Relax constraints on Y4M header parsing

Some refactoring and cleanup -- do not count the first 9 bytes against
the header limit. Add a unit test.

BUG=aomedia:2876

Change-Id: Id897d565e2917b48460cc77cd082cec4c98b42cb

3 files changed, 241 insertions, 190 deletions

diff --git a/test/y4m_test.cc b/test/y4m_test.cc
index 46cb5cff8..5df389f52 100644
--- a/test/y4m_test.cc
+++ b/test/y4m_test.cc
@@ -188,4 +188,29 @@ TEST_P(Y4mVideoWriteTest, WriteTest) {
 
 INSTANTIATE_TEST_SUITE_P(C, Y4mVideoWriteTest,
                          ::testing::ValuesIn(kY4mTestVectors));
+
+static const char kY4MRegularHeader[] =
+    "YUV4MPEG2 W4 H4 F30:1 Ip A0:0 C420jpeg XYSCSS=420JPEG\n"
+    "FRAME\n"
+    "012345678912345601230123";
+
+TEST(Y4MHeaderTest, RegularHeader) {
+  libvpx_test::TempOutFile f;
+  fwrite(kY4MRegularHeader, 1, sizeof(kY4MRegularHeader), f.file());
+  fflush(f.file());
+  EXPECT_EQ(0, fseek(f.file(), 0, 0));
+
+  y4m_input y4m;
+  EXPECT_EQ(y4m_input_open(&y4m, f.file(), /*skip_buffer=*/NULL,
+                           /*num_skip=*/0, /*only_420=*/0),
+            0);
+  EXPECT_EQ(y4m.pic_w, 4);
+  EXPECT_EQ(y4m.pic_h, 4);
+  EXPECT_EQ(y4m.fps_n, 30);
+  EXPECT_EQ(y4m.fps_d, 1);
+  EXPECT_EQ(y4m.interlace, 'p');
+  EXPECT_EQ(strcmp("420jpeg", y4m.chroma_type), 0);
+  y4m_input_close(&y4m);
+}
+
 }  // namespace
diff --git a/y4minput.c b/y4minput.c
index 007bd9971..68000768c 100644
--- a/y4minput.c
+++ b/y4minput.c
@@ -10,6 +10,7 @@
  *  Based on code from the OggTheora software codec source code,
  *  Copyright (C) 2002-2010 The Xiph.Org Foundation and contributors.
  */
+#include <assert.h>
 #include <errno.h>
 #include <stdlib.h>
 #include <string.h>
@@ -784,277 +785,294 @@ static void y4m_convert_null(y4m_input *_y4m, unsigned char *_dst,
   (void)_aux;
 }
 
-int y4m_input_open(y4m_input *_y4m, FILE *_fin, char *_skip, int _nskip,
-                   int only_420) {
+static const char TAG[] = "YUV4MPEG2";
+
+int y4m_input_open(y4m_input *y4m_ctx, FILE *file, char *skip_buffer,
+                   int num_skip, int only_420) {
+  // File must start with |TAG|.
+  char tag_buffer[9];  // 9 == strlen(TAG)
   char buffer[80] = { 0 };
   int ret;
   int i;
+  // Read as much as possible from |skip_buffer|, which were characters
+  // that were previously read from the file to do input-type detection.
+  assert(num_skip >= 0 && num_skip <= 8);
+  if (num_skip > 0) {
+    memcpy(tag_buffer, skip_buffer, num_skip);
+  }
+  // Start reading from the file now that the |skip_buffer| is depleted.
+  if (!file_read(tag_buffer + num_skip, 9 - num_skip, file)) {
+    return -1;
+  }
+  if (memcmp(TAG, tag_buffer, 9) != 0) {
+    fprintf(stderr, "Error parsing header: must start with %s\n", TAG);
+    return -1;
+  }
+  // Next character must be a space.
+  if (!file_read(buffer, 1, file) || buffer[0] != ' ') {
+    fprintf(stderr, "Error parsing header: space must follow %s\n", TAG);
+    return -1;
+  }
   /*Read until newline, or 80 cols, whichever happens first.*/
   for (i = 0; i < 79; i++) {
-    if (_nskip > 0) {
-      buffer[i] = *_skip++;
-      _nskip--;
-    } else {
-      if (!file_read(buffer + i, 1, _fin)) return -1;
-    }
+    if (!file_read(buffer + i, 1, file)) return -1;
     if (buffer[i] == '\n') break;
   }
   /*We skipped too much header data.*/
-  if (_nskip > 0) return -1;
   if (i == 79) {
-    fprintf(stderr, "Error parsing header; not a YUV2MPEG2 file?\n");
+    fprintf(stderr, "Error parsing header; not a YUV4MPEG2 file?\n");
     return -1;
   }
   buffer[i] = '\0';
-  if (memcmp(buffer, "YUV4MPEG", 8)) {
-    fprintf(stderr, "Incomplete magic for YUV4MPEG file.\n");
-    return -1;
-  }
-  if (buffer[8] != '2') {
-    fprintf(stderr, "Incorrect YUV input file version; YUV4MPEG2 required.\n");
-  }
-  ret = y4m_parse_tags(_y4m, buffer + 5);
+  ret = y4m_parse_tags(y4m_ctx, buffer);
   if (ret < 0) {
     fprintf(stderr, "Error parsing YUV4MPEG2 header.\n");
     return ret;
   }
-  if (_y4m->interlace == '?') {
+  if (y4m_ctx->interlace == '?') {
     fprintf(stderr,
             "Warning: Input video interlacing format unknown; "
             "assuming progressive scan.\n");
-  } else if (_y4m->interlace != 'p') {
+  } else if (y4m_ctx->interlace != 'p') {
     fprintf(stderr,
             "Input video is interlaced; "
             "Only progressive scan handled.\n");
     return -1;
   }
-  _y4m->vpx_fmt = VPX_IMG_FMT_I420;
-  _y4m->bps = 12;
-  _y4m->bit_depth = 8;
-  if (strcmp(_y4m->chroma_type, "420") == 0 ||
-      strcmp(_y4m->chroma_type, "420jpeg") == 0) {
-    _y4m->src_c_dec_h = _y4m->dst_c_dec_h = _y4m->src_c_dec_v =
-        _y4m->dst_c_dec_v = 2;
-    _y4m->dst_buf_read_sz =
-        _y4m->pic_w * _y4m->pic_h +
-        2 * ((_y4m->pic_w + 1) / 2) * ((_y4m->pic_h + 1) / 2);
+  y4m_ctx->vpx_fmt = VPX_IMG_FMT_I420;
+  y4m_ctx->bps = 12;
+  y4m_ctx->bit_depth = 8;
+  y4m_ctx->aux_buf = NULL;
+  y4m_ctx->dst_buf = NULL;
+  if (strcmp(y4m_ctx->chroma_type, "420") == 0 ||
+      strcmp(y4m_ctx->chroma_type, "420jpeg") == 0) {
+    y4m_ctx->src_c_dec_h = y4m_ctx->dst_c_dec_h = y4m_ctx->src_c_dec_v =
+        y4m_ctx->dst_c_dec_v = 2;
+    y4m_ctx->dst_buf_read_sz =
+        y4m_ctx->pic_w * y4m_ctx->pic_h +
+        2 * ((y4m_ctx->pic_w + 1) / 2) * ((y4m_ctx->pic_h + 1) / 2);
     /* Natively supported: no conversion required. */
-    _y4m->aux_buf_sz = _y4m->aux_buf_read_sz = 0;
-    _y4m->convert = y4m_convert_null;
-  } else if (strcmp(_y4m->chroma_type, "420p10") == 0) {
-    _y4m->src_c_dec_h = 2;
-    _y4m->dst_c_dec_h = 2;
-    _y4m->src_c_dec_v = 2;
-    _y4m->dst_c_dec_v = 2;
-    _y4m->dst_buf_read_sz =
-        2 * (_y4m->pic_w * _y4m->pic_h +
-             2 * ((_y4m->pic_w + 1) / 2) * ((_y4m->pic_h + 1) / 2));
+    y4m_ctx->aux_buf_sz = y4m_ctx->aux_buf_read_sz = 0;
+    y4m_ctx->convert = y4m_convert_null;
+  } else if (strcmp(y4m_ctx->chroma_type, "420p10") == 0) {
+    y4m_ctx->src_c_dec_h = 2;
+    y4m_ctx->dst_c_dec_h = 2;
+    y4m_ctx->src_c_dec_v = 2;
+    y4m_ctx->dst_c_dec_v = 2;
+    y4m_ctx->dst_buf_read_sz =
+        2 * (y4m_ctx->pic_w * y4m_ctx->pic_h +
+             2 * ((y4m_ctx->pic_w + 1) / 2) * ((y4m_ctx->pic_h + 1) / 2));
     /* Natively supported: no conversion required. */
-    _y4m->aux_buf_sz = _y4m->aux_buf_read_sz = 0;
-    _y4m->convert = y4m_convert_null;
-    _y4m->bit_depth = 10;
-    _y4m->bps = 15;
-    _y4m->vpx_fmt = VPX_IMG_FMT_I42016;
+    y4m_ctx->aux_buf_sz = y4m_ctx->aux_buf_read_sz = 0;
+    y4m_ctx->convert = y4m_convert_null;
+    y4m_ctx->bit_depth = 10;
+    y4m_ctx->bps = 15;
+    y4m_ctx->vpx_fmt = VPX_IMG_FMT_I42016;
     if (only_420) {
       fprintf(stderr, "Unsupported conversion from 420p10 to 420jpeg\n");
       return -1;
     }
-  } else if (strcmp(_y4m->chroma_type, "420p12") == 0) {
-    _y4m->src_c_dec_h = 2;
-    _y4m->dst_c_dec_h = 2;
-    _y4m->src_c_dec_v = 2;
-    _y4m->dst_c_dec_v = 2;
-    _y4m->dst_buf_read_sz =
-        2 * (_y4m->pic_w * _y4m->pic_h +
-             2 * ((_y4m->pic_w + 1) / 2) * ((_y4m->pic_h + 1) / 2));
+  } else if (strcmp(y4m_ctx->chroma_type, "420p12") == 0) {
+    y4m_ctx->src_c_dec_h = 2;
+    y4m_ctx->dst_c_dec_h = 2;
+    y4m_ctx->src_c_dec_v = 2;
+    y4m_ctx->dst_c_dec_v = 2;
+    y4m_ctx->dst_buf_read_sz =
+        2 * (y4m_ctx->pic_w * y4m_ctx->pic_h +
+             2 * ((y4m_ctx->pic_w + 1) / 2) * ((y4m_ctx->pic_h + 1) / 2));
     /* Natively supported: no conversion required. */
-    _y4m->aux_buf_sz = _y4m->aux_buf_read_sz = 0;
-    _y4m->convert = y4m_convert_null;
-    _y4m->bit_depth = 12;
-    _y4m->bps = 18;
-    _y4m->vpx_fmt = VPX_IMG_FMT_I42016;
+    y4m_ctx->aux_buf_sz = y4m_ctx->aux_buf_read_sz = 0;
+    y4m_ctx->convert = y4m_convert_null;
+    y4m_ctx->bit_depth = 12;
+    y4m_ctx->bps = 18;
+    y4m_ctx->vpx_fmt = VPX_IMG_FMT_I42016;
     if (only_420) {
       fprintf(stderr, "Unsupported conversion from 420p12 to 420jpeg\n");
       return -1;
     }
-  } else if (strcmp(_y4m->chroma_type, "420mpeg2") == 0) {
-    _y4m->src_c_dec_h = _y4m->dst_c_dec_h = _y4m->src_c_dec_v =
-        _y4m->dst_c_dec_v = 2;
-    _y4m->dst_buf_read_sz = _y4m->pic_w * _y4m->pic_h;
+  } else if (strcmp(y4m_ctx->chroma_type, "420mpeg2") == 0) {
+    y4m_ctx->src_c_dec_h = y4m_ctx->dst_c_dec_h = y4m_ctx->src_c_dec_v =
+        y4m_ctx->dst_c_dec_v = 2;
+    y4m_ctx->dst_buf_read_sz = y4m_ctx->pic_w * y4m_ctx->pic_h;
     /*Chroma filter required: read into the aux buf first.*/
-    _y4m->aux_buf_sz = _y4m->aux_buf_read_sz =
-        2 * ((_y4m->pic_w + 1) / 2) * ((_y4m->pic_h + 1) / 2);
-    _y4m->convert = y4m_convert_42xmpeg2_42xjpeg;
-  } else if (strcmp(_y4m->chroma_type, "420paldv") == 0) {
-    _y4m->src_c_dec_h = _y4m->dst_c_dec_h = _y4m->src_c_dec_v =
-        _y4m->dst_c_dec_v = 2;
-    _y4m->dst_buf_read_sz = _y4m->pic_w * _y4m->pic_h;
+    y4m_ctx->aux_buf_sz = y4m_ctx->aux_buf_read_sz =
+        2 * ((y4m_ctx->pic_w + 1) / 2) * ((y4m_ctx->pic_h + 1) / 2);
+    y4m_ctx->convert = y4m_convert_42xmpeg2_42xjpeg;
+  } else if (strcmp(y4m_ctx->chroma_type, "420paldv") == 0) {
+    y4m_ctx->src_c_dec_h = y4m_ctx->dst_c_dec_h = y4m_ctx->src_c_dec_v =
+        y4m_ctx->dst_c_dec_v = 2;
+    y4m_ctx->dst_buf_read_sz = y4m_ctx->pic_w * y4m_ctx->pic_h;
     /*Chroma filter required: read into the aux buf first.
       We need to make two filter passes, so we need some extra space in the
        aux buffer.*/
-    _y4m->aux_buf_sz = 3 * ((_y4m->pic_w + 1) / 2) * ((_y4m->pic_h + 1) / 2);
-    _y4m->aux_buf_read_sz =
-        2 * ((_y4m->pic_w + 1) / 2) * ((_y4m->pic_h + 1) / 2);
-    _y4m->convert = y4m_convert_42xpaldv_42xjpeg;
-  } else if (strcmp(_y4m->chroma_type, "422jpeg") == 0) {
-    _y4m->src_c_dec_h = _y4m->dst_c_dec_h = 2;
-    _y4m->src_c_dec_v = 1;
-    _y4m->dst_c_dec_v = 2;
-    _y4m->dst_buf_read_sz = _y4m->pic_w * _y4m->pic_h;
+    y4m_ctx->aux_buf_sz =
+        3 * ((y4m_ctx->pic_w + 1) / 2) * ((y4m_ctx->pic_h + 1) / 2);
+    y4m_ctx->aux_buf_read_sz =
+        2 * ((y4m_ctx->pic_w + 1) / 2) * ((y4m_ctx->pic_h + 1) / 2);
+    y4m_ctx->convert = y4m_convert_42xpaldv_42xjpeg;
+  } else if (strcmp(y4m_ctx->chroma_type, "422jpeg") == 0) {
+    y4m_ctx->src_c_dec_h = y4m_ctx->dst_c_dec_h = 2;
+    y4m_ctx->src_c_dec_v = 1;
+    y4m_ctx->dst_c_dec_v = 2;
+    y4m_ctx->dst_buf_read_sz = y4m_ctx->pic_w * y4m_ctx->pic_h;
     /*Chroma filter required: read into the aux buf first.*/
-    _y4m->aux_buf_sz = _y4m->aux_buf_read_sz =
-        2 * ((_y4m->pic_w + 1) / 2) * _y4m->pic_h;
-    _y4m->convert = y4m_convert_422jpeg_420jpeg;
-  } else if (strcmp(_y4m->chroma_type, "422") == 0) {
-    _y4m->src_c_dec_h = 2;
-    _y4m->src_c_dec_v = 1;
+    y4m_ctx->aux_buf_sz = y4m_ctx->aux_buf_read_sz =
+        2 * ((y4m_ctx->pic_w + 1) / 2) * y4m_ctx->pic_h;
+    y4m_ctx->convert = y4m_convert_422jpeg_420jpeg;
+  } else if (strcmp(y4m_ctx->chroma_type, "422") == 0) {
+    y4m_ctx->src_c_dec_h = 2;
+    y4m_ctx->src_c_dec_v = 1;
     if (only_420) {
-      _y4m->dst_c_dec_h = 2;
-      _y4m->dst_c_dec_v = 2;
-      _y4m->dst_buf_read_sz = _y4m->pic_w * _y4m->pic_h;
+      y4m_ctx->dst_c_dec_h = 2;
+      y4m_ctx->dst_c_dec_v = 2;
+      y4m_ctx->dst_buf_read_sz = y4m_ctx->pic_w * y4m_ctx->pic_h;
       /*Chroma filter required: read into the aux buf first.
         We need to make two filter passes, so we need some extra space in the
          aux buffer.*/
-      _y4m->aux_buf_read_sz = 2 * ((_y4m->pic_w + 1) / 2) * _y4m->pic_h;
-      _y4m->aux_buf_sz =
-          _y4m->aux_buf_read_sz + ((_y4m->pic_w + 1) / 2) * _y4m->pic_h;
-      _y4m->convert = y4m_convert_422_420jpeg;
+      y4m_ctx->aux_buf_read_sz =
+          2 * ((y4m_ctx->pic_w + 1) / 2) * y4m_ctx->pic_h;
+      y4m_ctx->aux_buf_sz = y4m_ctx->aux_buf_read_sz +
+                            ((y4m_ctx->pic_w + 1) / 2) * y4m_ctx->pic_h;
+      y4m_ctx->convert = y4m_convert_422_420jpeg;
     } else {
-      _y4m->vpx_fmt = VPX_IMG_FMT_I422;
-      _y4m->bps = 16;
-      _y4m->dst_c_dec_h = _y4m->src_c_dec_h;
-      _y4m->dst_c_dec_v = _y4m->src_c_dec_v;
-      _y4m->dst_buf_read_sz =
-          _y4m->pic_w * _y4m->pic_h + 2 * ((_y4m->pic_w + 1) / 2) * _y4m->pic_h;
+      y4m_ctx->vpx_fmt = VPX_IMG_FMT_I422;
+      y4m_ctx->bps = 16;
+      y4m_ctx->dst_c_dec_h = y4m_ctx->src_c_dec_h;
+      y4m_ctx->dst_c_dec_v = y4m_ctx->src_c_dec_v;
+      y4m_ctx->dst_buf_read_sz =
+          y4m_ctx->pic_w * y4m_ctx->pic_h +
+          2 * ((y4m_ctx->pic_w + 1) / 2) * y4m_ctx->pic_h;
       /*Natively supported: no conversion required.*/
-      _y4m->aux_buf_sz = _y4m->aux_buf_read_sz = 0;
-      _y4m->convert = y4m_convert_null;
+      y4m_ctx->aux_buf_sz = y4m_ctx->aux_buf_read_sz = 0;
+      y4m_ctx->convert = y4m_convert_null;
     }
-  } else if (strcmp(_y4m->chroma_type, "422p10") == 0) {
-    _y4m->src_c_dec_h = 2;
-    _y4m->src_c_dec_v = 1;
-    _y4m->vpx_fmt = VPX_IMG_FMT_I42216;
-    _y4m->bps = 20;
-    _y4m->bit_depth = 10;
-    _y4m->dst_c_dec_h = _y4m->src_c_dec_h;
-    _y4m->dst_c_dec_v = _y4m->src_c_dec_v;
-    _y4m->dst_buf_read_sz = 2 * (_y4m->pic_w * _y4m->pic_h +
-                                 2 * ((_y4m->pic_w + 1) / 2) * _y4m->pic_h);
-    _y4m->aux_buf_sz = _y4m->aux_buf_read_sz = 0;
-    _y4m->convert = y4m_convert_null;
+  } else if (strcmp(y4m_ctx->chroma_type, "422p10") == 0) {
+    y4m_ctx->src_c_dec_h = 2;
+    y4m_ctx->src_c_dec_v = 1;
+    y4m_ctx->vpx_fmt = VPX_IMG_FMT_I42216;
+    y4m_ctx->bps = 20;
+    y4m_ctx->bit_depth = 10;
+    y4m_ctx->dst_c_dec_h = y4m_ctx->src_c_dec_h;
+    y4m_ctx->dst_c_dec_v = y4m_ctx->src_c_dec_v;
+    y4m_ctx->dst_buf_read_sz =
+        2 * (y4m_ctx->pic_w * y4m_ctx->pic_h +
+             2 * ((y4m_ctx->pic_w + 1) / 2) * y4m_ctx->pic_h);
+    y4m_ctx->aux_buf_sz = y4m_ctx->aux_buf_read_sz = 0;
+    y4m_ctx->convert = y4m_convert_null;
     if (only_420) {
       fprintf(stderr, "Unsupported conversion from 422p10 to 420jpeg\n");
       return -1;
     }
-  } else if (strcmp(_y4m->chroma_type, "422p12") == 0) {
-    _y4m->src_c_dec_h = 2;
-    _y4m->src_c_dec_v = 1;
-    _y4m->vpx_fmt = VPX_IMG_FMT_I42216;
-    _y4m->bps = 24;
-    _y4m->bit_depth = 12;
-    _y4m->dst_c_dec_h = _y4m->src_c_dec_h;
-    _y4m->dst_c_dec_v = _y4m->src_c_dec_v;
-    _y4m->dst_buf_read_sz = 2 * (_y4m->pic_w * _y4m->pic_h +
-                                 2 * ((_y4m->pic_w + 1) / 2) * _y4m->pic_h);
-    _y4m->aux_buf_sz = _y4m->aux_buf_read_sz = 0;
-    _y4m->convert = y4m_convert_null;
+  } else if (strcmp(y4m_ctx->chroma_type, "422p12") == 0) {
+    y4m_ctx->src_c_dec_h = 2;
+    y4m_ctx->src_c_dec_v = 1;
+    y4m_ctx->vpx_fmt = VPX_IMG_FMT_I42216;
+    y4m_ctx->bps = 24;
+    y4m_ctx->bit_depth = 12;
+    y4m_ctx->dst_c_dec_h = y4m_ctx->src_c_dec_h;
+    y4m_ctx->dst_c_dec_v = y4m_ctx->src_c_dec_v;
+    y4m_ctx->dst_buf_read_sz =
+        2 * (y4m_ctx->pic_w * y4m_ctx->pic_h +
+             2 * ((y4m_ctx->pic_w + 1) / 2) * y4m_ctx->pic_h);
+    y4m_ctx->aux_buf_sz = y4m_ctx->aux_buf_read_sz = 0;
+    y4m_ctx->convert = y4m_convert_null;
     if (only_420) {
       fprintf(stderr, "Unsupported conversion from 422p12 to 420jpeg\n");
       return -1;
     }
-  } else if (strcmp(_y4m->chroma_type, "411") == 0) {
-    _y4m->src_c_dec_h = 4;
-    _y4m->dst_c_dec_h = 2;
-    _y4m->src_c_dec_v = 1;
-    _y4m->dst_c_dec_v = 2;
-    _y4m->dst_buf_read_sz = _y4m->pic_w * _y4m->pic_h;
+  } else if (strcmp(y4m_ctx->chroma_type, "411") == 0) {
+    y4m_ctx->src_c_dec_h = 4;
+    y4m_ctx->dst_c_dec_h = 2;
+    y4m_ctx->src_c_dec_v = 1;
+    y4m_ctx->dst_c_dec_v = 2;
+    y4m_ctx->dst_buf_read_sz = y4m_ctx->pic_w * y4m_ctx->pic_h;
     /*Chroma filter required: read into the aux buf first.
       We need to make two filter passes, so we need some extra space in the
        aux buffer.*/
-    _y4m->aux_buf_read_sz = 2 * ((_y4m->pic_w + 3) / 4) * _y4m->pic_h;
-    _y4m->aux_buf_sz =
-        _y4m->aux_buf_read_sz + ((_y4m->pic_w + 1) / 2) * _y4m->pic_h;
-    _y4m->convert = y4m_convert_411_420jpeg;
+    y4m_ctx->aux_buf_read_sz = 2 * ((y4m_ctx->pic_w + 3) / 4) * y4m_ctx->pic_h;
+    y4m_ctx->aux_buf_sz =
+        y4m_ctx->aux_buf_read_sz + ((y4m_ctx->pic_w + 1) / 2) * y4m_ctx->pic_h;
+    y4m_ctx->convert = y4m_convert_411_420jpeg;
     fprintf(stderr, "Unsupported conversion from yuv 411\n");
     return -1;
-  } else if (strcmp(_y4m->chroma_type, "444") == 0) {
-    _y4m->src_c_dec_h = 1;
-    _y4m->src_c_dec_v = 1;
+  } else if (strcmp(y4m_ctx->chroma_type, "444") == 0) {
+    y4m_ctx->src_c_dec_h = 1;
+    y4m_ctx->src_c_dec_v = 1;
     if (only_420) {
-      _y4m->dst_c_dec_h = 2;
-      _y4m->dst_c_dec_v = 2;
-      _y4m->dst_buf_read_sz = _y4m->pic_w * _y4m->pic_h;
+      y4m_ctx->dst_c_dec_h = 2;
+      y4m_ctx->dst_c_dec_v = 2;
+      y4m_ctx->dst_buf_read_sz = y4m_ctx->pic_w * y4m_ctx->pic_h;
       /*Chroma filter required: read into the aux buf first.
         We need to make two filter passes, so we need some extra space in the
          aux buffer.*/
-      _y4m->aux_buf_read_sz = 2 * _y4m->pic_w * _y4m->pic_h;
-      _y4m->aux_buf_sz =
-          _y4m->aux_buf_read_sz + ((_y4m->pic_w + 1) / 2) * _y4m->pic_h;
-      _y4m->convert = y4m_convert_444_420jpeg;
+      y4m_ctx->aux_buf_read_sz = 2 * y4m_ctx->pic_w * y4m_ctx->pic_h;
+      y4m_ctx->aux_buf_sz = y4m_ctx->aux_buf_read_sz +
+                            ((y4m_ctx->pic_w + 1) / 2) * y4m_ctx->pic_h;
+      y4m_ctx->convert = y4m_convert_444_420jpeg;
     } else {
-      _y4m->vpx_fmt = VPX_IMG_FMT_I444;
-      _y4m->bps = 24;
-      _y4m->dst_c_dec_h = _y4m->src_c_dec_h;
-      _y4m->dst_c_dec_v = _y4m->src_c_dec_v;
-      _y4m->dst_buf_read_sz = 3 * _y4m->pic_w * _y4m->pic_h;
+      y4m_ctx->vpx_fmt = VPX_IMG_FMT_I444;
+      y4m_ctx->bps = 24;
+      y4m_ctx->dst_c_dec_h = y4m_ctx->src_c_dec_h;
+      y4m_ctx->dst_c_dec_v = y4m_ctx->src_c_dec_v;
+      y4m_ctx->dst_buf_read_sz = 3 * y4m_ctx->pic_w * y4m_ctx->pic_h;
       /*Natively supported: no conversion required.*/
-      _y4m->aux_buf_sz = _y4m->aux_buf_read_sz = 0;
-      _y4m->convert = y4m_convert_null;
+      y4m_ctx->aux_buf_sz = y4m_ctx->aux_buf_read_sz = 0;
+      y4m_ctx->convert = y4m_convert_null;
     }
-  } else if (strcmp(_y4m->chroma_type, "444p10") == 0) {
-    _y4m->src_c_dec_h = 1;
-    _y4m->src_c_dec_v = 1;
-    _y4m->vpx_fmt = VPX_IMG_FMT_I44416;
-    _y4m->bps = 30;
-    _y4m->bit_depth = 10;
-    _y4m->dst_c_dec_h = _y4m->src_c_dec_h;
-    _y4m->dst_c_dec_v = _y4m->src_c_dec_v;
-    _y4m->dst_buf_read_sz = 2 * 3 * _y4m->pic_w * _y4m->pic_h;
-    _y4m->aux_buf_sz = _y4m->aux_buf_read_sz = 0;
-    _y4m->convert = y4m_convert_null;
+  } else if (strcmp(y4m_ctx->chroma_type, "444p10") == 0) {
+    y4m_ctx->src_c_dec_h = 1;
+    y4m_ctx->src_c_dec_v = 1;
+    y4m_ctx->vpx_fmt = VPX_IMG_FMT_I44416;
+    y4m_ctx->bps = 30;
+    y4m_ctx->bit_depth = 10;
+    y4m_ctx->dst_c_dec_h = y4m_ctx->src_c_dec_h;
+    y4m_ctx->dst_c_dec_v = y4m_ctx->src_c_dec_v;
+    y4m_ctx->dst_buf_read_sz = 2 * 3 * y4m_ctx->pic_w * y4m_ctx->pic_h;
+    y4m_ctx->aux_buf_sz = y4m_ctx->aux_buf_read_sz = 0;
+    y4m_ctx->convert = y4m_convert_null;
     if (only_420) {
       fprintf(stderr, "Unsupported conversion from 444p10 to 420jpeg\n");
       return -1;
     }
-  } else if (strcmp(_y4m->chroma_type, "444p12") == 0) {
-    _y4m->src_c_dec_h = 1;
-    _y4m->src_c_dec_v = 1;
-    _y4m->vpx_fmt = VPX_IMG_FMT_I44416;
-    _y4m->bps = 36;
-    _y4m->bit_depth = 12;
-    _y4m->dst_c_dec_h = _y4m->src_c_dec_h;
-    _y4m->dst_c_dec_v = _y4m->src_c_dec_v;
-    _y4m->dst_buf_read_sz = 2 * 3 * _y4m->pic_w * _y4m->pic_h;
-    _y4m->aux_buf_sz = _y4m->aux_buf_read_sz = 0;
-    _y4m->convert = y4m_convert_null;
+  } else if (strcmp(y4m_ctx->chroma_type, "444p12") == 0) {
+    y4m_ctx->src_c_dec_h = 1;
+    y4m_ctx->src_c_dec_v = 1;
+    y4m_ctx->vpx_fmt = VPX_IMG_FMT_I44416;
+    y4m_ctx->bps = 36;
+    y4m_ctx->bit_depth = 12;
+    y4m_ctx->dst_c_dec_h = y4m_ctx->src_c_dec_h;
+    y4m_ctx->dst_c_dec_v = y4m_ctx->src_c_dec_v;
+    y4m_ctx->dst_buf_read_sz = 2 * 3 * y4m_ctx->pic_w * y4m_ctx->pic_h;
+    y4m_ctx->aux_buf_sz = y4m_ctx->aux_buf_read_sz = 0;
+    y4m_ctx->convert = y4m_convert_null;
     if (only_420) {
       fprintf(stderr, "Unsupported conversion from 444p12 to 420jpeg\n");
       return -1;
     }
-  } else if (strcmp(_y4m->chroma_type, "mono") == 0) {
-    _y4m->src_c_dec_h = _y4m->src_c_dec_v = 0;
-    _y4m->dst_c_dec_h = _y4m->dst_c_dec_v = 2;
-    _y4m->dst_buf_read_sz = _y4m->pic_w * _y4m->pic_h;
+  } else if (strcmp(y4m_ctx->chroma_type, "mono") == 0) {
+    y4m_ctx->src_c_dec_h = y4m_ctx->src_c_dec_v = 0;
+    y4m_ctx->dst_c_dec_h = y4m_ctx->dst_c_dec_v = 2;
+    y4m_ctx->dst_buf_read_sz = y4m_ctx->pic_w * y4m_ctx->pic_h;
     /*No extra space required, but we need to clear the chroma planes.*/
-    _y4m->aux_buf_sz = _y4m->aux_buf_read_sz = 0;
-    _y4m->convert = y4m_convert_mono_420jpeg;
+    y4m_ctx->aux_buf_sz = y4m_ctx->aux_buf_read_sz = 0;
+    y4m_ctx->convert = y4m_convert_mono_420jpeg;
   } else {
-    fprintf(stderr, "Unknown chroma sampling type: %s\n", _y4m->chroma_type);
+    fprintf(stderr, "Unknown chroma sampling type: %s\n", y4m_ctx->chroma_type);
     return -1;
   }
   /*The size of the final frame buffers is always computed from the
      destination chroma decimation type.*/
-  _y4m->dst_buf_sz =
-      _y4m->pic_w * _y4m->pic_h +
-      2 * ((_y4m->pic_w + _y4m->dst_c_dec_h - 1) / _y4m->dst_c_dec_h) *
-          ((_y4m->pic_h + _y4m->dst_c_dec_v - 1) / _y4m->dst_c_dec_v);
-  if (_y4m->bit_depth == 8)
-    _y4m->dst_buf = (unsigned char *)malloc(_y4m->dst_buf_sz);
+  y4m_ctx->dst_buf_sz =
+      y4m_ctx->pic_w * y4m_ctx->pic_h +
+      2 * ((y4m_ctx->pic_w + y4m_ctx->dst_c_dec_h - 1) / y4m_ctx->dst_c_dec_h) *
+          ((y4m_ctx->pic_h + y4m_ctx->dst_c_dec_v - 1) / y4m_ctx->dst_c_dec_v);
+  if (y4m_ctx->bit_depth == 8)
+    y4m_ctx->dst_buf = (unsigned char *)malloc(y4m_ctx->dst_buf_sz);
   else
-    _y4m->dst_buf = (unsigned char *)malloc(2 * _y4m->dst_buf_sz);
+    y4m_ctx->dst_buf = (unsigned char *)malloc(2 * y4m_ctx->dst_buf_sz);
 
-  if (_y4m->aux_buf_sz > 0)
-    _y4m->aux_buf = (unsigned char *)malloc(_y4m->aux_buf_sz);
+  if (y4m_ctx->aux_buf_sz > 0)
+    y4m_ctx->aux_buf = (unsigned char *)malloc(y4m_ctx->aux_buf_sz);
   return 0;
 }
 
diff --git a/y4minput.h b/y4minput.h
index a4a8b18dc..573750d74 100644
--- a/y4minput.h
+++ b/y4minput.h
@@ -56,8 +56,16 @@ struct y4m_input {
   unsigned int bit_depth;
 };
 
-int y4m_input_open(y4m_input *_y4m, FILE *_fin, char *_skip, int _nskip,
-                   int only_420);
+/**
+ * Open the input file, treating it as Y4M. |y4m_ctx| is filled in after
+ * reading it. The |skip_buffer| indicates bytes that were previously read
+ * from |file|, to do input-type detection; this buffer will be read before
+ * the |file| is read. It is of size |num_skip|, which *must* be 8 or less.
+ *
+ * Returns 0 on success, -1 on failure.
+ */
+int y4m_input_open(y4m_input *y4m_ctx, FILE *file, char *skip_buffer,
+                   int num_skip, int only_420);
 void y4m_input_close(y4m_input *_y4m);
 int y4m_input_fetch_frame(y4m_input *_y4m, FILE *_fin, vpx_image_t *img);