diff --git a/applypatch/applypatch.cpp b/applypatch/applypatch.cpp
index bc8ea79..f9383dd 100644
--- a/applypatch/applypatch.cpp
+++ b/applypatch/applypatch.cpp
@@ -44,194 +44,130 @@
 #include "otautil/paths.h"
 #include "otautil/print_sha1.h"
 
-static int LoadPartitionContents(const std::string& filename, FileContents* file);
-static int GenerateTarget(const FileContents& source_file, const std::unique_ptr<Value>& patch,
-                          const std::string& target_filename,
-                          const uint8_t target_sha1[SHA_DIGEST_LENGTH], const Value* bonus_data);
+using namespace std::string_literals;
 
-int LoadFileContents(const std::string& filename, FileContents* file) {
-  // A special 'filename' beginning with "EMMC:" means to load the contents of a partition.
+static bool GenerateTarget(const Partition& target, const FileContents& source_file,
+                           const Value& patch, const Value* bonus_data);
+
+bool LoadFileContents(const std::string& filename, FileContents* file) {
+  // No longer allow loading contents from eMMC partitions.
   if (android::base::StartsWith(filename, "EMMC:")) {
-    return LoadPartitionContents(filename, file);
+    return false;
   }
 
   std::string data;
   if (!android::base::ReadFileToString(filename, &data)) {
     PLOG(ERROR) << "Failed to read \"" << filename << "\"";
-    return -1;
+    return false;
   }
 
   file->data = std::vector<unsigned char>(data.begin(), data.end());
   SHA1(file->data.data(), file->data.size(), file->sha1);
-  return 0;
+  return true;
 }
 
-// Loads the contents of an EMMC partition into the provided FileContents. filename should be a
-// string of the form "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. Returns 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.
-static int LoadPartitionContents(const std::string& filename, FileContents* file) {
-  std::vector<std::string> pieces = android::base::Split(filename, ":");
-  if (pieces.size() < 4 || pieces.size() % 2 != 0 || pieces[0] != "EMMC") {
-    LOG(ERROR) << "LoadPartitionContents called with bad filename \"" << filename << "\"";
-    return -1;
+// Reads the contents of a Partition to the given FileContents buffer.
+static bool ReadPartitionToBuffer(const Partition& partition, FileContents* out,
+                                  bool check_backup) {
+  uint8_t expected_sha1[SHA_DIGEST_LENGTH];
+  if (ParseSha1(partition.hash, expected_sha1) != 0) {
+    LOG(ERROR) << "Failed to parse target hash \"" << partition.hash << "\"";
+    return false;
   }
 
-  size_t pair_count = (pieces.size() - 2) / 2;  // # of (size, sha1) pairs in filename
-  std::vector<std::pair<size_t, std::string>> pairs;
-  for (size_t i = 0; i < pair_count; ++i) {
-    size_t size;
-    if (!android::base::ParseUint(pieces[i * 2 + 2], &size) || size == 0) {
-      LOG(ERROR) << "LoadPartitionContents called with bad size \"" << pieces[i * 2 + 2] << "\"";
-      return -1;
-    }
-    pairs.push_back({ size, pieces[i * 2 + 3] });
-  }
-
-  // Sort the pairs array so that they are in order of increasing size.
-  std::sort(pairs.begin(), pairs.end());
-
-  const char* partition = pieces[1].c_str();
-  android::base::unique_fd dev(open(partition, O_RDONLY));
-  if (dev == -1) {
+  android::base::unique_fd dev(open(partition.name.c_str(), O_RDONLY));
+  if (!dev) {
     PLOG(ERROR) << "Failed to open eMMC partition \"" << partition << "\"";
-    return -1;
-  }
-
-  SHA_CTX sha_ctx;
-  SHA1_Init(&sha_ctx);
-
-  // Allocate enough memory to hold the largest size.
-  std::vector<unsigned char> buffer(pairs[pair_count - 1].first);
-  size_t offset = 0;  // # bytes read so far
-  bool found = false;
-
-  for (const auto& pair : pairs) {
-    size_t current_size = pair.first;
-    const std::string& current_sha1 = pair.second;
-
-    // Read enough additional bytes to get us up to the next size. (Again,
-    // we're trying the possibilities in order of increasing size).
-    if (current_size - offset > 0) {
-      if (!android::base::ReadFully(dev, buffer.data() + offset, current_size - offset)) {
-        PLOG(ERROR) << "Failed to read " << current_size - offset << " bytes of data at offset "
-                    << offset << " for partition " << partition;
-        return -1;
+  } else {
+    std::vector<unsigned char> buffer(partition.size);
+    if (!android::base::ReadFully(dev, buffer.data(), buffer.size())) {
+      PLOG(ERROR) << "Failed to read " << buffer.size() << " bytes of data for partition "
+                  << partition;
+    } else {
+      SHA1(buffer.data(), buffer.size(), out->sha1);
+      if (memcmp(out->sha1, expected_sha1, SHA_DIGEST_LENGTH) == 0) {
+        out->data = std::move(buffer);
+        return true;
       }
-
-      SHA1_Update(&sha_ctx, buffer.data() + offset, current_size - offset);
-      offset = current_size;
-    }
-    // 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));
-    uint8_t sha_so_far[SHA_DIGEST_LENGTH];
-    SHA1_Final(sha_so_far, &temp_ctx);
-
-    uint8_t parsed_sha[SHA_DIGEST_LENGTH];
-    if (ParseSha1(current_sha1, parsed_sha) != 0) {
-      LOG(ERROR) << "Failed to parse SHA-1 \"" << current_sha1 << "\" in " << filename;
-      return -1;
-    }
-
-    if (memcmp(sha_so_far, parsed_sha, SHA_DIGEST_LENGTH) == 0) {
-      // We have a match. Stop reading the partition; we'll return the data we've read so far.
-      LOG(INFO) << "Partition read matched size " << current_size << " SHA-1 " << current_sha1;
-      found = true;
-      break;
     }
   }
 
-  if (!found) {
-    // Ran off the end of the list of (size, sha1) pairs without finding a match.
-    LOG(ERROR) << "Contents of partition \"" << partition << "\" didn't match " << filename;
-    return -1;
+  if (!check_backup) {
+    LOG(ERROR) << "Partition contents don't have the expected checksum";
+    return false;
   }
 
-  SHA1_Final(file->sha1, &sha_ctx);
+  if (LoadFileContents(Paths::Get().cache_temp_source(), out) &&
+      memcmp(out->sha1, expected_sha1, SHA_DIGEST_LENGTH) == 0) {
+    return true;
+  }
 
-  buffer.resize(offset);
-  file->data = std::move(buffer);
-
-  return 0;
+  LOG(ERROR) << "Both of partition contents and backup don't have the expected checksum";
+  return false;
 }
 
-int SaveFileContents(const std::string& filename, const FileContents* file) {
+bool SaveFileContents(const std::string& filename, const FileContents* file) {
   android::base::unique_fd fd(
       open(filename.c_str(), O_WRONLY | O_CREAT | O_TRUNC | O_SYNC, S_IRUSR | S_IWUSR));
   if (fd == -1) {
     PLOG(ERROR) << "Failed to open \"" << filename << "\" for write";
-    return -1;
+    return false;
   }
 
   if (!android::base::WriteFully(fd, file->data.data(), file->data.size())) {
     PLOG(ERROR) << "Failed to write " << file->data.size() << " bytes of data to " << filename;
-    return -1;
+    return false;
   }
 
   if (fsync(fd) != 0) {
     PLOG(ERROR) << "Failed to fsync \"" << filename << "\"";
-    return -1;
+    return false;
   }
 
   if (close(fd.release()) != 0) {
     PLOG(ERROR) << "Failed to close \"" << filename << "\"";
-    return -1;
+    return false;
   }
 
-  return 0;
+  return true;
 }
 
-// Writes a memory buffer to 'target' partition, a string of the form
-// "EMMC:<partition_device>[:...]". The target name might contain multiple colons, but
-// WriteToPartition() only uses the first two and ignores the rest. Returns 0 on success.
-static int WriteToPartition(const unsigned char* data, size_t len, const std::string& target) {
-  std::vector<std::string> pieces = android::base::Split(target, ":");
-  if (pieces.size() < 2 || pieces[0] != "EMMC") {
-    LOG(ERROR) << "WriteToPartition called with bad target \"" << target << "\"";
-    return -1;
-  }
-
+// Writes a memory buffer to 'target' Partition.
+static bool WriteBufferToPartition(const FileContents& file_contents, const Partition& partition) {
+  const unsigned char* data = file_contents.data.data();
+  size_t len = file_contents.data.size();
   size_t start = 0;
   bool success = false;
   for (size_t attempt = 0; attempt < 2; ++attempt) {
-    std::string partition = pieces[1];
-    android::base::unique_fd fd(open(partition.c_str(), O_RDWR));
+    android::base::unique_fd fd(open(partition.name.c_str(), O_RDWR));
     if (fd == -1) {
       PLOG(ERROR) << "Failed to open \"" << partition << "\"";
-      return -1;
+      return false;
     }
 
     if (TEMP_FAILURE_RETRY(lseek(fd, start, SEEK_SET)) == -1) {
       PLOG(ERROR) << "Failed to seek to " << start << " on \"" << partition << "\"";
-      return -1;
+      return false;
     }
 
     if (!android::base::WriteFully(fd, data + start, len - start)) {
       PLOG(ERROR) << "Failed to write " << len - start << " bytes to \"" << partition << "\"";
-      return -1;
+      return false;
     }
 
     if (fsync(fd) != 0) {
       PLOG(ERROR) << "Failed to sync \"" << partition << "\"";
-      return -1;
+      return false;
     }
     if (close(fd.release()) != 0) {
       PLOG(ERROR) << "Failed to close \"" << partition << "\"";
-      return -1;
+      return false;
     }
 
-    fd.reset(open(partition.c_str(), O_RDONLY));
+    fd.reset(open(partition.name.c_str(), O_RDONLY));
     if (fd == -1) {
       PLOG(ERROR) << "Failed to reopen \"" << partition << "\" for verification";
-      return -1;
+      return false;
     }
 
     // Drop caches so our subsequent verification read won't just be reading the cache.
@@ -247,7 +183,7 @@
     // Verify.
     if (TEMP_FAILURE_RETRY(lseek(fd, 0, SEEK_SET)) == -1) {
       PLOG(ERROR) << "Failed to seek to 0 on " << partition;
-      return -1;
+      return false;
     }
 
     unsigned char buffer[4096];
@@ -260,7 +196,7 @@
 
       if (!android::base::ReadFully(fd, buffer, to_read)) {
         PLOG(ERROR) << "Failed to verify-read " << partition << " at " << p;
-        return -1;
+        return false;
       }
 
       if (memcmp(buffer, data + p, to_read) != 0) {
@@ -278,18 +214,18 @@
 
     if (close(fd.release()) != 0) {
       PLOG(ERROR) << "Failed to close " << partition;
-      return -1;
+      return false;
     }
   }
 
   if (!success) {
     LOG(ERROR) << "Failed to verify after all attempts";
-    return -1;
+    return false;
   }
 
   sync();
 
-  return 0;
+  return true;
 }
 
 int ParseSha1(const std::string& str, uint8_t* digest) {
@@ -317,44 +253,11 @@
   return 0;
 }
 
-// Searches a vector of SHA-1 strings for one matching the given SHA-1. Returns the index of the
-// match on success, or -1 if no match is found.
-static int FindMatchingPatch(const uint8_t* sha1, const std::vector<std::string>& patch_sha1s) {
-  for (size_t i = 0; i < patch_sha1s.size(); ++i) {
-    uint8_t patch_sha1[SHA_DIGEST_LENGTH];
-    if (ParseSha1(patch_sha1s[i], patch_sha1) == 0 &&
-        memcmp(patch_sha1, sha1, SHA_DIGEST_LENGTH) == 0) {
-      return i;
-    }
-  }
-  return -1;
-}
-
-int applypatch_check(const std::string& filename, const std::vector<std::string>& sha1s) {
-  if (!android::base::StartsWith(filename, "EMMC:")) {
-    return 1;
-  }
-
-  // The check will pass if LoadPartitionContents is successful, because the filename already
-  // encodes the desired SHA-1s.
-  FileContents file;
-  if (LoadPartitionContents(filename, &file) != 0) {
-    LOG(INFO) << "\"" << filename << "\" doesn't have any of expected SHA-1 sums; checking cache";
-
-    // If the partition is corrupted, it might be because we were killed in the middle of patching
-    // it. A copy should have been made in cache_temp_source. If that file exists and matches the
-    // SHA-1 we're looking for, the check still passes.
-    if (LoadFileContents(Paths::Get().cache_temp_source(), &file) != 0) {
-      LOG(ERROR) << "Failed to load cache file";
-      return 1;
-    }
-
-    if (FindMatchingPatch(file.sha1, sha1s) < 0) {
-      LOG(ERROR) << "The cache bits don't match any SHA-1 for \"" << filename << "\"";
-      return 1;
-    }
-  }
-  return 0;
+bool PatchPartitionCheck(const Partition& target, const Partition& source) {
+  FileContents target_file;
+  FileContents source_file;
+  return (ReadPartitionToBuffer(target, &target_file, false) ||
+          ReadPartitionToBuffer(source, &source_file, true));
 }
 
 int ShowLicenses() {
@@ -362,124 +265,81 @@
   return 0;
 }
 
-int applypatch(const char* source_filename, const char* target_filename,
-               const char* target_sha1_str, size_t /* target_size */,
-               const std::vector<std::string>& patch_sha1s,
-               const std::vector<std::unique_ptr<Value>>& patch_data, const Value* bonus_data) {
-  LOG(INFO) << "Patching " << source_filename;
+bool PatchPartition(const Partition& target, const Partition& source, const Value& patch,
+                    const Value* bonus) {
+  LOG(INFO) << "Patching " << target.name;
 
-  if (target_filename[0] == '-' && target_filename[1] == '\0') {
-    target_filename = source_filename;
+  // We try to load and check against the target hash first.
+  FileContents target_file;
+  if (ReadPartitionToBuffer(target, &target_file, false)) {
+    // The early-exit case: the patch was already applied, this file has the desired hash, nothing
+    // for us to do.
+    LOG(INFO) << "  already " << target.hash.substr(0, 8);
+    return true;
   }
 
-  if (strncmp(target_filename, "EMMC:", 5) != 0) {
-    LOG(ERROR) << "Supporting patching EMMC targets only";
-    return 1;
-  }
-
-  uint8_t target_sha1[SHA_DIGEST_LENGTH];
-  if (ParseSha1(target_sha1_str, target_sha1) != 0) {
-    LOG(ERROR) << "Failed to parse target SHA-1 \"" << target_sha1_str << "\"";
-    return 1;
-  }
-
-  // We try to load the target file into the source_file object.
   FileContents source_file;
-  if (LoadFileContents(target_filename, &source_file) == 0) {
-    if (memcmp(source_file.sha1, target_sha1, SHA_DIGEST_LENGTH) == 0) {
-      // The early-exit case: the patch was already applied, this file has the desired hash, nothing
-      // for us to do.
-      LOG(INFO) << "  already " << short_sha1(target_sha1);
-      return 0;
-    }
+  if (ReadPartitionToBuffer(source, &source_file, true)) {
+    return GenerateTarget(target, source_file, patch, bonus);
   }
 
-  if (source_file.data.empty() ||
-      (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 expected.
-    source_file.data.clear();
-    LoadFileContents(source_filename, &source_file);
-  }
-
-  if (!source_file.data.empty()) {
-    int to_use = FindMatchingPatch(source_file.sha1, patch_sha1s);
-    if (to_use != -1) {
-      return GenerateTarget(source_file, patch_data[to_use], target_filename, target_sha1,
-                            bonus_data);
-    }
-  }
-
-  LOG(INFO) << "Source file is bad; trying copy";
-
-  FileContents copy_file;
-  if (LoadFileContents(Paths::Get().cache_temp_source(), &copy_file) < 0) {
-    LOG(ERROR) << "Failed to read copy file";
-    return 1;
-  }
-
-  int to_use = FindMatchingPatch(copy_file.sha1, patch_sha1s);
-  if (to_use == -1) {
-    LOG(ERROR) << "The copy on /cache doesn't match source SHA-1s either";
-    return 1;
-  }
-
-  return GenerateTarget(copy_file, patch_data[to_use], target_filename, target_sha1, bonus_data);
+  LOG(ERROR) << "Failed to find any match";
+  return false;
 }
 
-int applypatch_flash(const char* source_filename, const char* target_filename,
-                     const char* target_sha1_str, size_t target_size) {
-  LOG(INFO) << "Flashing " << target_filename;
+bool FlashPartition(const Partition& partition, const std::string& source_filename) {
+  LOG(INFO) << "Flashing " << partition;
 
-  uint8_t target_sha1[SHA_DIGEST_LENGTH];
-  if (ParseSha1(target_sha1_str, target_sha1) != 0) {
-    LOG(ERROR) << "Failed to parse target SHA-1 \"" << target_sha1_str << "\"";
-    return 1;
+  // We try to load and check against the target hash first.
+  FileContents target_file;
+  if (ReadPartitionToBuffer(partition, &target_file, false)) {
+    // The early-exit case: the patch was already applied, this file has the desired hash, nothing
+    // for us to do.
+    LOG(INFO) << "  already " << partition.hash.substr(0, 8);
+    return true;
   }
 
-  std::vector<std::string> pieces = android::base::Split(target_filename, ":");
-  if (pieces.size() != 4 || pieces[0] != "EMMC") {
-    LOG(ERROR) << "Invalid target name \"" << target_filename << "\"";
-    return 1;
-  }
-
-  // Load the target into the source_file object to see if already applied.
   FileContents source_file;
-  if (LoadPartitionContents(target_filename, &source_file) == 0 &&
-      memcmp(source_file.sha1, target_sha1, SHA_DIGEST_LENGTH) == 0) {
-    // The early-exit case: the image was already applied, this partition has the desired hash,
-    // nothing for us to do.
-    LOG(INFO) << "  already " << short_sha1(target_sha1);
-    return 0;
+  if (!LoadFileContents(source_filename, &source_file)) {
+    LOG(ERROR) << "Failed to load source file";
+    return false;
   }
 
-  if (LoadFileContents(source_filename, &source_file) == 0) {
-    if (memcmp(source_file.sha1, target_sha1, SHA_DIGEST_LENGTH) != 0) {
-      // The source doesn't have desired checksum.
-      LOG(ERROR) << "source \"" << source_filename << "\" doesn't have expected SHA-1 sum";
-      LOG(ERROR) << "expected: " << short_sha1(target_sha1)
-                 << ", found: " << short_sha1(source_file.sha1);
-      return 1;
-    }
+  uint8_t expected_sha1[SHA_DIGEST_LENGTH];
+  if (ParseSha1(partition.hash, expected_sha1) != 0) {
+    LOG(ERROR) << "Failed to parse source hash \"" << partition.hash << "\"";
+    return false;
   }
 
-  if (WriteToPartition(source_file.data.data(), target_size, target_filename) != 0) {
-    LOG(ERROR) << "Failed to write copied data to " << target_filename;
-    return 1;
+  if (memcmp(source_file.sha1, expected_sha1, SHA_DIGEST_LENGTH) != 0) {
+    // The source doesn't have desired checksum.
+    LOG(ERROR) << "source \"" << source_filename << "\" doesn't have expected SHA-1 sum";
+    LOG(ERROR) << "expected: " << partition.hash.substr(0, 8)
+               << ", found: " << short_sha1(source_file.sha1);
+    return false;
   }
-  return 0;
+  if (!WriteBufferToPartition(source_file, partition)) {
+    LOG(ERROR) << "Failed to write to " << partition;
+    return false;
+  }
+  return true;
 }
 
-static int GenerateTarget(const FileContents& source_file, const std::unique_ptr<Value>& patch,
-                          const std::string& target_filename,
-                          const uint8_t target_sha1[SHA_DIGEST_LENGTH], const Value* bonus_data) {
-  if (patch->type != Value::Type::BLOB) {
+static bool GenerateTarget(const Partition& target, const FileContents& source_file,
+                           const Value& patch, const Value* bonus_data) {
+  uint8_t expected_sha1[SHA_DIGEST_LENGTH];
+  if (ParseSha1(target.hash, expected_sha1) != 0) {
+    LOG(ERROR) << "Failed to parse target hash \"" << target.hash << "\"";
+    return false;
+  }
+
+  if (patch.type != Value::Type::BLOB) {
     LOG(ERROR) << "patch is not a blob";
-    return 1;
+    return false;
   }
 
-  const char* header = &patch->data[0];
-  size_t header_bytes_read = patch->data.size();
+  const char* header = patch.data.data();
+  size_t header_bytes_read = patch.data.size();
   bool use_bsdiff = false;
   if (header_bytes_read >= 8 && memcmp(header, "BSDIFF40", 8) == 0) {
     use_bsdiff = true;
@@ -487,57 +347,53 @@
     use_bsdiff = false;
   } else {
     LOG(ERROR) << "Unknown patch file format";
-    return 1;
+    return false;
   }
 
-  CHECK(android::base::StartsWith(target_filename, "EMMC:"));
-
   // We write the original source to cache, in case the partition write is interrupted.
   if (!CheckAndFreeSpaceOnCache(source_file.data.size())) {
     LOG(ERROR) << "Not enough free space on /cache";
-    return 1;
+    return false;
   }
-  if (SaveFileContents(Paths::Get().cache_temp_source(), &source_file) < 0) {
+  if (!SaveFileContents(Paths::Get().cache_temp_source(), &source_file)) {
     LOG(ERROR) << "Failed to back up source file";
-    return 1;
+    return false;
   }
 
   // We store the decoded output in memory.
-  std::string memory_sink_str;  // Don't need to reserve space.
+  FileContents patched;
   SHA_CTX ctx;
   SHA1_Init(&ctx);
-  SinkFn sink = [&memory_sink_str, &ctx](const unsigned char* data, size_t len) {
+  SinkFn sink = [&patched, &ctx](const unsigned char* data, size_t len) {
     SHA1_Update(&ctx, data, len);
-    memory_sink_str.append(reinterpret_cast<const char*>(data), len);
+    patched.data.insert(patched.data.end(), data, data + len);
     return len;
   };
 
   int result;
   if (use_bsdiff) {
-    result = ApplyBSDiffPatch(source_file.data.data(), source_file.data.size(), *patch, 0, sink);
+    result = ApplyBSDiffPatch(source_file.data.data(), source_file.data.size(), patch, 0, sink);
   } else {
     result =
-        ApplyImagePatch(source_file.data.data(), source_file.data.size(), *patch, sink, bonus_data);
+        ApplyImagePatch(source_file.data.data(), source_file.data.size(), patch, sink, bonus_data);
   }
 
   if (result != 0) {
     LOG(ERROR) << "Failed to apply the patch: " << result;
-    return 1;
+    return false;
   }
 
-  uint8_t current_target_sha1[SHA_DIGEST_LENGTH];
-  SHA1_Final(current_target_sha1, &ctx);
-  if (memcmp(current_target_sha1, target_sha1, SHA_DIGEST_LENGTH) != 0) {
-    LOG(ERROR) << "Patching did not produce the expected SHA-1 of " << short_sha1(target_sha1);
+  SHA1_Final(patched.sha1, &ctx);
+  if (memcmp(patched.sha1, expected_sha1, SHA_DIGEST_LENGTH) != 0) {
+    LOG(ERROR) << "Patching did not produce the expected SHA-1 of " << short_sha1(expected_sha1);
 
-    LOG(ERROR) << "target size " << memory_sink_str.size() << " SHA-1 "
-               << short_sha1(current_target_sha1);
+    LOG(ERROR) << "target size " << patched.data.size() << " SHA-1 " << short_sha1(patched.sha1);
     LOG(ERROR) << "source size " << source_file.data.size() << " SHA-1 "
                << short_sha1(source_file.sha1);
 
     uint8_t patch_digest[SHA_DIGEST_LENGTH];
-    SHA1(reinterpret_cast<const uint8_t*>(patch->data.data()), patch->data.size(), patch_digest);
-    LOG(ERROR) << "patch size " << patch->data.size() << " SHA-1 " << short_sha1(patch_digest);
+    SHA1(reinterpret_cast<const uint8_t*>(patch.data.data()), patch.data.size(), patch_digest);
+    LOG(ERROR) << "patch size " << patch.data.size() << " SHA-1 " << short_sha1(patch_digest);
 
     if (bonus_data != nullptr) {
       uint8_t bonus_digest[SHA_DIGEST_LENGTH];
@@ -547,21 +403,53 @@
                  << short_sha1(bonus_digest);
     }
 
-    return 1;
-  } else {
-    LOG(INFO) << "  now " << short_sha1(target_sha1);
+    return false;
   }
 
+  LOG(INFO) << "  now " << short_sha1(expected_sha1);
+
   // Write back the temp file to the partition.
-  if (WriteToPartition(reinterpret_cast<const unsigned char*>(memory_sink_str.c_str()),
-                       memory_sink_str.size(), target_filename) != 0) {
-    LOG(ERROR) << "Failed to write patched data to " << target_filename;
-    return 1;
+  if (!WriteBufferToPartition(patched, target)) {
+    LOG(ERROR) << "Failed to write patched data to " << target.name;
+    return false;
   }
 
   // Delete the backup copy of the source.
   unlink(Paths::Get().cache_temp_source().c_str());
 
   // Success!
-  return 0;
+  return true;
+}
+
+bool CheckPartition(const Partition& partition) {
+  FileContents target_file;
+  return ReadPartitionToBuffer(partition, &target_file, false);
+}
+
+Partition Partition::Parse(const std::string& input_str, std::string* err) {
+  std::vector<std::string> pieces = android::base::Split(input_str, ":");
+  if (pieces.size() != 4 || pieces[0] != "EMMC") {
+    *err = "Invalid number of tokens or non-eMMC target";
+    return {};
+  }
+
+  size_t size;
+  if (!android::base::ParseUint(pieces[2], &size) || size == 0) {
+    *err = "Failed to parse \"" + pieces[2] + "\" as byte count";
+    return {};
+  }
+
+  return Partition(pieces[1], size, pieces[3]);
+}
+
+std::string Partition::ToString() const {
+  if (*this) {
+    return "EMMC:"s + name + ":" + std::to_string(size) + ":" + hash;
+  }
+  return "<invalid-partition>";
+}
+
+std::ostream& operator<<(std::ostream& os, const Partition& partition) {
+  os << partition.ToString();
+  return os;
 }
diff --git a/applypatch/applypatch_main.cpp b/applypatch/applypatch_main.cpp
index 4fa73d3..92d2b3f 100644
--- a/applypatch/applypatch_main.cpp
+++ b/applypatch/applypatch_main.cpp
@@ -16,7 +16,10 @@
 
 #include "applypatch_modes.h"
 
+#include <android-base/logging.h>
+
 // See the comments for applypatch() function.
 int main(int argc, char** argv) {
+  android::base::InitLogging(argv);
   return applypatch_modes(argc, argv);
 }
diff --git a/applypatch/applypatch_modes.cpp b/applypatch/applypatch_modes.cpp
index f130ea2..b466598 100644
--- a/applypatch/applypatch_modes.cpp
+++ b/applypatch/applypatch_modes.cpp
@@ -35,56 +35,48 @@
 #include "applypatch/applypatch.h"
 #include "edify/expr.h"
 
-static int CheckMode(const std::string& target) {
-  return applypatch_check(target, {});
+static int CheckMode(const std::string& target_emmc) {
+  std::string err;
+  auto target = Partition::Parse(target_emmc, &err);
+  if (!target) {
+    LOG(ERROR) << "Failed to parse target \"" << target_emmc << "\": " << err;
+    return 2;
+  }
+  return CheckPartition(target) ? 0 : 1;
 }
 
 static int FlashMode(const std::string& target_emmc, const std::string& source_file) {
-  std::vector<std::string> pieces = android::base::Split(target_emmc, ":");
-  if (pieces.size() != 4 || pieces[0] != "EMMC") {
+  std::string err;
+  auto target = Partition::Parse(target_emmc, &err);
+  if (!target) {
+    LOG(ERROR) << "Failed to parse target \"" << target_emmc << "\": " << err;
     return 2;
   }
-  size_t target_size;
-  if (!android::base::ParseUint(pieces[2], &target_size) || target_size == 0) {
-    LOG(ERROR) << "Failed to parse \"" << pieces[2] << "\" as byte count";
-    return 1;
-  }
-  return applypatch_flash(source_file.c_str(), target_emmc.c_str(), pieces[3].c_str(), target_size);
+  return FlashPartition(target, source_file) ? 0 : 1;
 }
 
 static int PatchMode(const std::string& target_emmc, const std::string& source_emmc,
                      const std::string& patch_file, const std::string& bonus_file) {
-  std::vector<std::string> target_pieces = android::base::Split(target_emmc, ":");
-  if (target_pieces.size() != 4 || target_pieces[0] != "EMMC") {
+  std::string err;
+  auto target = Partition::Parse(target_emmc, &err);
+  if (!target) {
+    LOG(ERROR) << "Failed to parse target \"" << target_emmc << "\": " << err;
     return 2;
   }
 
-  size_t target_size;
-  if (!android::base::ParseUint(target_pieces[2], &target_size) || target_size == 0) {
-    LOG(ERROR) << "Failed to parse \"" << target_pieces[2] << "\" as byte count";
-    return 1;
-  }
-
-  std::vector<std::string> source_pieces = android::base::Split(source_emmc, ":");
-  if (source_pieces.size() != 4 || source_pieces[0] != "EMMC") {
+  auto source = Partition::Parse(source_emmc, &err);
+  if (!source) {
+    LOG(ERROR) << "Failed to parse source \"" << source_emmc << "\": " << err;
     return 2;
   }
 
-  size_t source_size;
-  if (!android::base::ParseUint(source_pieces[2], &source_size) || source_size == 0) {
-    LOG(ERROR) << "Failed to parse \"" << source_pieces[2] << "\" as byte count";
-    return 1;
-  }
-
-  std::string contents;
-  if (!android::base::ReadFileToString(patch_file, &contents)) {
+  std::string patch_contents;
+  if (!android::base::ReadFileToString(patch_file, &patch_contents)) {
     PLOG(ERROR) << "Failed to read patch file \"" << patch_file << "\"";
     return 1;
   }
-  std::vector<std::unique_ptr<Value>> patches;
-  patches.push_back(std::make_unique<Value>(Value::Type::BLOB, std::move(contents)));
-  std::vector<std::string> sha1s{ source_pieces[3] };
 
+  Value patch(Value::Type::BLOB, std::move(patch_contents));
   std::unique_ptr<Value> bonus;
   if (!bonus_file.empty()) {
     std::string bonus_contents;
@@ -95,8 +87,7 @@
     bonus = std::make_unique<Value>(Value::Type::BLOB, std::move(bonus_contents));
   }
 
-  return applypatch(source_emmc.c_str(), target_emmc.c_str(), target_pieces[3].c_str(), target_size,
-                    sha1s, patches, bonus.get());
+  return PatchPartition(target, source, patch, bonus.get()) ? 0 : 1;
 }
 
 static void Usage() {
diff --git a/applypatch/include/applypatch/applypatch.h b/applypatch/include/applypatch/applypatch.h
index cf65f3f..6fc6f0f 100644
--- a/applypatch/include/applypatch/applypatch.h
+++ b/applypatch/include/applypatch/applypatch.h
@@ -21,6 +21,7 @@
 
 #include <functional>
 #include <memory>
+#include <ostream>
 #include <string>
 #include <vector>
 
@@ -44,51 +45,58 @@
 // digest or be of the form "<digest>:<anything>". Returns 0 on success, or -1 on any error.
 int ParseSha1(const std::string& str, uint8_t* digest);
 
-// Applies binary patches to eMMC target 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 SHA-1 hash of 'target_filename' is 'target_sha1_string', does nothing and returns
-//   successfully.
-//
-// - Otherwise, if the SHA-1 hash of 'source_filename' is one of the entries in 'patch_sha1s', 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 data). If that new file has
-//   SHA-1 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' must refer to an eMMC partition to read the source data. See the comments for
-// the LoadPartitionContents() function for the format of such a filename. 'target_size' has become
-// obsolete since we have dropped the support for patching non-eMMC targets (eMMC targets have the
-// size embedded in the filename).
-int applypatch(const char* source_filename, const char* target_filename,
-               const char* target_sha1_str, size_t target_size,
-               const std::vector<std::string>& patch_sha1s,
-               const std::vector<std::unique_ptr<Value>>& patch_data, const Value* bonus_data);
+struct Partition {
+  Partition() = default;
 
-// Returns 0 if the contents of the eMMC target or the cached file match any of the given SHA-1's.
-// Returns nonzero otherwise. 'filename' must refer to an eMMC partition target. It would only use
-// 'sha1s' to find a match on /cache if the hashes embedded in the filename fail to match.
-int applypatch_check(const std::string& filename, const std::vector<std::string>& sha1s);
+  Partition(const std::string& name, size_t size, const std::string& hash)
+      : name(name), size(size), hash(hash) {}
 
-// Flashes a given image to the eMMC target partition. It verifies the target cheksum first, and
-// will return if target already has the desired hash. Otherwise it checks the checksum of the
-// given source image before flashing, and verifies the target partition afterwards.
-// 'target_filename' must refer to an eMMC partition, of the form "EMMC:<device>:<size>:<hash>".
-// The function is idempotent. Returns zero on success.
-int applypatch_flash(const char* source_filename, const char* target_filename,
-                     const char* target_sha1_str, size_t target_size);
+  // Parses and returns the given string into a Partition object. The input string is of the form
+  // "EMMC:<device>:<size>:<hash>". Returns the parsed Partition, or an empty object on error.
+  static Partition Parse(const std::string& partition, std::string* err);
 
-// Reads a file into memory; stores the file contents and associated metadata in *file. Returns 0
-// on success, or -1 on error.
-int LoadFileContents(const std::string& filename, FileContents* file);
+  std::string ToString() const;
 
-// Saves the given FileContents object to the given filename. Returns 0 on success, or -1 on error.
-int SaveFileContents(const std::string& filename, const FileContents* file);
+  // Returns whether the current Partition object is valid.
+  explicit operator bool() const {
+    return !name.empty();
+  }
+
+  std::string name;
+  size_t size;
+  std::string hash;
+};
+
+std::ostream& operator<<(std::ostream& os, const Partition& partition);
+
+// Applies the given 'patch' to the 'source' Partition, verifies then writes the patching result to
+// the 'target' Partition. While patching, it will backup the data on the source partition to
+// /cache, so that the patching could be resumed on interruption even if both of the source and
+// target partitions refer to the same device. The function is idempotent if called multiple times.
+// An optional arg 'bonus' can be provided, if the patch was generated with a bonus output.
+// Returns the patching result.
+bool PatchPartition(const Partition& target, const Partition& source, const Value& patch,
+                    const Value* bonus);
+
+// Returns whether the contents of the eMMC target or the cached file match the embedded hash.
+// It will look for the backup on /cache if the given partition doesn't match the checksum.
+bool PatchPartitionCheck(const Partition& target, const Partition& source);
+
+// Checks whether the contents of the given partition has the desired hash. It will NOT look for
+// the backup on /cache if the given partition doesn't have the expected checksum.
+bool CheckPartition(const Partition& target);
+
+// Flashes a given image in 'source_filename' to the eMMC target partition. It verifies the target
+// checksum first, and will return if target already has the desired hash. Otherwise it checks the
+// checksum of the given source image, flashes, and verifies the target partition afterwards. The
+// function is idempotent. Returns the flashing result.
+bool FlashPartition(const Partition& target, const std::string& source_filename);
+
+// Reads a file into memory; stores the file contents and associated metadata in *file.
+bool LoadFileContents(const std::string& filename, FileContents* file);
+
+// Saves the given FileContents object to the given filename.
+bool SaveFileContents(const std::string& filename, const FileContents* file);
 
 // bspatch.cpp
 
diff --git a/tests/component/updater_test.cpp b/tests/component/updater_test.cpp
index b253abc..24c63e7 100644
--- a/tests/component/updater_test.cpp
+++ b/tests/component/updater_test.cpp
@@ -41,6 +41,7 @@
 #include <ziparchive/zip_archive.h>
 #include <ziparchive/zip_writer.h>
 
+#include "applypatch/applypatch.h"
 #include "common/test_constants.h"
 #include "edify/expr.h"
 #include "otautil/error_code.h"
@@ -214,55 +215,47 @@
     expect(nullptr, "getprop(\"arg1\", \"arg2\")", kArgsParsingFailure);
 }
 
-TEST_F(UpdaterTest, apply_patch_check) {
-  // Zero-argument is not valid.
-  expect(nullptr, "apply_patch_check()", kArgsParsingFailure);
+TEST_F(UpdaterTest, patch_partition_check) {
+  // Zero argument is not valid.
+  expect(nullptr, "patch_partition_check()", kArgsParsingFailure);
 
-  // File not found.
-  expect("", "apply_patch_check(\"/doesntexist\")", kNoCause);
+  std::string source_file = from_testdata_base("boot.img");
+  std::string source_content;
+  ASSERT_TRUE(android::base::ReadFileToString(source_file, &source_content));
+  size_t source_size = source_content.size();
+  std::string source_hash = get_sha1(source_content);
+  Partition source(source_file, source_size, source_hash);
 
-  std::string src_file = from_testdata_base("boot.img");
-  std::string src_content;
-  ASSERT_TRUE(android::base::ReadFileToString(src_file, &src_content));
-  size_t src_size = src_content.size();
-  std::string src_hash = get_sha1(src_content);
+  std::string target_file = from_testdata_base("recovery.img");
+  std::string target_content;
+  ASSERT_TRUE(android::base::ReadFileToString(target_file, &target_content));
+  size_t target_size = target_content.size();
+  std::string target_hash = get_sha1(target_content);
+  Partition target(target_file, target_size, target_hash);
 
-  // One-argument with EMMC:file:size:sha1 should pass the check.
-  std::string filename = android::base::Join(
-      std::vector<std::string>{ "EMMC", src_file, std::to_string(src_size), src_hash }, ":");
-  std::string cmd = "apply_patch_check(\"" + filename + "\")";
+  // One argument is not valid.
+  expect(nullptr, "patch_partition_check(\"" + source.ToString() + "\")", kArgsParsingFailure);
+  expect(nullptr, "patch_partition_check(\"" + target.ToString() + "\")", kArgsParsingFailure);
+
+  // Both of the source and target have the desired checksum.
+  std::string cmd =
+      "patch_partition_check(\"" + source.ToString() + "\", \"" + target.ToString() + "\")";
   expect("t", cmd, kNoCause);
 
-  // EMMC:file:(size-1):sha1:(size+1):sha1 should fail the check.
-  std::string filename_bad = android::base::Join(
-      std::vector<std::string>{ "EMMC", src_file, std::to_string(src_size - 1), src_hash,
-                                std::to_string(src_size + 1), src_hash },
-      ":");
-  cmd = "apply_patch_check(\"" + filename_bad + "\")";
+  // Only source partition has the desired checksum.
+  Partition bad_target(target_file, target_size - 1, target_hash);
+  cmd = "patch_partition_check(\"" + source.ToString() + "\", \"" + bad_target.ToString() + "\")";
+  expect("t", cmd, kNoCause);
+
+  // Only target partition has the desired checksum.
+  Partition bad_source(source_file, source_size + 1, source_hash);
+  cmd = "patch_partition_check(\"" + bad_source.ToString() + "\", \"" + target.ToString() + "\")";
+  expect("t", cmd, kNoCause);
+
+  // Neither of the source or target has the desired checksum.
+  cmd =
+      "patch_partition_check(\"" + bad_source.ToString() + "\", \"" + bad_target.ToString() + "\")";
   expect("", cmd, kNoCause);
-
-  // EMMC:file:(size-1):sha1:size:sha1:(size+1):sha1 should pass the check.
-  filename_bad =
-      android::base::Join(std::vector<std::string>{ "EMMC", src_file, std::to_string(src_size - 1),
-                                                    src_hash, std::to_string(src_size), src_hash,
-                                                    std::to_string(src_size + 1), src_hash },
-                          ":");
-  cmd = "apply_patch_check(\"" + filename_bad + "\")";
-  expect("t", cmd, kNoCause);
-
-  // Multiple arguments.
-  // As long as it successfully loads the partition specified in filename, it won't check against
-  // any given SHAs.
-  cmd = "apply_patch_check(\"" + filename + "\", \"wrong_sha1\", \"wrong_sha2\")";
-  expect("t", cmd, kNoCause);
-
-  cmd = "apply_patch_check(\"" + filename + "\", \"wrong_sha1\", \"" + src_hash +
-        "\", \"wrong_sha2\")";
-  expect("t", cmd, kNoCause);
-
-  cmd = "apply_patch_check(\"" + filename_bad + "\", \"wrong_sha1\", \"" + src_hash +
-        "\", \"wrong_sha2\")";
-  expect("t", cmd, kNoCause);
 }
 
 TEST_F(UpdaterTest, file_getprop) {
diff --git a/tests/unit/applypatch_test.cpp b/tests/unit/applypatch_test.cpp
index fe00e1e..066f981 100644
--- a/tests/unit/applypatch_test.cpp
+++ b/tests/unit/applypatch_test.cpp
@@ -38,6 +38,7 @@
 
 #include "applypatch/applypatch.h"
 #include "common/test_constants.h"
+#include "edify/expr.h"
 #include "otautil/paths.h"
 #include "otautil/print_sha1.h"
 
@@ -48,22 +49,29 @@
   void SetUp() override {
     source_file = from_testdata_base("boot.img");
     FileContents boot_fc;
-    ASSERT_EQ(0, LoadFileContents(source_file, &boot_fc));
+    ASSERT_TRUE(LoadFileContents(source_file, &boot_fc));
     source_size = boot_fc.data.size();
     source_sha1 = print_sha1(boot_fc.sha1);
 
     target_file = from_testdata_base("recovery.img");
     FileContents recovery_fc;
-    ASSERT_EQ(0, LoadFileContents(target_file, &recovery_fc));
+    ASSERT_TRUE(LoadFileContents(target_file, &recovery_fc));
     target_size = recovery_fc.data.size();
     target_sha1 = print_sha1(recovery_fc.sha1);
 
+    source_partition = Partition(source_file, source_size, source_sha1);
+    target_partition = Partition(partition_file.path, target_size, target_sha1);
+
     srand(time(nullptr));
     bad_sha1_a = android::base::StringPrintf("%040x", rand());
     bad_sha1_b = android::base::StringPrintf("%040x", rand());
 
     // Reset the cache backup file.
-    Paths::Get().set_cache_temp_source("/cache/saved.file");
+    Paths::Get().set_cache_temp_source(cache_temp_source.path);
+  }
+
+  void TearDown() override {
+    ASSERT_TRUE(android::base::RemoveFileIfExists(cache_temp_source.path));
   }
 
   std::string source_file;
@@ -76,74 +84,75 @@
 
   std::string bad_sha1_a;
   std::string bad_sha1_b;
+
+  Partition source_partition;
+  Partition target_partition;
+
+ private:
+  TemporaryFile partition_file;
+  TemporaryFile cache_temp_source;
 };
 
-TEST_F(ApplyPatchTest, CheckMode) {
-  std::string partition =
-      "EMMC:" + source_file + ":" + std::to_string(source_size) + ":" + source_sha1;
-  ASSERT_EQ(0, applypatch_check(partition, {}));
-  ASSERT_EQ(0, applypatch_check(partition, { source_sha1 }));
-  ASSERT_EQ(0, applypatch_check(partition, { bad_sha1_a, bad_sha1_b }));
-  ASSERT_EQ(0, applypatch_check(partition, { bad_sha1_a, source_sha1, bad_sha1_b }));
+TEST_F(ApplyPatchTest, CheckPartition) {
+  ASSERT_TRUE(CheckPartition(source_partition));
 }
 
-TEST_F(ApplyPatchTest, CheckMode_NonEmmcTarget) {
-  ASSERT_NE(0, applypatch_check(source_file, {}));
-  ASSERT_NE(0, applypatch_check(source_file, { source_sha1 }));
-  ASSERT_NE(0, applypatch_check(source_file, { bad_sha1_a, bad_sha1_b }));
-  ASSERT_NE(0, applypatch_check(source_file, { bad_sha1_a, source_sha1, bad_sha1_b }));
+TEST_F(ApplyPatchTest, CheckPartition_Mismatching) {
+  ASSERT_FALSE(CheckPartition(Partition(source_file, target_size, target_sha1)));
+  ASSERT_FALSE(CheckPartition(Partition(source_file, source_size, bad_sha1_a)));
+
+  ASSERT_FALSE(CheckPartition(Partition(source_file, source_size - 1, source_sha1)));
+  ASSERT_FALSE(CheckPartition(Partition(source_file, source_size + 1, source_sha1)));
 }
 
-TEST_F(ApplyPatchTest, CheckMode_EmmcTarget) {
-  // EMMC:source_file:size:sha1 should pass the check.
-  std::string src_file =
-      "EMMC:" + source_file + ":" + std::to_string(source_size) + ":" + source_sha1;
-  ASSERT_EQ(0, applypatch_check(src_file, {}));
+TEST_F(ApplyPatchTest, PatchPartitionCheck) {
+  ASSERT_TRUE(PatchPartitionCheck(target_partition, source_partition));
 
-  // EMMC:source_file:(size-1):sha1:(size+1):sha1 should fail the check.
-  src_file = "EMMC:" + source_file + ":" + std::to_string(source_size - 1) + ":" + source_sha1 +
-             ":" + std::to_string(source_size + 1) + ":" + source_sha1;
-  ASSERT_NE(0, applypatch_check(src_file, {}));
+  ASSERT_TRUE(
+      PatchPartitionCheck(Partition(source_file, source_size - 1, source_sha1), source_partition));
 
-  // EMMC:source_file:(size-1):sha1:size:sha1:(size+1):sha1 should pass the check.
-  src_file = "EMMC:" + source_file + ":" + std::to_string(source_size - 1) + ":" + source_sha1 +
-             ":" + std::to_string(source_size) + ":" + source_sha1 + ":" +
-             std::to_string(source_size + 1) + ":" + source_sha1;
-  ASSERT_EQ(0, applypatch_check(src_file, {}));
-
-  // EMMC:source_file:(size+1):sha1:(size-1):sha1:size:sha1 should pass the check.
-  src_file = "EMMC:" + source_file + ":" + std::to_string(source_size + 1) + ":" + source_sha1 +
-             ":" + std::to_string(source_size - 1) + ":" + source_sha1 + ":" +
-             std::to_string(source_size) + ":" + source_sha1;
-  ASSERT_EQ(0, applypatch_check(src_file, {}));
-
-  // EMMC:target_file:(size+1):source_sha1:(size-1):source_sha1:size:source_sha1:size:target_sha1
-  // should pass the check.
-  src_file = "EMMC:" + target_file + ":" + std::to_string(source_size + 1) + ":" + source_sha1 +
-             ":" + std::to_string(source_size - 1) + ":" + source_sha1 + ":" +
-             std::to_string(source_size) + ":" + source_sha1 + ":" + std::to_string(target_size) +
-             ":" + target_sha1;
-  ASSERT_EQ(0, applypatch_check(src_file, {}));
+  ASSERT_TRUE(
+      PatchPartitionCheck(Partition(source_file, source_size + 1, source_sha1), source_partition));
 }
 
-TEST_F(ApplyPatchTest, CheckMode_UseBackup) {
-  std::string corrupted =
-      "EMMC:" + source_file + ":" + std::to_string(source_size) + ":" + bad_sha1_a;
-  ASSERT_NE(0, applypatch_check(corrupted, { source_sha1 }));
+TEST_F(ApplyPatchTest, PatchPartitionCheck_UseBackup) {
+  ASSERT_FALSE(
+      PatchPartitionCheck(target_partition, Partition(target_file, source_size, source_sha1)));
 
   Paths::Get().set_cache_temp_source(source_file);
-  ASSERT_EQ(0, applypatch_check(corrupted, { source_sha1 }));
-  ASSERT_EQ(0, applypatch_check(corrupted, { bad_sha1_a, source_sha1, bad_sha1_b }));
+  ASSERT_TRUE(
+      PatchPartitionCheck(target_partition, Partition(target_file, source_size, source_sha1)));
 }
 
-TEST_F(ApplyPatchTest, CheckMode_UseBackup_BothCorrupted) {
-  std::string corrupted =
-      "EMMC:" + source_file + ":" + std::to_string(source_size) + ":" + bad_sha1_a;
-  ASSERT_NE(0, applypatch_check(corrupted, {}));
-  ASSERT_NE(0, applypatch_check(corrupted, { source_sha1 }));
+TEST_F(ApplyPatchTest, PatchPartitionCheck_UseBackup_BothCorrupted) {
+  ASSERT_FALSE(
+      PatchPartitionCheck(target_partition, Partition(target_file, source_size, source_sha1)));
 
-  Paths::Get().set_cache_temp_source(source_file);
-  ASSERT_NE(0, applypatch_check(corrupted, { bad_sha1_a, bad_sha1_b }));
+  Paths::Get().set_cache_temp_source(target_file);
+  ASSERT_FALSE(
+      PatchPartitionCheck(target_partition, Partition(target_file, source_size, source_sha1)));
+}
+
+TEST_F(ApplyPatchTest, PatchPartition) {
+  FileContents patch_fc;
+  ASSERT_TRUE(LoadFileContents(from_testdata_base("recovery-from-boot.p"), &patch_fc));
+  Value patch(Value::Type::BLOB, std::string(patch_fc.data.cbegin(), patch_fc.data.cend()));
+
+  FileContents bonus_fc;
+  ASSERT_TRUE(LoadFileContents(from_testdata_base("bonus.file"), &bonus_fc));
+  Value bonus(Value::Type::BLOB, std::string(bonus_fc.data.cbegin(), bonus_fc.data.cend()));
+
+  ASSERT_TRUE(PatchPartition(target_partition, source_partition, patch, &bonus));
+}
+
+// Tests patching an eMMC target without a separate bonus file (i.e. recovery-from-boot patch has
+// everything).
+TEST_F(ApplyPatchTest, PatchPartitionWithoutBonusFile) {
+  FileContents patch_fc;
+  ASSERT_TRUE(LoadFileContents(from_testdata_base("recovery-from-boot-with-bonus.p"), &patch_fc));
+  Value patch(Value::Type::BLOB, std::string(patch_fc.data.cbegin(), patch_fc.data.cend()));
+
+  ASSERT_TRUE(PatchPartition(target_partition, source_partition, patch, nullptr));
 }
 
 class FreeCacheTest : public ::testing::Test {
diff --git a/updater/install.cpp b/updater/install.cpp
index 34514b6..deb7a2b 100644
--- a/updater/install.cpp
+++ b/updater/install.cpp
@@ -196,94 +196,82 @@
   }
 }
 
-// apply_patch(src_file, tgt_file, tgt_sha1, tgt_size, patch1_sha1, patch1_blob, [...])
-//   Applies a binary patch to the src_file to produce the tgt_file. If the desired target is the
-//   same as the source, pass "-" for tgt_file. tgt_sha1 and tgt_size are the expected final SHA1
-//   hash and size of the target file. The remaining arguments must come in pairs: a SHA1 hash (a
-//   40-character hex string) and a blob. The blob is the patch to be applied when the source
-//   file's current contents have the given SHA1.
+// patch_partition_check(target_partition, source_partition)
+//   Checks if the target and source partitions have the desired checksums to be patched. It returns
+//   directly, if the target partition already has the expected checksum. Otherwise it in turn
+//   checks the integrity of the source partition and the backup file on /cache.
 //
-//   The patching is done in a safe manner that guarantees the target file either has the desired
-//   SHA1 hash and size, or it is untouched -- it will not be left in an unrecoverable intermediate
-//   state. If the process is interrupted during patching, the target file may be in an intermediate
-//   state; a copy exists in the cache partition so restarting the update can successfully update
-//   the file.
-Value* ApplyPatchFn(const char* name, State* state,
-                    const std::vector<std::unique_ptr<Expr>>& argv) {
-  if (argv.size() < 6 || (argv.size() % 2) == 1) {
+// For example, patch_partition_check(
+//     "EMMC:/dev/block/boot:12342568:8aaacf187a6929d0e9c3e9e46ea7ff495b43424d",
+//     "EMMC:/dev/block/boot:12363048:06b0b16299dcefc94900efed01e0763ff644ffa4")
+Value* PatchPartitionCheckFn(const char* name, State* state,
+                             const std::vector<std::unique_ptr<Expr>>& argv) {
+  if (argv.size() != 2) {
     return ErrorAbort(state, kArgsParsingFailure,
-                      "%s(): expected at least 6 args and an "
-                      "even number, got %zu",
-                      name, argv.size());
+                      "%s(): Invalid number of args (expected 2, got %zu)", name, argv.size());
   }
 
   std::vector<std::string> args;
-  if (!ReadArgs(state, argv, &args, 0, 4)) {
-    return ErrorAbort(state, kArgsParsingFailure, "%s() Failed to parse the argument(s)", name);
-  }
-  const std::string& source_filename = args[0];
-  const std::string& target_filename = args[1];
-  const std::string& target_sha1 = args[2];
-  const std::string& target_size_str = args[3];
-
-  size_t target_size;
-  if (!android::base::ParseUint(target_size_str.c_str(), &target_size)) {
-    return ErrorAbort(state, kArgsParsingFailure, "%s(): can't parse \"%s\" as byte count", name,
-                      target_size_str.c_str());
+  if (!ReadArgs(state, argv, &args, 0, 2)) {
+    return ErrorAbort(state, kArgsParsingFailure, "%s(): Failed to parse the argument(s)", name);
   }
 
-  int patchcount = (argv.size() - 4) / 2;
-  std::vector<std::unique_ptr<Value>> arg_values;
-  if (!ReadValueArgs(state, argv, &arg_values, 4, argv.size() - 4)) {
-    return nullptr;
+  std::string err;
+  auto target = Partition::Parse(args[0], &err);
+  if (!target) {
+    return ErrorAbort(state, kArgsParsingFailure, "%s(): Failed to parse target \"%s\": %s", name,
+                      args[0].c_str(), err.c_str());
   }
 
-  for (int i = 0; i < patchcount; ++i) {
-    if (arg_values[i * 2]->type != Value::Type::STRING) {
-      return ErrorAbort(state, kArgsParsingFailure, "%s(): sha-1 #%d is not string", name, i * 2);
-    }
-    if (arg_values[i * 2 + 1]->type != Value::Type::BLOB) {
-      return ErrorAbort(state, kArgsParsingFailure, "%s(): patch #%d is not blob", name, i * 2 + 1);
-    }
+  auto source = Partition::Parse(args[1], &err);
+  if (!source) {
+    return ErrorAbort(state, kArgsParsingFailure, "%s(): Failed to parse source \"%s\": %s", name,
+                      args[1].c_str(), err.c_str());
   }
 
-  std::vector<std::string> patch_sha_str;
-  std::vector<std::unique_ptr<Value>> patches;
-  for (int i = 0; i < patchcount; ++i) {
-    patch_sha_str.push_back(arg_values[i * 2]->data);
-    patches.push_back(std::move(arg_values[i * 2 + 1]));
-  }
-
-  int result = applypatch(source_filename.c_str(), target_filename.c_str(), target_sha1.c_str(),
-                          target_size, patch_sha_str, patches, nullptr);
-
-  return StringValue(result == 0 ? "t" : "");
+  bool result = PatchPartitionCheck(target, source);
+  return StringValue(result ? "t" : "");
 }
 
-// apply_patch_check(filename, [sha1, ...])
-//   Returns true if the contents of filename or the temporary copy in the cache partition (if
-//   present) have a SHA-1 checksum equal to one of the given sha1 values. sha1 values are
-//   specified as 40 hex digits.
-Value* ApplyPatchCheckFn(const char* name, State* state,
-                         const std::vector<std::unique_ptr<Expr>>& argv) {
-  if (argv.size() < 1) {
-    return ErrorAbort(state, kArgsParsingFailure, "%s(): expected at least 1 arg, got %zu", name,
-                      argv.size());
+// patch_partition(target, source, patch)
+//   Applies the given patch to the source partition, and writes the result to the target partition.
+//
+// For example, patch_partition(
+//     "EMMC:/dev/block/boot:12342568:8aaacf187a6929d0e9c3e9e46ea7ff495b43424d",
+//     "EMMC:/dev/block/boot:12363048:06b0b16299dcefc94900efed01e0763ff644ffa4",
+//     package_extract_file("boot.img.p"))
+Value* PatchPartitionFn(const char* name, State* state,
+                        const std::vector<std::unique_ptr<Expr>>& argv) {
+  if (argv.size() != 3) {
+    return ErrorAbort(state, kArgsParsingFailure,
+                      "%s(): Invalid number of args (expected 3, got %zu)", name, argv.size());
   }
 
   std::vector<std::string> args;
-  if (!ReadArgs(state, argv, &args, 0, 1)) {
-    return ErrorAbort(state, kArgsParsingFailure, "%s() Failed to parse the argument(s)", name);
+  if (!ReadArgs(state, argv, &args, 0, 2)) {
+    return ErrorAbort(state, kArgsParsingFailure, "%s(): Failed to parse the argument(s)", name);
   }
-  const std::string& filename = args[0];
 
-  std::vector<std::string> sha1s;
-  if (argv.size() > 1 && !ReadArgs(state, argv, &sha1s, 1, argv.size() - 1)) {
-    return ErrorAbort(state, kArgsParsingFailure, "%s() Failed to parse the argument(s)", name);
+  std::string err;
+  auto target = Partition::Parse(args[0], &err);
+  if (!target) {
+    return ErrorAbort(state, kArgsParsingFailure, "%s(): Failed to parse target \"%s\": %s", name,
+                      args[0].c_str(), err.c_str());
   }
-  int result = applypatch_check(filename.c_str(), sha1s);
 
-  return StringValue(result == 0 ? "t" : "");
+  auto source = Partition::Parse(args[1], &err);
+  if (!source) {
+    return ErrorAbort(state, kArgsParsingFailure, "%s(): Failed to parse source \"%s\": %s", name,
+                      args[1].c_str(), err.c_str());
+  }
+
+  std::vector<std::unique_ptr<Value>> values;
+  if (!ReadValueArgs(state, argv, &values, 2, 1) || values[0]->type != Value::Type::BLOB) {
+    return ErrorAbort(state, kArgsParsingFailure, "%s(): Invalid patch arg", name);
+  }
+
+  bool result = PatchPartition(target, source, *values[0], nullptr);
+  return StringValue(result ? "t" : "");
 }
 
 // mount(fs_type, partition_type, location, mount_point)
@@ -956,9 +944,9 @@
   RegisterFunction("getprop", GetPropFn);
   RegisterFunction("file_getprop", FileGetPropFn);
 
-  RegisterFunction("apply_patch", ApplyPatchFn);
-  RegisterFunction("apply_patch_check", ApplyPatchCheckFn);
   RegisterFunction("apply_patch_space", ApplyPatchSpaceFn);
+  RegisterFunction("patch_partition", PatchPartitionFn);
+  RegisterFunction("patch_partition_check", PatchPartitionCheckFn);
 
   RegisterFunction("wipe_block_device", WipeBlockDeviceFn);