
+
+  size_t num_blends = sizeof(blends) / sizeof(blends[0]);
+  wuffs_base__pixel_blend blend = blends[(0xFF & (hash >> 16)) % num_blends];
+
+  size_t dst_len = 0xFF & (hash >> 24);
+  size_t src_len = 0xFF & (hash >> 32);
+
+  wuffs_base__pixel_swizzler swizzler;
+  wuffs_base__status status = wuffs_base__pixel_swizzler__prepare(
+      &swizzler, dst_pixfmt, dst_palette, src_pixfmt, src_palette, blend);
+  if (status.repr) {
+    return wuffs_base__status__message(&status);
+  }
+
+  const char* ret = NULL;
+  wuffs_base__slice_u8 dst_slice = ((wuffs_base__slice_u8){});
+  wuffs_base__slice_u8 src_slice = ((wuffs_base__slice_u8){});
+
+  // Use a {} code block so that "goto exit" doesn't trigger "jump bypasses
+  // variable initialization" warnings.
+  {
+    if ((src->meta.wi - src->meta.ri) < dst_len) {
+      ret = "not enough data";
+      goto exit;
+    }
+    dst_slice = wuffs_base__malloc_slice_u8(malloc, dst_len);
+    if (!dst_slice.ptr) {
+      ret = "out of memory";
+      goto exit;
+    }
+    memcpy(dst_slice.ptr, src->data.ptr + src->meta.ri, dst_len);
+    src->meta.ri += dst_len;
+
+    if ((src->meta.wi - src->meta.ri) < src_len) {
+      ret = "not enough data";
+      goto exit;
+    }
+    src_slice = wuffs_base__malloc_slice_u8(malloc, src_len);
+    if (!src_slice.ptr) {
+      ret = "out of memory";
+      goto exit;
+    }
+    memcpy(src_slice.ptr, src->data.ptr + src->meta.ri, src_len);
+    src->meta.ri += src_len;
+
+    wuffs_base__pixel_swizzler__swizzle_interleaved_from_slice(
+        &swizzler, dst_slice, dst_palette, src_slice);
+  }
+
+exit:
+  free(src_slice.ptr);
+  free(dst_slice.ptr);
+  return ret;
+}
diff --git a/fuzz/c/std/seed_corpora.txt b/fuzz/c/std/seed_corpora.txt
index ad886a2..3537325 100644
--- a/fuzz/c/std/seed_corpora.txt
+++ b/fuzz/c/std/seed_corpora.txt
@@ -22,3 +22,7 @@
 png:    test/data/*.png   ../pngsuite_corpus/*.png
 wbmp:   test/data/*.wbmp
 zlib:   test/data/*.zlib
+
+# Wuffs' pixel_swizzler doesn't process any particular file format. We just
+# want some random inputs and bricks* is as good a seed corpus as any.
+pixel_swizzler: test/data/bricks*