chibipub

blake3.c (27003B)

---

 #include <assert.h>
 #include <stdbool.h>
 #include <string.h>
 
 #include "blake3.h"
 #include "blake3_impl.h"
 
 const char * blake3_version(void) {
   return BLAKE3_VERSION_STRING;
 }
 
 INLINE void chunk_state_init(blake3_chunk_state *self, const uint32_t key[8],
                              uint8_t flags) {
   memcpy(self->cv, key, BLAKE3_KEY_LEN);
   self->chunk_counter = 0;
   memset(self->buf, 0, BLAKE3_BLOCK_LEN);
   self->buf_len = 0;
   self->blocks_compressed = 0;
   self->flags = flags;
 }
 
 INLINE void chunk_state_reset(blake3_chunk_state *self, const uint32_t key[8],
                               uint64_t chunk_counter) {
   memcpy(self->cv, key, BLAKE3_KEY_LEN);
   self->chunk_counter = chunk_counter;
   self->blocks_compressed = 0;
   memset(self->buf, 0, BLAKE3_BLOCK_LEN);
   self->buf_len = 0;
 }
 
 INLINE size_t chunk_state_len(const blake3_chunk_state *self) {
   return (BLAKE3_BLOCK_LEN * (size_t)self->blocks_compressed) +
          ((size_t)self->buf_len);
 }
 
 INLINE size_t chunk_state_fill_buf(blake3_chunk_state *self,
                                    const uint8_t *input, size_t input_len) {
   size_t take = BLAKE3_BLOCK_LEN - ((size_t)self->buf_len);
   if (take > input_len) {
     take = input_len;
   }
   uint8_t *dest = self->buf + ((size_t)self->buf_len);
   memcpy(dest, input, take);
   self->buf_len += (uint8_t)take;
   return take;
 }
 
 INLINE uint8_t chunk_state_maybe_start_flag(const blake3_chunk_state *self) {
   if (self->blocks_compressed == 0) {
     return CHUNK_START;
   } else {
     return 0;
   }
 }
 
 typedef struct {
   uint32_t input_cv[8];
   uint64_t counter;
   uint8_t block[BLAKE3_BLOCK_LEN];
   uint8_t block_len;
   uint8_t flags;
 } output_t;
 
 INLINE output_t make_output(const uint32_t input_cv[8],
                             const uint8_t block[BLAKE3_BLOCK_LEN],
                             uint8_t block_len, uint64_t counter,
                             uint8_t flags) {
   output_t ret;
   memcpy(ret.input_cv, input_cv, 32);
   memcpy(ret.block, block, BLAKE3_BLOCK_LEN);
   ret.block_len = block_len;
   ret.counter = counter;
   ret.flags = flags;
   return ret;
 }
 
 // Chaining values within a given chunk (specifically the compress_in_place
 // interface) are represented as words. This avoids unnecessary bytes<->words
 // conversion overhead in the portable implementation. However, the hash_many
 // interface handles both user input and parent node blocks, so it accepts
 // bytes. For that reason, chaining values in the CV stack are represented as
 // bytes.
 INLINE void output_chaining_value(const output_t *self, uint8_t cv[32]) {
   uint32_t cv_words[8];
   memcpy(cv_words, self->input_cv, 32);
   blake3_compress_in_place(cv_words, self->block, self->block_len,
                            self->counter, self->flags);
   store_cv_words(cv, cv_words);
 }
 
 INLINE void output_root_bytes(const output_t *self, uint64_t seek, uint8_t *out,
                               size_t out_len) {
   uint64_t output_block_counter = seek / 64;
   size_t offset_within_block = seek % 64;
   uint8_t wide_buf[64];
   while (out_len > 0) {
     blake3_compress_xof(self->input_cv, self->block, self->block_len,
                         output_block_counter, self->flags | ROOT, wide_buf);
     size_t available_bytes = 64 - offset_within_block;
     size_t memcpy_len;
     if (out_len > available_bytes) {
       memcpy_len = available_bytes;
     } else {
       memcpy_len = out_len;
     }
     memcpy(out, wide_buf + offset_within_block, memcpy_len);
     out += memcpy_len;
     out_len -= memcpy_len;
     output_block_counter += 1;
     offset_within_block = 0;
   }
 }
 
 INLINE void chunk_state_update(blake3_chunk_state *self, const uint8_t *input,
                                size_t input_len) {
   if (self->buf_len > 0) {
     size_t take = chunk_state_fill_buf(self, input, input_len);
     input += take;
     input_len -= take;
     if (input_len > 0) {
       blake3_compress_in_place(
           self->cv, self->buf, BLAKE3_BLOCK_LEN, self->chunk_counter,
           self->flags | chunk_state_maybe_start_flag(self));
       self->blocks_compressed += 1;
       self->buf_len = 0;
       memset(self->buf, 0, BLAKE3_BLOCK_LEN);
     }
   }
 
   while (input_len > BLAKE3_BLOCK_LEN) {
     blake3_compress_in_place(self->cv, input, BLAKE3_BLOCK_LEN,
                              self->chunk_counter,
                              self->flags | chunk_state_maybe_start_flag(self));
     self->blocks_compressed += 1;
     input += BLAKE3_BLOCK_LEN;
     input_len -= BLAKE3_BLOCK_LEN;
   }
 
   size_t take = chunk_state_fill_buf(self, input, input_len);
   input += take;
   input_len -= take;
 }
 
 INLINE output_t chunk_state_output(const blake3_chunk_state *self) {
   uint8_t block_flags =
       self->flags | chunk_state_maybe_start_flag(self) | CHUNK_END;
   return make_output(self->cv, self->buf, self->buf_len, self->chunk_counter,
                      block_flags);
 }
 
 INLINE output_t parent_output(const uint8_t block[BLAKE3_BLOCK_LEN],
                               const uint32_t key[8], uint8_t flags) {
   return make_output(key, block, BLAKE3_BLOCK_LEN, 0, flags | PARENT);
 }
 
 // Given some input larger than one chunk, return the number of bytes that
 // should go in the left subtree. This is the largest power-of-2 number of
 // chunks that leaves at least 1 byte for the right subtree.
 INLINE size_t left_len(size_t content_len) {
   // Subtract 1 to reserve at least one byte for the right side. content_len
   // should always be greater than BLAKE3_CHUNK_LEN.
   size_t full_chunks = (content_len - 1) / BLAKE3_CHUNK_LEN;
   return round_down_to_power_of_2(full_chunks) * BLAKE3_CHUNK_LEN;
 }
 
 // Use SIMD parallelism to hash up to MAX_SIMD_DEGREE chunks at the same time
 // on a single thread. Write out the chunk chaining values and return the
 // number of chunks hashed. These chunks are never the root and never empty;
 // those cases use a different codepath.
 INLINE size_t compress_chunks_parallel(const uint8_t *input, size_t input_len,
                                        const uint32_t key[8],
                                        uint64_t chunk_counter, uint8_t flags,
                                        uint8_t *out) {
 #if defined(BLAKE3_TESTING)
   assert(0 < input_len);
   assert(input_len <= MAX_SIMD_DEGREE * BLAKE3_CHUNK_LEN);
 #endif
 
   const uint8_t *chunks_array[MAX_SIMD_DEGREE];
   size_t input_position = 0;
   size_t chunks_array_len = 0;
   while (input_len - input_position >= BLAKE3_CHUNK_LEN) {
     chunks_array[chunks_array_len] = &input[input_position];
     input_position += BLAKE3_CHUNK_LEN;
     chunks_array_len += 1;
   }
 
   blake3_hash_many(chunks_array, chunks_array_len,
                    BLAKE3_CHUNK_LEN / BLAKE3_BLOCK_LEN, key, chunk_counter,
                    true, flags, CHUNK_START, CHUNK_END, out);
 
   // Hash the remaining partial chunk, if there is one. Note that the empty
   // chunk (meaning the empty message) is a different codepath.
   if (input_len > input_position) {
     uint64_t counter = chunk_counter + (uint64_t)chunks_array_len;
     blake3_chunk_state chunk_state;
     chunk_state_init(&chunk_state, key, flags);
     chunk_state.chunk_counter = counter;
     chunk_state_update(&chunk_state, &input[input_position],
                        input_len - input_position);
     output_t output = chunk_state_output(&chunk_state);
     output_chaining_value(&output, &out[chunks_array_len * BLAKE3_OUT_LEN]);
     return chunks_array_len + 1;
   } else {
     return chunks_array_len;
   }
 }
 
 // Use SIMD parallelism to hash up to MAX_SIMD_DEGREE parents at the same time
 // on a single thread. Write out the parent chaining values and return the
 // number of parents hashed. (If there's an odd input chaining value left over,
 // return it as an additional output.) These parents are never the root and
 // never empty; those cases use a different codepath.
 INLINE size_t compress_parents_parallel(const uint8_t *child_chaining_values,
                                         size_t num_chaining_values,
                                         const uint32_t key[8], uint8_t flags,
                                         uint8_t *out) {
 #if defined(BLAKE3_TESTING)
   assert(2 <= num_chaining_values);
   assert(num_chaining_values <= 2 * MAX_SIMD_DEGREE_OR_2);
 #endif
 
   const uint8_t *parents_array[MAX_SIMD_DEGREE_OR_2];
   size_t parents_array_len = 0;
   while (num_chaining_values - (2 * parents_array_len) >= 2) {
     parents_array[parents_array_len] =
         &child_chaining_values[2 * parents_array_len * BLAKE3_OUT_LEN];
     parents_array_len += 1;
   }
 
   blake3_hash_many(parents_array, parents_array_len, 1, key,
                    0, // Parents always use counter 0.
                    false, flags | PARENT,
                    0, // Parents have no start flags.
                    0, // Parents have no end flags.
                    out);
 
   // If there's an odd child left over, it becomes an output.
   if (num_chaining_values > 2 * parents_array_len) {
     memcpy(&out[parents_array_len * BLAKE3_OUT_LEN],
            &child_chaining_values[2 * parents_array_len * BLAKE3_OUT_LEN],
            BLAKE3_OUT_LEN);
     return parents_array_len + 1;
   } else {
     return parents_array_len;
   }
 }
 
 // The wide helper function returns (writes out) an array of chaining values
 // and returns the length of that array. The number of chaining values returned
 // is the dyanmically detected SIMD degree, at most MAX_SIMD_DEGREE. Or fewer,
 // if the input is shorter than that many chunks. The reason for maintaining a
 // wide array of chaining values going back up the tree, is to allow the
 // implementation to hash as many parents in parallel as possible.
 //
 // As a special case when the SIMD degree is 1, this function will still return
 // at least 2 outputs. This guarantees that this function doesn't perform the
 // root compression. (If it did, it would use the wrong flags, and also we
 // wouldn't be able to implement exendable ouput.) Note that this function is
 // not used when the whole input is only 1 chunk long; that's a different
 // codepath.
 //
 // Why not just have the caller split the input on the first update(), instead
 // of implementing this special rule? Because we don't want to limit SIMD or
 // multi-threading parallelism for that update().
 static size_t blake3_compress_subtree_wide(const uint8_t *input,
                                            size_t input_len,
                                            const uint32_t key[8],
                                            uint64_t chunk_counter,
                                            uint8_t flags, uint8_t *out) {
   // Note that the single chunk case does *not* bump the SIMD degree up to 2
   // when it is 1. If this implementation adds multi-threading in the future,
   // this gives us the option of multi-threading even the 2-chunk case, which
   // can help performance on smaller platforms.
   if (input_len <= blake3_simd_degree() * BLAKE3_CHUNK_LEN) {
     return compress_chunks_parallel(input, input_len, key, chunk_counter, flags,
                                     out);
   }
 
   // With more than simd_degree chunks, we need to recurse. Start by dividing
   // the input into left and right subtrees. (Note that this is only optimal
   // as long as the SIMD degree is a power of 2. If we ever get a SIMD degree
   // of 3 or something, we'll need a more complicated strategy.)
   size_t left_input_len = left_len(input_len);
   size_t right_input_len = input_len - left_input_len;
   const uint8_t *right_input = &input[left_input_len];
   uint64_t right_chunk_counter =
       chunk_counter + (uint64_t)(left_input_len / BLAKE3_CHUNK_LEN);
 
   // Make space for the child outputs. Here we use MAX_SIMD_DEGREE_OR_2 to
   // account for the special case of returning 2 outputs when the SIMD degree
   // is 1.
   uint8_t cv_array[2 * MAX_SIMD_DEGREE_OR_2 * BLAKE3_OUT_LEN];
   size_t degree = blake3_simd_degree();
   if (left_input_len > BLAKE3_CHUNK_LEN && degree == 1) {
     // The special case: We always use a degree of at least two, to make
     // sure there are two outputs. Except, as noted above, at the chunk
     // level, where we allow degree=1. (Note that the 1-chunk-input case is
     // a different codepath.)
     degree = 2;
   }
   uint8_t *right_cvs = &cv_array[degree * BLAKE3_OUT_LEN];
 
   // Recurse! If this implementation adds multi-threading support in the
   // future, this is where it will go.
   size_t left_n = blake3_compress_subtree_wide(input, left_input_len, key,
                                                chunk_counter, flags, cv_array);
   size_t right_n = blake3_compress_subtree_wide(
       right_input, right_input_len, key, right_chunk_counter, flags, right_cvs);
 
   // The special case again. If simd_degree=1, then we'll have left_n=1 and
   // right_n=1. Rather than compressing them into a single output, return
   // them directly, to make sure we always have at least two outputs.
   if (left_n == 1) {
     memcpy(out, cv_array, 2 * BLAKE3_OUT_LEN);
     return 2;
   }
 
   // Otherwise, do one layer of parent node compression.
   size_t num_chaining_values = left_n + right_n;
   return compress_parents_parallel(cv_array, num_chaining_values, key, flags,
                                    out);
 }
 
 // Hash a subtree with compress_subtree_wide(), and then condense the resulting
 // list of chaining values down to a single parent node. Don't compress that
 // last parent node, however. Instead, return its message bytes (the
 // concatenated chaining values of its children). This is necessary when the
 // first call to update() supplies a complete subtree, because the topmost
 // parent node of that subtree could end up being the root. It's also necessary
 // for extended output in the general case.
 //
 // As with compress_subtree_wide(), this function is not used on inputs of 1
 // chunk or less. That's a different codepath.
 INLINE void compress_subtree_to_parent_node(
     const uint8_t *input, size_t input_len, const uint32_t key[8],
     uint64_t chunk_counter, uint8_t flags, uint8_t out[2 * BLAKE3_OUT_LEN]) {
 #if defined(BLAKE3_TESTING)
   assert(input_len > BLAKE3_CHUNK_LEN);
 #endif
 
   uint8_t cv_array[MAX_SIMD_DEGREE_OR_2 * BLAKE3_OUT_LEN];
   size_t num_cvs = blake3_compress_subtree_wide(input, input_len, key,
                                                 chunk_counter, flags, cv_array);
 
   // If MAX_SIMD_DEGREE is greater than 2 and there's enough input,
   // compress_subtree_wide() returns more than 2 chaining values. Condense
   // them into 2 by forming parent nodes repeatedly.
   uint8_t out_array[MAX_SIMD_DEGREE_OR_2 * BLAKE3_OUT_LEN / 2];
   while (num_cvs > 2) {
     num_cvs =
         compress_parents_parallel(cv_array, num_cvs, key, flags, out_array);
     memcpy(cv_array, out_array, num_cvs * BLAKE3_OUT_LEN);
   }
   memcpy(out, cv_array, 2 * BLAKE3_OUT_LEN);
 }
 
 INLINE void hasher_init_base(blake3_hasher *self, const uint32_t key[8],
                              uint8_t flags) {
   memcpy(self->key, key, BLAKE3_KEY_LEN);
   chunk_state_init(&self->chunk, key, flags);
   self->cv_stack_len = 0;
 }
 
 void blake3_hasher_init(blake3_hasher *self) { hasher_init_base(self, IV, 0); }
 
 void blake3_hasher_init_keyed(blake3_hasher *self,
                               const uint8_t key[BLAKE3_KEY_LEN]) {
   uint32_t key_words[8];
   load_key_words(key, key_words);
   hasher_init_base(self, key_words, KEYED_HASH);
 }
 
 void blake3_hasher_init_derive_key_raw(blake3_hasher *self, const void *context,
                                        size_t context_len) {
   blake3_hasher context_hasher;
   hasher_init_base(&context_hasher, IV, DERIVE_KEY_CONTEXT);
   blake3_hasher_update(&context_hasher, context, context_len);
   uint8_t context_key[BLAKE3_KEY_LEN];
   blake3_hasher_finalize(&context_hasher, context_key, BLAKE3_KEY_LEN);
   uint32_t context_key_words[8];
   load_key_words(context_key, context_key_words);
   hasher_init_base(self, context_key_words, DERIVE_KEY_MATERIAL);
 }
 
 void blake3_hasher_init_derive_key(blake3_hasher *self, const char *context) {
   blake3_hasher_init_derive_key_raw(self, context, strlen(context));
 }
 
 // As described in hasher_push_cv() below, we do "lazy merging", delaying
 // merges until right before the next CV is about to be added. This is
 // different from the reference implementation. Another difference is that we
 // aren't always merging 1 chunk at a time. Instead, each CV might represent
 // any power-of-two number of chunks, as long as the smaller-above-larger stack
 // order is maintained. Instead of the "count the trailing 0-bits" algorithm
 // described in the spec, we use a "count the total number of 1-bits" variant
 // that doesn't require us to retain the subtree size of the CV on top of the
 // stack. The principle is the same: each CV that should remain in the stack is
 // represented by a 1-bit in the total number of chunks (or bytes) so far.
 INLINE void hasher_merge_cv_stack(blake3_hasher *self, uint64_t total_len) {
   size_t post_merge_stack_len = (size_t)popcnt(total_len);
   while (self->cv_stack_len > post_merge_stack_len) {
     uint8_t *parent_node =
         &self->cv_stack[(self->cv_stack_len - 2) * BLAKE3_OUT_LEN];
     output_t output = parent_output(parent_node, self->key, self->chunk.flags);
     output_chaining_value(&output, parent_node);
     self->cv_stack_len -= 1;
   }
 }
 
 // In reference_impl.rs, we merge the new CV with existing CVs from the stack
 // before pushing it. We can do that because we know more input is coming, so
 // we know none of the merges are root.
 //
 // This setting is different. We want to feed as much input as possible to
 // compress_subtree_wide(), without setting aside anything for the chunk_state.
 // If the user gives us 64 KiB, we want to parallelize over all 64 KiB at once
 // as a single subtree, if at all possible.
 //
 // This leads to two problems:
 // 1) This 64 KiB input might be the only call that ever gets made to update.
 //    In this case, the root node of the 64 KiB subtree would be the root node
 //    of the whole tree, and it would need to be ROOT finalized. We can't
 //    compress it until we know.
 // 2) This 64 KiB input might complete a larger tree, whose root node is
 //    similarly going to be the the root of the whole tree. For example, maybe
 //    we have 196 KiB (that is, 128 + 64) hashed so far. We can't compress the
 //    node at the root of the 256 KiB subtree until we know how to finalize it.
 //
 // The second problem is solved with "lazy merging". That is, when we're about
 // to add a CV to the stack, we don't merge it with anything first, as the
 // reference impl does. Instead we do merges using the *previous* CV that was
 // added, which is sitting on top of the stack, and we put the new CV
 // (unmerged) on top of the stack afterwards. This guarantees that we never
 // merge the root node until finalize().
 //
 // Solving the first problem requires an additional tool,
 // compress_subtree_to_parent_node(). That function always returns the top
 // *two* chaining values of the subtree it's compressing. We then do lazy
 // merging with each of them separately, so that the second CV will always
 // remain unmerged. (That also helps us support extendable output when we're
 // hashing an input all-at-once.)
 INLINE void hasher_push_cv(blake3_hasher *self, uint8_t new_cv[BLAKE3_OUT_LEN],
                            uint64_t chunk_counter) {
   hasher_merge_cv_stack(self, chunk_counter);
   memcpy(&self->cv_stack[self->cv_stack_len * BLAKE3_OUT_LEN], new_cv,
          BLAKE3_OUT_LEN);
   self->cv_stack_len += 1;
 }
 
 void blake3_hasher_update(blake3_hasher *self, const void *input,
                           size_t input_len) {
   // Explicitly checking for zero avoids causing UB by passing a null pointer
   // to memcpy. This comes up in practice with things like:
   //   std::vector<uint8_t> v;
   //   blake3_hasher_update(&hasher, v.data(), v.size());
   if (input_len == 0) {
     return;
   }
 
   const uint8_t *input_bytes = (const uint8_t *)input;
 
   // If we have some partial chunk bytes in the internal chunk_state, we need
   // to finish that chunk first.
   if (chunk_state_len(&self->chunk) > 0) {
     size_t take = BLAKE3_CHUNK_LEN - chunk_state_len(&self->chunk);
     if (take > input_len) {
       take = input_len;
     }
     chunk_state_update(&self->chunk, input_bytes, take);
     input_bytes += take;
     input_len -= take;
     // If we've filled the current chunk and there's more coming, finalize this
     // chunk and proceed. In this case we know it's not the root.
     if (input_len > 0) {
       output_t output = chunk_state_output(&self->chunk);
       uint8_t chunk_cv[32];
       output_chaining_value(&output, chunk_cv);
       hasher_push_cv(self, chunk_cv, self->chunk.chunk_counter);
       chunk_state_reset(&self->chunk, self->key, self->chunk.chunk_counter + 1);
     } else {
       return;
     }
   }
 
   // Now the chunk_state is clear, and we have more input. If there's more than
   // a single chunk (so, definitely not the root chunk), hash the largest whole
   // subtree we can, with the full benefits of SIMD (and maybe in the future,
   // multi-threading) parallelism. Two restrictions:
   // - The subtree has to be a power-of-2 number of chunks. Only subtrees along
   //   the right edge can be incomplete, and we don't know where the right edge
   //   is going to be until we get to finalize().
   // - The subtree must evenly divide the total number of chunks up until this
   //   point (if total is not 0). If the current incomplete subtree is only
   //   waiting for 1 more chunk, we can't hash a subtree of 4 chunks. We have
   //   to complete the current subtree first.
   // Because we might need to break up the input to form powers of 2, or to
   // evenly divide what we already have, this part runs in a loop.
   while (input_len > BLAKE3_CHUNK_LEN) {
     size_t subtree_len = round_down_to_power_of_2(input_len);
     uint64_t count_so_far = self->chunk.chunk_counter * BLAKE3_CHUNK_LEN;
     // Shrink the subtree_len until it evenly divides the count so far. We know
     // that subtree_len itself is a power of 2, so we can use a bitmasking
     // trick instead of an actual remainder operation. (Note that if the caller
     // consistently passes power-of-2 inputs of the same size, as is hopefully
     // typical, this loop condition will always fail, and subtree_len will
     // always be the full length of the input.)
     //
     // An aside: We don't have to shrink subtree_len quite this much. For
     // example, if count_so_far is 1, we could pass 2 chunks to
     // compress_subtree_to_parent_node. Since we'll get 2 CVs back, we'll still
     // get the right answer in the end, and we might get to use 2-way SIMD
     // parallelism. The problem with this optimization, is that it gets us
     // stuck always hashing 2 chunks. The total number of chunks will remain
     // odd, and we'll never graduate to higher degrees of parallelism. See
     // https://github.com/BLAKE3-team/BLAKE3/issues/69.
     while ((((uint64_t)(subtree_len - 1)) & count_so_far) != 0) {
       subtree_len /= 2;
     }
     // The shrunken subtree_len might now be 1 chunk long. If so, hash that one
     // chunk by itself. Otherwise, compress the subtree into a pair of CVs.
     uint64_t subtree_chunks = subtree_len / BLAKE3_CHUNK_LEN;
     if (subtree_len <= BLAKE3_CHUNK_LEN) {
       blake3_chunk_state chunk_state;
       chunk_state_init(&chunk_state, self->key, self->chunk.flags);
       chunk_state.chunk_counter = self->chunk.chunk_counter;
       chunk_state_update(&chunk_state, input_bytes, subtree_len);
       output_t output = chunk_state_output(&chunk_state);
       uint8_t cv[BLAKE3_OUT_LEN];
       output_chaining_value(&output, cv);
       hasher_push_cv(self, cv, chunk_state.chunk_counter);
     } else {
       // This is the high-performance happy path, though getting here depends
       // on the caller giving us a long enough input.
       uint8_t cv_pair[2 * BLAKE3_OUT_LEN];
       compress_subtree_to_parent_node(input_bytes, subtree_len, self->key,
                                       self->chunk.chunk_counter,
                                       self->chunk.flags, cv_pair);
       hasher_push_cv(self, cv_pair, self->chunk.chunk_counter);
       hasher_push_cv(self, &cv_pair[BLAKE3_OUT_LEN],
                      self->chunk.chunk_counter + (subtree_chunks / 2));
     }
     self->chunk.chunk_counter += subtree_chunks;
     input_bytes += subtree_len;
     input_len -= subtree_len;
   }
 
   // If there's any remaining input less than a full chunk, add it to the chunk
   // state. In that case, also do a final merge loop to make sure the subtree
   // stack doesn't contain any unmerged pairs. The remaining input means we
   // know these merges are non-root. This merge loop isn't strictly necessary
   // here, because hasher_push_chunk_cv already does its own merge loop, but it
   // simplifies blake3_hasher_finalize below.
   if (input_len > 0) {
     chunk_state_update(&self->chunk, input_bytes, input_len);
     hasher_merge_cv_stack(self, self->chunk.chunk_counter);
   }
 }
 
 void blake3_hasher_finalize(const blake3_hasher *self, uint8_t *out,
                             size_t out_len) {
   blake3_hasher_finalize_seek(self, 0, out, out_len);
 }
 
 void blake3_hasher_finalize_seek(const blake3_hasher *self, uint64_t seek,
                                  uint8_t *out, size_t out_len) {
   // Explicitly checking for zero avoids causing UB by passing a null pointer
   // to memcpy. This comes up in practice with things like:
   //   std::vector<uint8_t> v;
   //   blake3_hasher_finalize(&hasher, v.data(), v.size());
   if (out_len == 0) {
     return;
   }
 
   // If the subtree stack is empty, then the current chunk is the root.
   if (self->cv_stack_len == 0) {
     output_t output = chunk_state_output(&self->chunk);
     output_root_bytes(&output, seek, out, out_len);
     return;
   }
   // If there are any bytes in the chunk state, finalize that chunk and do a
   // roll-up merge between that chunk hash and every subtree in the stack. In
   // this case, the extra merge loop at the end of blake3_hasher_update
   // guarantees that none of the subtrees in the stack need to be merged with
   // each other first. Otherwise, if there are no bytes in the chunk state,
   // then the top of the stack is a chunk hash, and we start the merge from
   // that.
   output_t output;
   size_t cvs_remaining;
   if (chunk_state_len(&self->chunk) > 0) {
     cvs_remaining = self->cv_stack_len;
     output = chunk_state_output(&self->chunk);
   } else {
     // There are always at least 2 CVs in the stack in this case.
     cvs_remaining = self->cv_stack_len - 2;
     output = parent_output(&self->cv_stack[cvs_remaining * 32], self->key,
                            self->chunk.flags);
   }
   while (cvs_remaining > 0) {
     cvs_remaining -= 1;
     uint8_t parent_block[BLAKE3_BLOCK_LEN];
     memcpy(parent_block, &self->cv_stack[cvs_remaining * 32], 32);
     output_chaining_value(&output, &parent_block[32]);
     output = parent_output(parent_block, self->key, self->chunk.flags);
   }
   output_root_bytes(&output, seek, out, out_len);
 }