diff --git a/applypatch/imgdiff.c b/applypatch/imgdiff.c
new file mode 100644
index 0000000..6b9ebee
--- /dev/null
+++ b/applypatch/imgdiff.c
@@ -0,0 +1,1010 @@
+/*
+ * Copyright (C) 2009 The Android Open Source Project
+ *
+ * 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
+ *
+ *      http://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.
+ */
+
+/*
+ * This program constructs binary patches for images -- such as boot.img
+ * and recovery.img -- that consist primarily of large chunks of gzipped
+ * data interspersed with uncompressed data.  Doing a naive bsdiff of
+ * these files is not useful because small changes in the data lead to
+ * large changes in the compressed bitstream; bsdiff patches of gzipped
+ * data are typically as large as the data itself.
+ *
+ * To patch these usefully, we break the source and target images up into
+ * chunks of two types: "normal" and "gzip".  Normal chunks are simply
+ * patched using a plain bsdiff.  Gzip chunks are first expanded, then a
+ * bsdiff is applied to the uncompressed data, then the patched data is
+ * gzipped using the same encoder parameters.  Patched chunks are
+ * concatenated together to create the output file; the output image
+ * should be *exactly* the same series of bytes as the target image used
+ * originally to generate the patch.
+ *
+ * To work well with this tool, the gzipped sections of the target
+ * image must have been generated using the same deflate encoder that
+ * is available in applypatch, namely, the one in the zlib library.
+ * In practice this means that images should be compressed using the
+ * "minigzip" tool included in the zlib distribution, not the GNU gzip
+ * program.
+ *
+ * An "imgdiff" patch consists of a header describing the chunk structure
+ * of the file and any encoding parameters needed for the gzipped
+ * chunks, followed by N bsdiff patches, one per chunk.
+ *
+ * For a diff to be generated, the source and target images must have the
+ * same "chunk" structure: that is, the same number of gzipped and normal
+ * chunks in the same order.  Android boot and recovery images currently
+ * consist of five chunks:  a small normal header, a gzipped kernel, a
+ * small normal section, a gzipped ramdisk, and finally a small normal
+ * footer.
+ *
+ * Caveats:  we locate gzipped sections within the source and target
+ * images by searching for the byte sequence 1f8b0800:  1f8b is the gzip
+ * magic number; 08 specifies the "deflate" encoding [the only encoding
+ * supported by the gzip standard]; and 00 is the flags byte.  We do not
+ * currently support any extra header fields (which would be indicated by
+ * a nonzero flags byte).  We also don't handle the case when that byte
+ * sequence appears spuriously in the file.  (Note that it would have to
+ * occur spuriously within a normal chunk to be a problem.)
+ *
+ *
+ * The imgdiff patch header looks like this:
+ *
+ *    "IMGDIFF1"                  (8)   [magic number and version]
+ *    chunk count                 (4)
+ *    for each chunk:
+ *        chunk type              (4)   [CHUNK_{NORMAL, GZIP, DEFLATE, RAW}]
+ *        if chunk type == CHUNK_NORMAL:
+ *           source start         (8)
+ *           source len           (8)
+ *           bsdiff patch offset  (8)   [from start of patch file]
+ *        if chunk type == CHUNK_GZIP:      (version 1 only)
+ *           source start         (8)
+ *           source len           (8)
+ *           bsdiff patch offset  (8)   [from start of patch file]
+ *           source expanded len  (8)   [size of uncompressed source]
+ *           target expected len  (8)   [size of uncompressed target]
+ *           gzip level           (4)
+ *                method          (4)
+ *                windowBits      (4)
+ *                memLevel        (4)
+ *                strategy        (4)
+ *           gzip header len      (4)
+ *           gzip header          (gzip header len)
+ *           gzip footer          (8)
+ *        if chunk type == CHUNK_DEFLATE:   (version 2 only)
+ *           source start         (8)
+ *           source len           (8)
+ *           bsdiff patch offset  (8)   [from start of patch file]
+ *           source expanded len  (8)   [size of uncompressed source]
+ *           target expected len  (8)   [size of uncompressed target]
+ *           gzip level           (4)
+ *                method          (4)
+ *                windowBits      (4)
+ *                memLevel        (4)
+ *                strategy        (4)
+ *        if chunk type == RAW:             (version 2 only)
+ *           target len           (4)
+ *           data                 (target len)
+ *
+ * All integers are little-endian.  "source start" and "source len"
+ * specify the section of the input image that comprises this chunk,
+ * including the gzip header and footer for gzip chunks.  "source
+ * expanded len" is the size of the uncompressed source data.  "target
+ * expected len" is the size of the uncompressed data after applying
+ * the bsdiff patch.  The next five parameters specify the zlib
+ * parameters to be used when compressing the patched data, and the
+ * next three specify the header and footer to be wrapped around the
+ * compressed data to create the output chunk (so that header contents
+ * like the timestamp are recreated exactly).
+ *
+ * After the header there are 'chunk count' bsdiff patches; the offset
+ * of each from the beginning of the file is specified in the header.
+ */
+
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include <sys/types.h>
+
+#include "zlib.h"
+#include "imgdiff.h"
+#include "utils.h"
+
+typedef struct {
+  int type;             // CHUNK_NORMAL, CHUNK_DEFLATE
+  size_t start;         // offset of chunk in original image file
+
+  size_t len;
+  unsigned char* data;  // data to be patched (uncompressed, for deflate chunks)
+
+  size_t source_start;
+  size_t source_len;
+
+  off_t* I;             // used by bsdiff
+
+  // --- for CHUNK_DEFLATE chunks only: ---
+
+  // original (compressed) deflate data
+  size_t deflate_len;
+  unsigned char* deflate_data;
+
+  char* filename;       // used for zip entries
+
+  // deflate encoder parameters
+  int level, method, windowBits, memLevel, strategy;
+
+  size_t source_uncompressed_len;
+} ImageChunk;
+
+typedef struct {
+  int data_offset;
+  int deflate_len;
+  int uncomp_len;
+  char* filename;
+} ZipFileEntry;
+
+static int fileentry_compare(const void* a, const void* b) {
+  int ao = ((ZipFileEntry*)a)->data_offset;
+  int bo = ((ZipFileEntry*)b)->data_offset;
+  if (ao < bo) {
+    return -1;
+  } else if (ao > bo) {
+    return 1;
+  } else {
+    return 0;
+  }
+}
+
+// from bsdiff.c
+int bsdiff(u_char* old, off_t oldsize, off_t** IP, u_char* new, off_t newsize,
+           const char* patch_filename);
+
+unsigned char* ReadZip(const char* filename,
+                       int* num_chunks, ImageChunk** chunks,
+                       int include_pseudo_chunk) {
+  struct stat st;
+  if (stat(filename, &st) != 0) {
+    printf("failed to stat \"%s\": %s\n", filename, strerror(errno));
+    return NULL;
+  }
+
+  unsigned char* img = malloc(st.st_size);
+  FILE* f = fopen(filename, "rb");
+  if (fread(img, 1, st.st_size, f) != st.st_size) {
+    printf("failed to read \"%s\" %s\n", filename, strerror(errno));
+    fclose(f);
+    return NULL;
+  }
+  fclose(f);
+
+  // look for the end-of-central-directory record.
+
+  int i;
+  for (i = st.st_size-20; i >= 0 && i > st.st_size - 65600; --i) {
+    if (img[i] == 0x50 && img[i+1] == 0x4b &&
+        img[i+2] == 0x05 && img[i+3] == 0x06) {
+      break;
+    }
+  }
+  // double-check: this archive consists of a single "disk"
+  if (!(img[i+4] == 0 && img[i+5] == 0 && img[i+6] == 0 && img[i+7] == 0)) {
+    printf("can't process multi-disk archive\n");
+    return NULL;
+  }
+
+  int cdcount = Read2(img+i+8);
+  int cdoffset = Read4(img+i+16);
+
+  ZipFileEntry* temp_entries = malloc(cdcount * sizeof(ZipFileEntry));
+  int entrycount = 0;
+
+  unsigned char* cd = img+cdoffset;
+  for (i = 0; i < cdcount; ++i) {
+    if (!(cd[0] == 0x50 && cd[1] == 0x4b && cd[2] == 0x01 && cd[3] == 0x02)) {
+      printf("bad central directory entry %d\n", i);
+      return NULL;
+    }
+
+    int clen = Read4(cd+20);   // compressed len
+    int ulen = Read4(cd+24);   // uncompressed len
+    int nlen = Read2(cd+28);   // filename len
+    int xlen = Read2(cd+30);   // extra field len
+    int mlen = Read2(cd+32);   // file comment len
+    int hoffset = Read4(cd+42);   // local header offset
+
+    char* filename = malloc(nlen+1);
+    memcpy(filename, cd+46, nlen);
+    filename[nlen] = '\0';
+
+    int method = Read2(cd+10);
+
+    cd += 46 + nlen + xlen + mlen;
+
+    if (method != 8) {  // 8 == deflate
+      free(filename);
+      continue;
+    }
+
+    unsigned char* lh = img + hoffset;
+
+    if (!(lh[0] == 0x50 && lh[1] == 0x4b && lh[2] == 0x03 && lh[3] == 0x04)) {
+      printf("bad local file header entry %d\n", i);
+      return NULL;
+    }
+
+    if (Read2(lh+26) != nlen || memcmp(lh+30, filename, nlen) != 0) {
+      printf("central dir filename doesn't match local header\n");
+      return NULL;
+    }
+
+    xlen = Read2(lh+28);   // extra field len; might be different from CD entry?
+
+    temp_entries[entrycount].data_offset = hoffset+30+nlen+xlen;
+    temp_entries[entrycount].deflate_len = clen;
+    temp_entries[entrycount].uncomp_len = ulen;
+    temp_entries[entrycount].filename = filename;
+    ++entrycount;
+  }
+
+  qsort(temp_entries, entrycount, sizeof(ZipFileEntry), fileentry_compare);
+
+#if 0
+  printf("found %d deflated entries\n", entrycount);
+  for (i = 0; i < entrycount; ++i) {
+    printf("off %10d  len %10d unlen %10d   %p %s\n",
+           temp_entries[i].data_offset,
+           temp_entries[i].deflate_len,
+           temp_entries[i].uncomp_len,
+           temp_entries[i].filename,
+           temp_entries[i].filename);
+  }
+#endif
+
+  *num_chunks = 0;
+  *chunks = malloc((entrycount*2+2) * sizeof(ImageChunk));
+  ImageChunk* curr = *chunks;
+
+  if (include_pseudo_chunk) {
+    curr->type = CHUNK_NORMAL;
+    curr->start = 0;
+    curr->len = st.st_size;
+    curr->data = img;
+    curr->filename = NULL;
+    curr->I = NULL;
+    ++curr;
+    ++*num_chunks;
+  }
+
+  int pos = 0;
+  int nextentry = 0;
+
+  while (pos < st.st_size) {
+    if (nextentry < entrycount && pos == temp_entries[nextentry].data_offset) {
+      curr->type = CHUNK_DEFLATE;
+      curr->start = pos;
+      curr->deflate_len = temp_entries[nextentry].deflate_len;
+      curr->deflate_data = img + pos;
+      curr->filename = temp_entries[nextentry].filename;
+      curr->I = NULL;
+
+      curr->len = temp_entries[nextentry].uncomp_len;
+      curr->data = malloc(curr->len);
+
+      z_stream strm;
+      strm.zalloc = Z_NULL;
+      strm.zfree = Z_NULL;
+      strm.opaque = Z_NULL;
+      strm.avail_in = curr->deflate_len;
+      strm.next_in = curr->deflate_data;
+
+      // -15 means we are decoding a 'raw' deflate stream; zlib will
+      // not expect zlib headers.
+      int ret = inflateInit2(&strm, -15);
+
+      strm.avail_out = curr->len;
+      strm.next_out = curr->data;
+      ret = inflate(&strm, Z_NO_FLUSH);
+      if (ret != Z_STREAM_END) {
+        printf("failed to inflate \"%s\"; %d\n", curr->filename, ret);
+        return NULL;
+      }
+
+      inflateEnd(&strm);
+
+      pos += curr->deflate_len;
+      ++nextentry;
+      ++*num_chunks;
+      ++curr;
+      continue;
+    }
+
+    // use a normal chunk to take all the data up to the start of the
+    // next deflate section.
+
+    curr->type = CHUNK_NORMAL;
+    curr->start = pos;
+    if (nextentry < entrycount) {
+      curr->len = temp_entries[nextentry].data_offset - pos;
+    } else {
+      curr->len = st.st_size - pos;
+    }
+    curr->data = img + pos;
+    curr->filename = NULL;
+    curr->I = NULL;
+    pos += curr->len;
+
+    ++*num_chunks;
+    ++curr;
+  }
+
+  free(temp_entries);
+  return img;
+}
+
+/*
+ * Read the given file and break it up into chunks, putting the number
+ * of chunks and their info in *num_chunks and **chunks,
+ * respectively.  Returns a malloc'd block of memory containing the
+ * contents of the file; various pointers in the output chunk array
+ * will point into this block of memory.  The caller should free the
+ * return value when done with all the chunks.  Returns NULL on
+ * failure.
+ */
+unsigned char* ReadImage(const char* filename,
+                         int* num_chunks, ImageChunk** chunks) {
+  struct stat st;
+  if (stat(filename, &st) != 0) {
+    printf("failed to stat \"%s\": %s\n", filename, strerror(errno));
+    return NULL;
+  }
+
+  unsigned char* img = malloc(st.st_size + 4);
+  FILE* f = fopen(filename, "rb");
+  if (fread(img, 1, st.st_size, f) != st.st_size) {
+    printf("failed to read \"%s\" %s\n", filename, strerror(errno));
+    fclose(f);
+    return NULL;
+  }
+  fclose(f);
+
+  // append 4 zero bytes to the data so we can always search for the
+  // four-byte string 1f8b0800 starting at any point in the actual
+  // file data, without special-casing the end of the data.
+  memset(img+st.st_size, 0, 4);
+
+  size_t pos = 0;
+
+  *num_chunks = 0;
+  *chunks = NULL;
+
+  while (pos < st.st_size) {
+    unsigned char* p = img+pos;
+
+    if (st.st_size - pos >= 4 &&
+        p[0] == 0x1f && p[1] == 0x8b &&
+        p[2] == 0x08 &&    // deflate compression
+        p[3] == 0x00) {    // no header flags
+      // 'pos' is the offset of the start of a gzip chunk.
+
+      *num_chunks += 3;
+      *chunks = realloc(*chunks, *num_chunks * sizeof(ImageChunk));
+      ImageChunk* curr = *chunks + (*num_chunks-3);
+
+      // create a normal chunk for the header.
+      curr->start = pos;
+      curr->type = CHUNK_NORMAL;
+      curr->len = GZIP_HEADER_LEN;
+      curr->data = p;
+      curr->I = NULL;
+
+      pos += curr->len;
+      p += curr->len;
+      ++curr;
+
+      curr->type = CHUNK_DEFLATE;
+      curr->filename = NULL;
+      curr->I = NULL;
+
+      // We must decompress this chunk in order to discover where it
+      // ends, and so we can put the uncompressed data and its length
+      // into curr->data and curr->len.
+
+      size_t allocated = 32768;
+      curr->len = 0;
+      curr->data = malloc(allocated);
+      curr->start = pos;
+      curr->deflate_data = p;
+
+      z_stream strm;
+      strm.zalloc = Z_NULL;
+      strm.zfree = Z_NULL;
+      strm.opaque = Z_NULL;
+      strm.avail_in = st.st_size - pos;
+      strm.next_in = p;
+
+      // -15 means we are decoding a 'raw' deflate stream; zlib will
+      // not expect zlib headers.
+      int ret = inflateInit2(&strm, -15);
+
+      do {
+        strm.avail_out = allocated - curr->len;
+        strm.next_out = curr->data + curr->len;
+        ret = inflate(&strm, Z_NO_FLUSH);
+        curr->len = allocated - strm.avail_out;
+        if (strm.avail_out == 0) {
+          allocated *= 2;
+          curr->data = realloc(curr->data, allocated);
+        }
+      } while (ret != Z_STREAM_END);
+
+      curr->deflate_len = st.st_size - strm.avail_in - pos;
+      inflateEnd(&strm);
+      pos += curr->deflate_len;
+      p += curr->deflate_len;
+      ++curr;
+
+      // create a normal chunk for the footer
+
+      curr->type = CHUNK_NORMAL;
+      curr->start = pos;
+      curr->len = GZIP_FOOTER_LEN;
+      curr->data = img+pos;
+      curr->I = NULL;
+
+      pos += curr->len;
+      p += curr->len;
+      ++curr;
+
+      // The footer (that we just skipped over) contains the size of
+      // the uncompressed data.  Double-check to make sure that it
+      // matches the size of the data we got when we actually did
+      // the decompression.
+      size_t footer_size = Read4(p-4);
+      if (footer_size != curr[-2].len) {
+        printf("Error: footer size %d != decompressed size %d\n",
+                footer_size, curr[-2].len);
+        free(img);
+        return NULL;
+      }
+    } else {
+      // Reallocate the list for every chunk; we expect the number of
+      // chunks to be small (5 for typical boot and recovery images).
+      ++*num_chunks;
+      *chunks = realloc(*chunks, *num_chunks * sizeof(ImageChunk));
+      ImageChunk* curr = *chunks + (*num_chunks-1);
+      curr->start = pos;
+      curr->I = NULL;
+
+      // 'pos' is not the offset of the start of a gzip chunk, so scan
+      // forward until we find a gzip header.
+      curr->type = CHUNK_NORMAL;
+      curr->data = p;
+
+      for (curr->len = 0; curr->len < (st.st_size - pos); ++curr->len) {
+        if (p[curr->len] == 0x1f &&
+            p[curr->len+1] == 0x8b &&
+            p[curr->len+2] == 0x08 &&
+            p[curr->len+3] == 0x00) {
+          break;
+        }
+      }
+      pos += curr->len;
+    }
+  }
+
+  return img;
+}
+
+#define BUFFER_SIZE 32768
+
+/*
+ * Takes the uncompressed data stored in the chunk, compresses it
+ * using the zlib parameters stored in the chunk, and checks that it
+ * matches exactly the compressed data we started with (also stored in
+ * the chunk).  Return 0 on success.
+ */
+int TryReconstruction(ImageChunk* chunk, unsigned char* out) {
+  size_t p = 0;
+
+#if 0
+  printf("trying %d %d %d %d %d\n",
+          chunk->level, chunk->method, chunk->windowBits,
+          chunk->memLevel, chunk->strategy);
+#endif
+
+  z_stream strm;
+  strm.zalloc = Z_NULL;
+  strm.zfree = Z_NULL;
+  strm.opaque = Z_NULL;
+  strm.avail_in = chunk->len;
+  strm.next_in = chunk->data;
+  int ret;
+  ret = deflateInit2(&strm, chunk->level, chunk->method, chunk->windowBits,
+                     chunk->memLevel, chunk->strategy);
+  do {
+    strm.avail_out = BUFFER_SIZE;
+    strm.next_out = out;
+    ret = deflate(&strm, Z_FINISH);
+    size_t have = BUFFER_SIZE - strm.avail_out;
+
+    if (memcmp(out, chunk->deflate_data+p, have) != 0) {
+      // mismatch; data isn't the same.
+      deflateEnd(&strm);
+      return -1;
+    }
+    p += have;
+  } while (ret != Z_STREAM_END);
+  deflateEnd(&strm);
+  if (p != chunk->deflate_len) {
+    // mismatch; ran out of data before we should have.
+    return -1;
+  }
+  return 0;
+}
+
+/*
+ * Verify that we can reproduce exactly the same compressed data that
+ * we started with.  Sets the level, method, windowBits, memLevel, and
+ * strategy fields in the chunk to the encoding parameters needed to
+ * produce the right output.  Returns 0 on success.
+ */
+int ReconstructDeflateChunk(ImageChunk* chunk) {
+  if (chunk->type != CHUNK_DEFLATE) {
+    printf("attempt to reconstruct non-deflate chunk\n");
+    return -1;
+  }
+
+  size_t p = 0;
+  unsigned char* out = malloc(BUFFER_SIZE);
+
+  // We only check two combinations of encoder parameters:  level 6
+  // (the default) and level 9 (the maximum).
+  for (chunk->level = 6; chunk->level <= 9; chunk->level += 3) {
+    chunk->windowBits = -15;  // 32kb window; negative to indicate a raw stream.
+    chunk->memLevel = 8;      // the default value.
+    chunk->method = Z_DEFLATED;
+    chunk->strategy = Z_DEFAULT_STRATEGY;
+
+    if (TryReconstruction(chunk, out) == 0) {
+      free(out);
+      return 0;
+    }
+  }
+
+  free(out);
+  return -1;
+}
+
+/*
+ * Given source and target chunks, compute a bsdiff patch between them
+ * by running bsdiff in a subprocess.  Return the patch data, placing
+ * its length in *size.  Return NULL on failure.  We expect the bsdiff
+ * program to be in the path.
+ */
+unsigned char* MakePatch(ImageChunk* src, ImageChunk* tgt, size_t* size) {
+  if (tgt->type == CHUNK_NORMAL) {
+    if (tgt->len <= 160) {
+      tgt->type = CHUNK_RAW;
+      *size = tgt->len;
+      return tgt->data;
+    }
+  }
+
+  char ptemp[] = "/tmp/imgdiff-patch-XXXXXX";
+  mkstemp(ptemp);
+
+  int r = bsdiff(src->data, src->len, &(src->I), tgt->data, tgt->len, ptemp);
+  if (r != 0) {
+    printf("bsdiff() failed: %d\n", r);
+    return NULL;
+  }
+
+  struct stat st;
+  if (stat(ptemp, &st) != 0) {
+    printf("failed to stat patch file %s: %s\n",
+            ptemp, strerror(errno));
+    return NULL;
+  }
+
+  unsigned char* data = malloc(st.st_size);
+
+  if (tgt->type == CHUNK_NORMAL && tgt->len <= st.st_size) {
+    unlink(ptemp);
+
+    tgt->type = CHUNK_RAW;
+    *size = tgt->len;
+    return tgt->data;
+  }
+
+  *size = st.st_size;
+
+  FILE* f = fopen(ptemp, "rb");
+  if (f == NULL) {
+    printf("failed to open patch %s: %s\n", ptemp, strerror(errno));
+    return NULL;
+  }
+  if (fread(data, 1, st.st_size, f) != st.st_size) {
+    printf("failed to read patch %s: %s\n", ptemp, strerror(errno));
+    return NULL;
+  }
+  fclose(f);
+
+  unlink(ptemp);
+
+  tgt->source_start = src->start;
+  switch (tgt->type) {
+    case CHUNK_NORMAL:
+      tgt->source_len = src->len;
+      break;
+    case CHUNK_DEFLATE:
+      tgt->source_len = src->deflate_len;
+      tgt->source_uncompressed_len = src->len;
+      break;
+  }
+
+  return data;
+}
+
+/*
+ * Cause a gzip chunk to be treated as a normal chunk (ie, as a blob
+ * of uninterpreted data).  The resulting patch will likely be about
+ * as big as the target file, but it lets us handle the case of images
+ * where some gzip chunks are reconstructible but others aren't (by
+ * treating the ones that aren't as normal chunks).
+ */
+void ChangeDeflateChunkToNormal(ImageChunk* ch) {
+  if (ch->type != CHUNK_DEFLATE) return;
+  ch->type = CHUNK_NORMAL;
+  free(ch->data);
+  ch->data = ch->deflate_data;
+  ch->len = ch->deflate_len;
+}
+
+/*
+ * Return true if the data in the chunk is identical (including the
+ * compressed representation, for gzip chunks).
+ */
+int AreChunksEqual(ImageChunk* a, ImageChunk* b) {
+    if (a->type != b->type) return 0;
+
+    switch (a->type) {
+        case CHUNK_NORMAL:
+            return a->len == b->len && memcmp(a->data, b->data, a->len) == 0;
+
+        case CHUNK_DEFLATE:
+            return a->deflate_len == b->deflate_len &&
+                memcmp(a->deflate_data, b->deflate_data, a->deflate_len) == 0;
+
+        default:
+            printf("unknown chunk type %d\n", a->type);
+            return 0;
+    }
+}
+
+/*
+ * Look for runs of adjacent normal chunks and compress them down into
+ * a single chunk.  (Such runs can be produced when deflate chunks are
+ * changed to normal chunks.)
+ */
+void MergeAdjacentNormalChunks(ImageChunk* chunks, int* num_chunks) {
+  int out = 0;
+  int in_start = 0, in_end;
+  while (in_start < *num_chunks) {
+    if (chunks[in_start].type != CHUNK_NORMAL) {
+      in_end = in_start+1;
+    } else {
+      // in_start is a normal chunk.  Look for a run of normal chunks
+      // that constitute a solid block of data (ie, each chunk begins
+      // where the previous one ended).
+      for (in_end = in_start+1;
+           in_end < *num_chunks && chunks[in_end].type == CHUNK_NORMAL &&
+             (chunks[in_end].start ==
+              chunks[in_end-1].start + chunks[in_end-1].len &&
+              chunks[in_end].data ==
+              chunks[in_end-1].data + chunks[in_end-1].len);
+           ++in_end);
+    }
+
+    if (in_end == in_start+1) {
+#if 0
+      printf("chunk %d is now %d\n", in_start, out);
+#endif
+      if (out != in_start) {
+        memcpy(chunks+out, chunks+in_start, sizeof(ImageChunk));
+      }
+    } else {
+#if 0
+      printf("collapse normal chunks %d-%d into %d\n", in_start, in_end-1, out);
+#endif
+
+      // Merge chunks [in_start, in_end-1] into one chunk.  Since the
+      // data member of each chunk is just a pointer into an in-memory
+      // copy of the file, this can be done without recopying (the
+      // output chunk has the first chunk's start location and data
+      // pointer, and length equal to the sum of the input chunk
+      // lengths).
+      chunks[out].type = CHUNK_NORMAL;
+      chunks[out].start = chunks[in_start].start;
+      chunks[out].data = chunks[in_start].data;
+      chunks[out].len = chunks[in_end-1].len +
+        (chunks[in_end-1].start - chunks[in_start].start);
+    }
+
+    ++out;
+    in_start = in_end;
+  }
+  *num_chunks = out;
+}
+
+ImageChunk* FindChunkByName(const char* name,
+                            ImageChunk* chunks, int num_chunks) {
+  int i;
+  for (i = 0; i < num_chunks; ++i) {
+    if (chunks[i].type == CHUNK_DEFLATE && chunks[i].filename &&
+        strcmp(name, chunks[i].filename) == 0) {
+      return chunks+i;
+    }
+  }
+  return NULL;
+}
+
+void DumpChunks(ImageChunk* chunks, int num_chunks) {
+    int i;
+    for (i = 0; i < num_chunks; ++i) {
+        printf("chunk %d: type %d start %d len %d\n",
+               i, chunks[i].type, chunks[i].start, chunks[i].len);
+    }
+}
+
+int main(int argc, char** argv) {
+  if (argc != 4 && argc != 5) {
+    usage:
+    printf("usage: %s [-z] <src-img> <tgt-img> <patch-file>\n",
+            argv[0]);
+    return 2;
+  }
+
+  int zip_mode = 0;
+
+  if (strcmp(argv[1], "-z") == 0) {
+    zip_mode = 1;
+    --argc;
+    ++argv;
+  }
+
+
+  int num_src_chunks;
+  ImageChunk* src_chunks;
+  int num_tgt_chunks;
+  ImageChunk* tgt_chunks;
+  int i;
+
+  if (zip_mode) {
+    if (ReadZip(argv[1], &num_src_chunks, &src_chunks, 1) == NULL) {
+      printf("failed to break apart source zip file\n");
+      return 1;
+    }
+    if (ReadZip(argv[2], &num_tgt_chunks, &tgt_chunks, 0) == NULL) {
+      printf("failed to break apart target zip file\n");
+      return 1;
+    }
+  } else {
+    if (ReadImage(argv[1], &num_src_chunks, &src_chunks) == NULL) {
+      printf("failed to break apart source image\n");
+      return 1;
+    }
+    if (ReadImage(argv[2], &num_tgt_chunks, &tgt_chunks) == NULL) {
+      printf("failed to break apart target image\n");
+      return 1;
+    }
+
+    // Verify that the source and target images have the same chunk
+    // structure (ie, the same sequence of deflate and normal chunks).
+
+    if (!zip_mode) {
+        // Merge the gzip header and footer in with any adjacent
+        // normal chunks.
+        MergeAdjacentNormalChunks(tgt_chunks, &num_tgt_chunks);
+        MergeAdjacentNormalChunks(src_chunks, &num_src_chunks);
+    }
+
+    if (num_src_chunks != num_tgt_chunks) {
+      printf("source and target don't have same number of chunks!\n");
+      printf("source chunks:\n");
+      DumpChunks(src_chunks, num_src_chunks);
+      printf("target chunks:\n");
+      DumpChunks(tgt_chunks, num_tgt_chunks);
+      return 1;
+    }
+    for (i = 0; i < num_src_chunks; ++i) {
+      if (src_chunks[i].type != tgt_chunks[i].type) {
+        printf("source and target don't have same chunk "
+                "structure! (chunk %d)\n", i);
+        printf("source chunks:\n");
+        DumpChunks(src_chunks, num_src_chunks);
+        printf("target chunks:\n");
+        DumpChunks(tgt_chunks, num_tgt_chunks);
+        return 1;
+      }
+    }
+  }
+
+  for (i = 0; i < num_tgt_chunks; ++i) {
+    if (tgt_chunks[i].type == CHUNK_DEFLATE) {
+      // Confirm that given the uncompressed chunk data in the target, we
+      // can recompress it and get exactly the same bits as are in the
+      // input target image.  If this fails, treat the chunk as a normal
+      // non-deflated chunk.
+      if (ReconstructDeflateChunk(tgt_chunks+i) < 0) {
+        printf("failed to reconstruct target deflate chunk %d [%s]; "
+               "treating as normal\n", i, tgt_chunks[i].filename);
+        ChangeDeflateChunkToNormal(tgt_chunks+i);
+        if (zip_mode) {
+          ImageChunk* src = FindChunkByName(tgt_chunks[i].filename, src_chunks, num_src_chunks);
+          if (src) {
+            ChangeDeflateChunkToNormal(src);
+          }
+        } else {
+          ChangeDeflateChunkToNormal(src_chunks+i);
+        }
+        continue;
+      }
+
+      // If two deflate chunks are identical (eg, the kernel has not
+      // changed between two builds), treat them as normal chunks.
+      // This makes applypatch much faster -- it can apply a trivial
+      // patch to the compressed data, rather than uncompressing and
+      // recompressing to apply the trivial patch to the uncompressed
+      // data.
+      ImageChunk* src;
+      if (zip_mode) {
+        src = FindChunkByName(tgt_chunks[i].filename, src_chunks, num_src_chunks);
+      } else {
+        src = src_chunks+i;
+      }
+
+      if (src == NULL || AreChunksEqual(tgt_chunks+i, src)) {
+        ChangeDeflateChunkToNormal(tgt_chunks+i);
+        if (src) {
+          ChangeDeflateChunkToNormal(src);
+        }
+      }
+    }
+  }
+
+  // Merging neighboring normal chunks.
+  if (zip_mode) {
+    // For zips, we only need to do this to the target:  deflated
+    // chunks are matched via filename, and normal chunks are patched
+    // using the entire source file as the source.
+    MergeAdjacentNormalChunks(tgt_chunks, &num_tgt_chunks);
+  } else {
+    // For images, we need to maintain the parallel structure of the
+    // chunk lists, so do the merging in both the source and target
+    // lists.
+    MergeAdjacentNormalChunks(tgt_chunks, &num_tgt_chunks);
+    MergeAdjacentNormalChunks(src_chunks, &num_src_chunks);
+    if (num_src_chunks != num_tgt_chunks) {
+      // This shouldn't happen.
+      printf("merging normal chunks went awry\n");
+      return 1;
+    }
+  }
+
+  // Compute bsdiff patches for each chunk's data (the uncompressed
+  // data, in the case of deflate chunks).
+
+  printf("Construct patches for %d chunks...\n", num_tgt_chunks);
+  unsigned char** patch_data = malloc(num_tgt_chunks * sizeof(unsigned char*));
+  size_t* patch_size = malloc(num_tgt_chunks * sizeof(size_t));
+  for (i = 0; i < num_tgt_chunks; ++i) {
+    if (zip_mode) {
+      ImageChunk* src;
+      if (tgt_chunks[i].type == CHUNK_DEFLATE &&
+          (src = FindChunkByName(tgt_chunks[i].filename, src_chunks,
+                                 num_src_chunks))) {
+        patch_data[i] = MakePatch(src, tgt_chunks+i, patch_size+i);
+      } else {
+        patch_data[i] = MakePatch(src_chunks, tgt_chunks+i, patch_size+i);
+      }
+    } else {
+      patch_data[i] = MakePatch(src_chunks+i, tgt_chunks+i, patch_size+i);
+    }
+    printf("patch %3d is %d bytes (of %d)\n",
+           i, patch_size[i], tgt_chunks[i].source_len);
+  }
+
+  // Figure out how big the imgdiff file header is going to be, so
+  // that we can correctly compute the offset of each bsdiff patch
+  // within the file.
+
+  size_t total_header_size = 12;
+  for (i = 0; i < num_tgt_chunks; ++i) {
+    total_header_size += 4;
+    switch (tgt_chunks[i].type) {
+      case CHUNK_NORMAL:
+        total_header_size += 8*3;
+        break;
+      case CHUNK_DEFLATE:
+        total_header_size += 8*5 + 4*5;
+        break;
+      case CHUNK_RAW:
+        total_header_size += 4 + patch_size[i];
+        break;
+    }
+  }
+
+  size_t offset = total_header_size;
+
+  FILE* f = fopen(argv[3], "wb");
+
+  // Write out the headers.
+
+  fwrite("IMGDIFF2", 1, 8, f);
+  Write4(num_tgt_chunks, f);
+  for (i = 0; i < num_tgt_chunks; ++i) {
+    Write4(tgt_chunks[i].type, f);
+
+    switch (tgt_chunks[i].type) {
+      case CHUNK_NORMAL:
+        printf("chunk %3d: normal   (%10d, %10d)  %10d\n", i,
+               tgt_chunks[i].start, tgt_chunks[i].len, patch_size[i]);
+        Write8(tgt_chunks[i].source_start, f);
+        Write8(tgt_chunks[i].source_len, f);
+        Write8(offset, f);
+        offset += patch_size[i];
+        break;
+
+      case CHUNK_DEFLATE:
+        printf("chunk %3d: deflate  (%10d, %10d)  %10d  %s\n", i,
+               tgt_chunks[i].start, tgt_chunks[i].deflate_len, patch_size[i],
+               tgt_chunks[i].filename);
+        Write8(tgt_chunks[i].source_start, f);
+        Write8(tgt_chunks[i].source_len, f);
+        Write8(offset, f);
+        Write8(tgt_chunks[i].source_uncompressed_len, f);
+        Write8(tgt_chunks[i].len, f);
+        Write4(tgt_chunks[i].level, f);
+        Write4(tgt_chunks[i].method, f);
+        Write4(tgt_chunks[i].windowBits, f);
+        Write4(tgt_chunks[i].memLevel, f);
+        Write4(tgt_chunks[i].strategy, f);
+        offset += patch_size[i];
+        break;
+
+      case CHUNK_RAW:
+        printf("chunk %3d: raw      (%10d, %10d)\n", i,
+               tgt_chunks[i].start, tgt_chunks[i].len);
+        Write4(patch_size[i], f);
+        fwrite(patch_data[i], 1, patch_size[i], f);
+        break;
+    }
+  }
+
+  // Append each chunk's bsdiff patch, in order.
+
+  for (i = 0; i < num_tgt_chunks; ++i) {
+    if (tgt_chunks[i].type != CHUNK_RAW) {
+      fwrite(patch_data[i], 1, patch_size[i], f);
+    }
+  }
+
+  fclose(f);
+
+  return 0;
+}