diff --git a/applypatch/applypatch.cpp b/applypatch/applypatch.cpp
new file mode 100644
index 0000000..96bd88e
--- /dev/null
+++ b/applypatch/applypatch.cpp
@@ -0,0 +1,1025 @@
+/*
+ * Copyright (C) 2008 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.
+ */
+
+#include <errno.h>
+#include <fcntl.h>
+#include <libgen.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <sys/statfs.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include "mincrypt/sha.h"
+#include "applypatch.h"
+#include "mtdutils/mtdutils.h"
+#include "edify/expr.h"
+
+static int LoadPartitionContents(const char* filename, FileContents* file);
+static ssize_t FileSink(const unsigned char* data, ssize_t len, void* token);
+static int GenerateTarget(FileContents* source_file,
+                          const Value* source_patch_value,
+                          FileContents* copy_file,
+                          const Value* copy_patch_value,
+                          const char* source_filename,
+                          const char* target_filename,
+                          const uint8_t target_sha1[SHA_DIGEST_SIZE],
+                          size_t target_size,
+                          const Value* bonus_data);
+
+static int mtd_partitions_scanned = 0;
+
+// Read a file into memory; store the file contents and associated
+// metadata in *file.
+//
+// Return 0 on success.
+int LoadFileContents(const char* filename, FileContents* file) {
+    file->data = NULL;
+
+    // A special 'filename' beginning with "MTD:" or "EMMC:" means to
+    // load the contents of a partition.
+    if (strncmp(filename, "MTD:", 4) == 0 ||
+        strncmp(filename, "EMMC:", 5) == 0) {
+        return LoadPartitionContents(filename, file);
+    }
+
+    if (stat(filename, &file->st) != 0) {
+        printf("failed to stat \"%s\": %s\n", filename, strerror(errno));
+        return -1;
+    }
+
+    file->size = file->st.st_size;
+    file->data = reinterpret_cast<unsigned char*>(malloc(file->size));
+
+    FILE* f = fopen(filename, "rb");
+    if (f == NULL) {
+        printf("failed to open \"%s\": %s\n", filename, strerror(errno));
+        free(file->data);
+        file->data = NULL;
+        return -1;
+    }
+
+    size_t bytes_read = fread(file->data, 1, file->size, f);
+    if (bytes_read != static_cast<size_t>(file->size)) {
+        printf("short read of \"%s\" (%zu bytes of %zd)\n", filename, bytes_read, file->size);
+        free(file->data);
+        file->data = NULL;
+        return -1;
+    }
+    fclose(f);
+
+    SHA_hash(file->data, file->size, file->sha1);
+    return 0;
+}
+
+static size_t* size_array;
+// comparison function for qsort()ing an int array of indexes into
+// size_array[].
+static int compare_size_indices(const void* a, const void* b) {
+    const int aa = *reinterpret_cast<const int*>(a);
+    const int bb = *reinterpret_cast<const int*>(b);
+    if (size_array[aa] < size_array[bb]) {
+        return -1;
+    } else if (size_array[aa] > size_array[bb]) {
+        return 1;
+    } else {
+        return 0;
+    }
+}
+
+// Load the contents of an MTD or EMMC partition into the provided
+// FileContents.  filename should be a string of the form
+// "MTD:<partition_name>:<size_1>:<sha1_1>:<size_2>:<sha1_2>:..."  (or
+// "EMMC:<partition_device>:...").  The smallest size_n bytes for
+// which that prefix of the partition contents has the corresponding
+// sha1 hash will be loaded.  It is acceptable for a size value to be
+// repeated with different sha1s.  Will return 0 on success.
+//
+// This complexity is needed because if an OTA installation is
+// interrupted, the partition might contain either the source or the
+// target data, which might be of different lengths.  We need to know
+// the length in order to read from a partition (there is no
+// "end-of-file" marker), so the caller must specify the possible
+// lengths and the hash of the data, and we'll do the load expecting
+// to find one of those hashes.
+enum PartitionType { MTD, EMMC };
+
+static int LoadPartitionContents(const char* filename, FileContents* file) {
+    char* copy = strdup(filename);
+    const char* magic = strtok(copy, ":");
+
+    enum PartitionType type;
+
+    if (strcmp(magic, "MTD") == 0) {
+        type = MTD;
+    } else if (strcmp(magic, "EMMC") == 0) {
+        type = EMMC;
+    } else {
+        printf("LoadPartitionContents called with bad filename (%s)\n", filename);
+        return -1;
+    }
+    const char* partition = strtok(NULL, ":");
+
+    int i;
+    int colons = 0;
+    for (i = 0; filename[i] != '\0'; ++i) {
+        if (filename[i] == ':') {
+            ++colons;
+        }
+    }
+    if (colons < 3 || colons%2 == 0) {
+        printf("LoadPartitionContents called with bad filename (%s)\n",
+               filename);
+    }
+
+    int pairs = (colons-1)/2;     // # of (size,sha1) pairs in filename
+    int* index = reinterpret_cast<int*>(malloc(pairs * sizeof(int)));
+    size_t* size = reinterpret_cast<size_t*>(malloc(pairs * sizeof(size_t)));
+    char** sha1sum = reinterpret_cast<char**>(malloc(pairs * sizeof(char*)));
+
+    for (i = 0; i < pairs; ++i) {
+        const char* size_str = strtok(NULL, ":");
+        size[i] = strtol(size_str, NULL, 10);
+        if (size[i] == 0) {
+            printf("LoadPartitionContents called with bad size (%s)\n", filename);
+            return -1;
+        }
+        sha1sum[i] = strtok(NULL, ":");
+        index[i] = i;
+    }
+
+    // sort the index[] array so it indexes the pairs in order of
+    // increasing size.
+    size_array = size;
+    qsort(index, pairs, sizeof(int), compare_size_indices);
+
+    MtdReadContext* ctx = NULL;
+    FILE* dev = NULL;
+
+    switch (type) {
+        case MTD: {
+            if (!mtd_partitions_scanned) {
+                mtd_scan_partitions();
+                mtd_partitions_scanned = 1;
+            }
+
+            const MtdPartition* mtd = mtd_find_partition_by_name(partition);
+            if (mtd == NULL) {
+                printf("mtd partition \"%s\" not found (loading %s)\n",
+                       partition, filename);
+                return -1;
+            }
+
+            ctx = mtd_read_partition(mtd);
+            if (ctx == NULL) {
+                printf("failed to initialize read of mtd partition \"%s\"\n",
+                       partition);
+                return -1;
+            }
+            break;
+        }
+
+        case EMMC:
+            dev = fopen(partition, "rb");
+            if (dev == NULL) {
+                printf("failed to open emmc partition \"%s\": %s\n",
+                       partition, strerror(errno));
+                return -1;
+            }
+    }
+
+    SHA_CTX sha_ctx;
+    SHA_init(&sha_ctx);
+    uint8_t parsed_sha[SHA_DIGEST_SIZE];
+
+    // allocate enough memory to hold the largest size.
+    file->data = reinterpret_cast<unsigned char*>(malloc(size[index[pairs-1]]));
+    char* p = (char*)file->data;
+    file->size = 0;                // # bytes read so far
+
+    for (i = 0; i < pairs; ++i) {
+        // Read enough additional bytes to get us up to the next size
+        // (again, we're trying the possibilities in order of increasing
+        // size).
+        size_t next = size[index[i]] - file->size;
+        size_t read = 0;
+        if (next > 0) {
+            switch (type) {
+                case MTD:
+                    read = mtd_read_data(ctx, p, next);
+                    break;
+
+                case EMMC:
+                    read = fread(p, 1, next, dev);
+                    break;
+            }
+            if (next != read) {
+                printf("short read (%zu bytes of %zu) for partition \"%s\"\n",
+                       read, next, partition);
+                free(file->data);
+                file->data = NULL;
+                return -1;
+            }
+            SHA_update(&sha_ctx, p, read);
+            file->size += read;
+        }
+
+        // Duplicate the SHA context and finalize the duplicate so we can
+        // check it against this pair's expected hash.
+        SHA_CTX temp_ctx;
+        memcpy(&temp_ctx, &sha_ctx, sizeof(SHA_CTX));
+        const uint8_t* sha_so_far = SHA_final(&temp_ctx);
+
+        if (ParseSha1(sha1sum[index[i]], parsed_sha) != 0) {
+            printf("failed to parse sha1 %s in %s\n", sha1sum[index[i]], filename);
+            free(file->data);
+            file->data = NULL;
+            return -1;
+        }
+
+        if (memcmp(sha_so_far, parsed_sha, SHA_DIGEST_SIZE) == 0) {
+            // we have a match.  stop reading the partition; we'll return
+            // the data we've read so far.
+            printf("partition read matched size %zu sha %s\n",
+                   size[index[i]], sha1sum[index[i]]);
+            break;
+        }
+
+        p += read;
+    }
+
+    switch (type) {
+        case MTD:
+            mtd_read_close(ctx);
+            break;
+
+        case EMMC:
+            fclose(dev);
+            break;
+    }
+
+
+    if (i == pairs) {
+        // Ran off the end of the list of (size,sha1) pairs without
+        // finding a match.
+        printf("contents of partition \"%s\" didn't match %s\n", partition, filename);
+        free(file->data);
+        file->data = NULL;
+        return -1;
+    }
+
+    const uint8_t* sha_final = SHA_final(&sha_ctx);
+    for (size_t i = 0; i < SHA_DIGEST_SIZE; ++i) {
+        file->sha1[i] = sha_final[i];
+    }
+
+    // Fake some stat() info.
+    file->st.st_mode = 0644;
+    file->st.st_uid = 0;
+    file->st.st_gid = 0;
+
+    free(copy);
+    free(index);
+    free(size);
+    free(sha1sum);
+
+    return 0;
+}
+
+
+// Save the contents of the given FileContents object under the given
+// filename.  Return 0 on success.
+int SaveFileContents(const char* filename, const FileContents* file) {
+    int fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC | O_SYNC, S_IRUSR | S_IWUSR);
+    if (fd < 0) {
+        printf("failed to open \"%s\" for write: %s\n", filename, strerror(errno));
+        return -1;
+    }
+
+    ssize_t bytes_written = FileSink(file->data, file->size, &fd);
+    if (bytes_written != file->size) {
+        printf("short write of \"%s\" (%zd bytes of %zd) (%s)\n",
+               filename, bytes_written, file->size, strerror(errno));
+        close(fd);
+        return -1;
+    }
+    if (fsync(fd) != 0) {
+        printf("fsync of \"%s\" failed: %s\n", filename, strerror(errno));
+        return -1;
+    }
+    if (close(fd) != 0) {
+        printf("close of \"%s\" failed: %s\n", filename, strerror(errno));
+        return -1;
+    }
+
+    if (chmod(filename, file->st.st_mode) != 0) {
+        printf("chmod of \"%s\" failed: %s\n", filename, strerror(errno));
+        return -1;
+    }
+    if (chown(filename, file->st.st_uid, file->st.st_gid) != 0) {
+        printf("chown of \"%s\" failed: %s\n", filename, strerror(errno));
+        return -1;
+    }
+
+    return 0;
+}
+
+// Write a memory buffer to 'target' partition, a string of the form
+// "MTD:<partition>[:...]" or "EMMC:<partition_device>:".  Return 0 on
+// success.
+int WriteToPartition(unsigned char* data, size_t len, const char* target) {
+    char* copy = strdup(target);
+    const char* magic = strtok(copy, ":");
+
+    enum PartitionType type;
+    if (strcmp(magic, "MTD") == 0) {
+        type = MTD;
+    } else if (strcmp(magic, "EMMC") == 0) {
+        type = EMMC;
+    } else {
+        printf("WriteToPartition called with bad target (%s)\n", target);
+        return -1;
+    }
+    const char* partition = strtok(NULL, ":");
+
+    if (partition == NULL) {
+        printf("bad partition target name \"%s\"\n", target);
+        return -1;
+    }
+
+    switch (type) {
+        case MTD: {
+            if (!mtd_partitions_scanned) {
+                mtd_scan_partitions();
+                mtd_partitions_scanned = 1;
+            }
+
+            const MtdPartition* mtd = mtd_find_partition_by_name(partition);
+            if (mtd == NULL) {
+                printf("mtd partition \"%s\" not found for writing\n", partition);
+                return -1;
+            }
+
+            MtdWriteContext* ctx = mtd_write_partition(mtd);
+            if (ctx == NULL) {
+                printf("failed to init mtd partition \"%s\" for writing\n", partition);
+                return -1;
+            }
+
+            size_t written = mtd_write_data(ctx, reinterpret_cast<char*>(data), len);
+            if (written != len) {
+                printf("only wrote %zu of %zu bytes to MTD %s\n", written, len, partition);
+                mtd_write_close(ctx);
+                return -1;
+            }
+
+            if (mtd_erase_blocks(ctx, -1) < 0) {
+                printf("error finishing mtd write of %s\n", partition);
+                mtd_write_close(ctx);
+                return -1;
+            }
+
+            if (mtd_write_close(ctx)) {
+                printf("error closing mtd write of %s\n", partition);
+                return -1;
+            }
+            break;
+        }
+
+        case EMMC: {
+            size_t start = 0;
+            bool success = false;
+            int fd = open(partition, O_RDWR | O_SYNC);
+            if (fd < 0) {
+                printf("failed to open %s: %s\n", partition, strerror(errno));
+                return -1;
+            }
+
+            for (int attempt = 0; attempt < 2; ++attempt) {
+                if (TEMP_FAILURE_RETRY(lseek(fd, start, SEEK_SET)) == -1) {
+                    printf("failed seek on %s: %s\n", partition, strerror(errno));
+                    return -1;
+                }
+                while (start < len) {
+                    size_t to_write = len - start;
+                    if (to_write > 1<<20) to_write = 1<<20;
+
+                    ssize_t written = TEMP_FAILURE_RETRY(write(fd, data+start, to_write));
+                    if (written == -1) {
+                        printf("failed write writing to %s: %s\n", partition, strerror(errno));
+                        return -1;
+                    }
+                    start += written;
+                }
+                if (fsync(fd) != 0) {
+                   printf("failed to sync to %s (%s)\n", partition, strerror(errno));
+                   return -1;
+                }
+                if (close(fd) != 0) {
+                   printf("failed to close %s (%s)\n", partition, strerror(errno));
+                   return -1;
+                }
+                fd = open(partition, O_RDONLY);
+                if (fd < 0) {
+                   printf("failed to reopen %s for verify (%s)\n", partition, strerror(errno));
+                   return -1;
+                }
+
+                // Drop caches so our subsequent verification read
+                // won't just be reading the cache.
+                sync();
+                int dc = open("/proc/sys/vm/drop_caches", O_WRONLY);
+                if (TEMP_FAILURE_RETRY(write(dc, "3\n", 2)) == -1) {
+                    printf("write to /proc/sys/vm/drop_caches failed: %s\n", strerror(errno));
+                } else {
+                    printf("  caches dropped\n");
+                }
+                close(dc);
+                sleep(1);
+
+                // verify
+                if (TEMP_FAILURE_RETRY(lseek(fd, 0, SEEK_SET)) == -1) {
+                    printf("failed to seek back to beginning of %s: %s\n",
+                           partition, strerror(errno));
+                    return -1;
+                }
+                unsigned char buffer[4096];
+                start = len;
+                for (size_t p = 0; p < len; p += sizeof(buffer)) {
+                    size_t to_read = len - p;
+                    if (to_read > sizeof(buffer)) {
+                        to_read = sizeof(buffer);
+                    }
+
+                    size_t so_far = 0;
+                    while (so_far < to_read) {
+                        ssize_t read_count =
+                                TEMP_FAILURE_RETRY(read(fd, buffer+so_far, to_read-so_far));
+                        if (read_count == -1) {
+                            printf("verify read error %s at %zu: %s\n",
+                                   partition, p, strerror(errno));
+                            return -1;
+                        }
+                        if (static_cast<size_t>(read_count) < to_read) {
+                            printf("short verify read %s at %zu: %zd %zu %s\n",
+                                   partition, p, read_count, to_read, strerror(errno));
+                        }
+                        so_far += read_count;
+                    }
+
+                    if (memcmp(buffer, data+p, to_read) != 0) {
+                        printf("verification failed starting at %zu\n", p);
+                        start = p;
+                        break;
+                    }
+                }
+
+                if (start == len) {
+                    printf("verification read succeeded (attempt %d)\n", attempt+1);
+                    success = true;
+                    break;
+                }
+            }
+
+            if (!success) {
+                printf("failed to verify after all attempts\n");
+                return -1;
+            }
+
+            if (close(fd) != 0) {
+                printf("error closing %s (%s)\n", partition, strerror(errno));
+                return -1;
+            }
+            sync();
+            break;
+        }
+    }
+
+    free(copy);
+    return 0;
+}
+
+
+// Take a string 'str' of 40 hex digits and parse it into the 20
+// byte array 'digest'.  'str' may contain only the digest or be of
+// the form "<digest>:<anything>".  Return 0 on success, -1 on any
+// error.
+int ParseSha1(const char* str, uint8_t* digest) {
+    const char* ps = str;
+    uint8_t* pd = digest;
+    for (int i = 0; i < SHA_DIGEST_SIZE * 2; ++i, ++ps) {
+        int digit;
+        if (*ps >= '0' && *ps <= '9') {
+            digit = *ps - '0';
+        } else if (*ps >= 'a' && *ps <= 'f') {
+            digit = *ps - 'a' + 10;
+        } else if (*ps >= 'A' && *ps <= 'F') {
+            digit = *ps - 'A' + 10;
+        } else {
+            return -1;
+        }
+        if (i % 2 == 0) {
+            *pd = digit << 4;
+        } else {
+            *pd |= digit;
+            ++pd;
+        }
+    }
+    if (*ps != '\0') return -1;
+    return 0;
+}
+
+// Search an array of sha1 strings for one matching the given sha1.
+// Return the index of the match on success, or -1 if no match is
+// found.
+int FindMatchingPatch(uint8_t* sha1, char* const * const patch_sha1_str,
+                      int num_patches) {
+    uint8_t patch_sha1[SHA_DIGEST_SIZE];
+    for (int i = 0; i < num_patches; ++i) {
+        if (ParseSha1(patch_sha1_str[i], patch_sha1) == 0 &&
+            memcmp(patch_sha1, sha1, SHA_DIGEST_SIZE) == 0) {
+            return i;
+        }
+    }
+    return -1;
+}
+
+// Returns 0 if the contents of the file (argv[2]) or the cached file
+// match any of the sha1's on the command line (argv[3:]).  Returns
+// nonzero otherwise.
+int applypatch_check(const char* filename, int num_patches,
+                     char** const patch_sha1_str) {
+    FileContents file;
+    file.data = NULL;
+
+    // It's okay to specify no sha1s; the check will pass if the
+    // LoadFileContents is successful.  (Useful for reading
+    // partitions, where the filename encodes the sha1s; no need to
+    // check them twice.)
+    if (LoadFileContents(filename, &file) != 0 ||
+        (num_patches > 0 &&
+         FindMatchingPatch(file.sha1, patch_sha1_str, num_patches) < 0)) {
+        printf("file \"%s\" doesn't have any of expected "
+               "sha1 sums; checking cache\n", filename);
+
+        free(file.data);
+        file.data = NULL;
+
+        // If the source file is missing or corrupted, it might be because
+        // we were killed in the middle of patching it.  A copy of it
+        // should have been made in CACHE_TEMP_SOURCE.  If that file
+        // exists and matches the sha1 we're looking for, the check still
+        // passes.
+
+        if (LoadFileContents(CACHE_TEMP_SOURCE, &file) != 0) {
+            printf("failed to load cache file\n");
+            return 1;
+        }
+
+        if (FindMatchingPatch(file.sha1, patch_sha1_str, num_patches) < 0) {
+            printf("cache bits don't match any sha1 for \"%s\"\n", filename);
+            free(file.data);
+            return 1;
+        }
+    }
+
+    free(file.data);
+    return 0;
+}
+
+int ShowLicenses() {
+    ShowBSDiffLicense();
+    return 0;
+}
+
+ssize_t FileSink(const unsigned char* data, ssize_t len, void* token) {
+    int fd = *reinterpret_cast<int *>(token);
+    ssize_t done = 0;
+    ssize_t wrote;
+    while (done < len) {
+        wrote = TEMP_FAILURE_RETRY(write(fd, data+done, len-done));
+        if (wrote == -1) {
+            printf("error writing %zd bytes: %s\n", (len-done), strerror(errno));
+            return done;
+        }
+        done += wrote;
+    }
+    return done;
+}
+
+typedef struct {
+    unsigned char* buffer;
+    ssize_t size;
+    ssize_t pos;
+} MemorySinkInfo;
+
+ssize_t MemorySink(const unsigned char* data, ssize_t len, void* token) {
+    MemorySinkInfo* msi = reinterpret_cast<MemorySinkInfo*>(token);
+    if (msi->size - msi->pos < len) {
+        return -1;
+    }
+    memcpy(msi->buffer + msi->pos, data, len);
+    msi->pos += len;
+    return len;
+}
+
+// Return the amount of free space (in bytes) on the filesystem
+// containing filename.  filename must exist.  Return -1 on error.
+size_t FreeSpaceForFile(const char* filename) {
+    struct statfs sf;
+    if (statfs(filename, &sf) != 0) {
+        printf("failed to statfs %s: %s\n", filename, strerror(errno));
+        return -1;
+    }
+    return sf.f_bsize * sf.f_bavail;
+}
+
+int CacheSizeCheck(size_t bytes) {
+    if (MakeFreeSpaceOnCache(bytes) < 0) {
+        printf("unable to make %ld bytes available on /cache\n", (long)bytes);
+        return 1;
+    } else {
+        return 0;
+    }
+}
+
+static void print_short_sha1(const uint8_t sha1[SHA_DIGEST_SIZE]) {
+    const char* hex = "0123456789abcdef";
+    for (size_t i = 0; i < 4; ++i) {
+        putchar(hex[(sha1[i]>>4) & 0xf]);
+        putchar(hex[sha1[i] & 0xf]);
+    }
+}
+
+// This function applies binary patches to files in a way that is safe
+// (the original file is not touched until we have the desired
+// replacement for it) and idempotent (it's okay to run this program
+// multiple times).
+//
+// - if the sha1 hash of <target_filename> is <target_sha1_string>,
+//   does nothing and exits successfully.
+//
+// - otherwise, if the sha1 hash of <source_filename> is one of the
+//   entries in <patch_sha1_str>, the corresponding patch from
+//   <patch_data> (which must be a VAL_BLOB) is applied to produce a
+//   new file (the type of patch is automatically detected from the
+//   blob daat).  If that new file has sha1 hash <target_sha1_str>,
+//   moves it to replace <target_filename>, and exits successfully.
+//   Note that if <source_filename> and <target_filename> are not the
+//   same, <source_filename> is NOT deleted on success.
+//   <target_filename> may be the string "-" to mean "the same as
+//   source_filename".
+//
+// - otherwise, or if any error is encountered, exits with non-zero
+//   status.
+//
+// <source_filename> may refer to a partition to read the source data.
+// See the comments for the LoadPartition Contents() function above
+// for the format of such a filename.
+
+int applypatch(const char* source_filename,
+               const char* target_filename,
+               const char* target_sha1_str,
+               size_t target_size,
+               int num_patches,
+               char** const patch_sha1_str,
+               Value** patch_data,
+               Value* bonus_data) {
+    printf("patch %s: ", source_filename);
+
+    if (target_filename[0] == '-' && target_filename[1] == '\0') {
+        target_filename = source_filename;
+    }
+
+    uint8_t target_sha1[SHA_DIGEST_SIZE];
+    if (ParseSha1(target_sha1_str, target_sha1) != 0) {
+        printf("failed to parse tgt-sha1 \"%s\"\n", target_sha1_str);
+        return 1;
+    }
+
+    FileContents copy_file;
+    FileContents source_file;
+    copy_file.data = NULL;
+    source_file.data = NULL;
+    const Value* source_patch_value = NULL;
+    const Value* copy_patch_value = NULL;
+
+    // We try to load the target file into the source_file object.
+    if (LoadFileContents(target_filename, &source_file) == 0) {
+        if (memcmp(source_file.sha1, target_sha1, SHA_DIGEST_SIZE) == 0) {
+            // The early-exit case:  the patch was already applied, this file
+            // has the desired hash, nothing for us to do.
+            printf("already ");
+            print_short_sha1(target_sha1);
+            putchar('\n');
+            free(source_file.data);
+            return 0;
+        }
+    }
+
+    if (source_file.data == NULL ||
+        (target_filename != source_filename &&
+         strcmp(target_filename, source_filename) != 0)) {
+        // Need to load the source file:  either we failed to load the
+        // target file, or we did but it's different from the source file.
+        free(source_file.data);
+        source_file.data = NULL;
+        LoadFileContents(source_filename, &source_file);
+    }
+
+    if (source_file.data != NULL) {
+        int to_use = FindMatchingPatch(source_file.sha1, patch_sha1_str, num_patches);
+        if (to_use >= 0) {
+            source_patch_value = patch_data[to_use];
+        }
+    }
+
+    if (source_patch_value == NULL) {
+        free(source_file.data);
+        source_file.data = NULL;
+        printf("source file is bad; trying copy\n");
+
+        if (LoadFileContents(CACHE_TEMP_SOURCE, &copy_file) < 0) {
+            // fail.
+            printf("failed to read copy file\n");
+            return 1;
+        }
+
+        int to_use = FindMatchingPatch(copy_file.sha1, patch_sha1_str, num_patches);
+        if (to_use >= 0) {
+            copy_patch_value = patch_data[to_use];
+        }
+
+        if (copy_patch_value == NULL) {
+            // fail.
+            printf("copy file doesn't match source SHA-1s either\n");
+            free(copy_file.data);
+            return 1;
+        }
+    }
+
+    int result = GenerateTarget(&source_file, source_patch_value,
+                                &copy_file, copy_patch_value,
+                                source_filename, target_filename,
+                                target_sha1, target_size, bonus_data);
+    free(source_file.data);
+    free(copy_file.data);
+
+    return result;
+}
+
+static int GenerateTarget(FileContents* source_file,
+                          const Value* source_patch_value,
+                          FileContents* copy_file,
+                          const Value* copy_patch_value,
+                          const char* source_filename,
+                          const char* target_filename,
+                          const uint8_t target_sha1[SHA_DIGEST_SIZE],
+                          size_t target_size,
+                          const Value* bonus_data) {
+    int retry = 1;
+    SHA_CTX ctx;
+    int output;
+    MemorySinkInfo msi;
+    FileContents* source_to_use;
+    char* outname;
+    int made_copy = 0;
+
+    // assume that target_filename (eg "/system/app/Foo.apk") is located
+    // on the same filesystem as its top-level directory ("/system").
+    // We need something that exists for calling statfs().
+    char target_fs[strlen(target_filename)+1];
+    char* slash = strchr(target_filename+1, '/');
+    if (slash != NULL) {
+        int count = slash - target_filename;
+        strncpy(target_fs, target_filename, count);
+        target_fs[count] = '\0';
+    } else {
+        strcpy(target_fs, target_filename);
+    }
+
+    do {
+        // Is there enough room in the target filesystem to hold the patched
+        // file?
+
+        if (strncmp(target_filename, "MTD:", 4) == 0 ||
+            strncmp(target_filename, "EMMC:", 5) == 0) {
+            // If the target is a partition, we're actually going to
+            // write the output to /tmp and then copy it to the
+            // partition.  statfs() always returns 0 blocks free for
+            // /tmp, so instead we'll just assume that /tmp has enough
+            // space to hold the file.
+
+            // We still write the original source to cache, in case
+            // the partition write is interrupted.
+            if (MakeFreeSpaceOnCache(source_file->size) < 0) {
+                printf("not enough free space on /cache\n");
+                return 1;
+            }
+            if (SaveFileContents(CACHE_TEMP_SOURCE, source_file) < 0) {
+                printf("failed to back up source file\n");
+                return 1;
+            }
+            made_copy = 1;
+            retry = 0;
+        } else {
+            int enough_space = 0;
+            if (retry > 0) {
+                size_t free_space = FreeSpaceForFile(target_fs);
+                enough_space =
+                    (free_space > (256 << 10)) &&          // 256k (two-block) minimum
+                    (free_space > (target_size * 3 / 2));  // 50% margin of error
+                if (!enough_space) {
+                    printf("target %zu bytes; free space %zu bytes; retry %d; enough %d\n",
+                           target_size, free_space, retry, enough_space);
+                }
+            }
+
+            if (!enough_space) {
+                retry = 0;
+            }
+
+            if (!enough_space && source_patch_value != NULL) {
+                // Using the original source, but not enough free space.  First
+                // copy the source file to cache, then delete it from the original
+                // location.
+
+                if (strncmp(source_filename, "MTD:", 4) == 0 ||
+                    strncmp(source_filename, "EMMC:", 5) == 0) {
+                    // It's impossible to free space on the target filesystem by
+                    // deleting the source if the source is a partition.  If
+                    // we're ever in a state where we need to do this, fail.
+                    printf("not enough free space for target but source is partition\n");
+                    return 1;
+                }
+
+                if (MakeFreeSpaceOnCache(source_file->size) < 0) {
+                    printf("not enough free space on /cache\n");
+                    return 1;
+                }
+
+                if (SaveFileContents(CACHE_TEMP_SOURCE, source_file) < 0) {
+                    printf("failed to back up source file\n");
+                    return 1;
+                }
+                made_copy = 1;
+                unlink(source_filename);
+
+                size_t free_space = FreeSpaceForFile(target_fs);
+                printf("(now %zu bytes free for target) ", free_space);
+            }
+        }
+
+        const Value* patch;
+        if (source_patch_value != NULL) {
+            source_to_use = source_file;
+            patch = source_patch_value;
+        } else {
+            source_to_use = copy_file;
+            patch = copy_patch_value;
+        }
+
+        if (patch->type != VAL_BLOB) {
+            printf("patch is not a blob\n");
+            return 1;
+        }
+
+        SinkFn sink = NULL;
+        void* token = NULL;
+        output = -1;
+        outname = NULL;
+        if (strncmp(target_filename, "MTD:", 4) == 0 ||
+            strncmp(target_filename, "EMMC:", 5) == 0) {
+            // We store the decoded output in memory.
+            msi.buffer = reinterpret_cast<unsigned char*>(malloc(target_size));
+            if (msi.buffer == NULL) {
+                printf("failed to alloc %zu bytes for output\n", target_size);
+                return 1;
+            }
+            msi.pos = 0;
+            msi.size = target_size;
+            sink = MemorySink;
+            token = &msi;
+        } else {
+            // We write the decoded output to "<tgt-file>.patch".
+            outname = reinterpret_cast<char*>(malloc(strlen(target_filename) + 10));
+            strcpy(outname, target_filename);
+            strcat(outname, ".patch");
+
+            output = open(outname, O_WRONLY | O_CREAT | O_TRUNC | O_SYNC, S_IRUSR | S_IWUSR);
+            if (output < 0) {
+                printf("failed to open output file %s: %s\n",
+                       outname, strerror(errno));
+                return 1;
+            }
+            sink = FileSink;
+            token = &output;
+        }
+
+        char* header = patch->data;
+        ssize_t header_bytes_read = patch->size;
+
+        SHA_init(&ctx);
+
+        int result;
+
+        if (header_bytes_read >= 8 &&
+            memcmp(header, "BSDIFF40", 8) == 0) {
+            result = ApplyBSDiffPatch(source_to_use->data, source_to_use->size,
+                                      patch, 0, sink, token, &ctx);
+        } else if (header_bytes_read >= 8 &&
+                   memcmp(header, "IMGDIFF2", 8) == 0) {
+            result = ApplyImagePatch(source_to_use->data, source_to_use->size,
+                                     patch, sink, token, &ctx, bonus_data);
+        } else {
+            printf("Unknown patch file format\n");
+            return 1;
+        }
+
+        if (output >= 0) {
+            if (fsync(output) != 0) {
+                printf("failed to fsync file \"%s\" (%s)\n", outname, strerror(errno));
+                result = 1;
+            }
+            if (close(output) != 0) {
+                printf("failed to close file \"%s\" (%s)\n", outname, strerror(errno));
+                result = 1;
+            }
+        }
+
+        if (result != 0) {
+            if (retry == 0) {
+                printf("applying patch failed\n");
+                return result != 0;
+            } else {
+                printf("applying patch failed; retrying\n");
+            }
+            if (outname != NULL) {
+                unlink(outname);
+            }
+        } else {
+            // succeeded; no need to retry
+            break;
+        }
+    } while (retry-- > 0);
+
+    const uint8_t* current_target_sha1 = SHA_final(&ctx);
+    if (memcmp(current_target_sha1, target_sha1, SHA_DIGEST_SIZE) != 0) {
+        printf("patch did not produce expected sha1\n");
+        return 1;
+    } else {
+        printf("now ");
+        print_short_sha1(target_sha1);
+        putchar('\n');
+    }
+
+    if (output < 0) {
+        // Copy the temp file to the partition.
+        if (WriteToPartition(msi.buffer, msi.pos, target_filename) != 0) {
+            printf("write of patched data to %s failed\n", target_filename);
+            return 1;
+        }
+        free(msi.buffer);
+    } else {
+        // Give the .patch file the same owner, group, and mode of the
+        // original source file.
+        if (chmod(outname, source_to_use->st.st_mode) != 0) {
+            printf("chmod of \"%s\" failed: %s\n", outname, strerror(errno));
+            return 1;
+        }
+        if (chown(outname, source_to_use->st.st_uid, source_to_use->st.st_gid) != 0) {
+            printf("chown of \"%s\" failed: %s\n", outname, strerror(errno));
+            return 1;
+        }
+
+        // Finally, rename the .patch file to replace the target file.
+        if (rename(outname, target_filename) != 0) {
+            printf("rename of .patch to \"%s\" failed: %s\n", target_filename, strerror(errno));
+            return 1;
+        }
+    }
+
+    // If this run of applypatch created the copy, and we're here, we
+    // can delete it.
+    if (made_copy) {
+        unlink(CACHE_TEMP_SOURCE);
+    }
+
+    // Success!
+    return 0;
+}