refactor: make flat_example an implementation detail of ksvm (#4505)

* refactor!: make flat_example an implementation detail of ksvm

* Update memory_tree.cc

* Absorb flat_example into svm_example

* revert "Absorb flat_example into svm_example"

This reverts commit b063feb.

vowpalwabbit/core/include/vw/core/example.h

@@ -143,26 +143,8 @@ class example : public example_predict  // core example datatype.
 
 class workspace;
 
-class flat_example
-{
-public:
-  polylabel l;
-  reduction_features ex_reduction_features;
-
-  VW::v_array<char> tag;  // An identifier for the example.
-
-  size_t example_counter;
-  uint64_t ft_offset;
-  float global_weight;
-
-  size_t num_features;      // precomputed, cause it's fast&easy.
-  float total_sum_feat_sq;  // precomputed, cause it's kind of fast & easy.
-  features fs;              // all the features
-};
-
-flat_example* flatten_example(VW::workspace& all, example* ec);
-flat_example* flatten_sort_example(VW::workspace& all, example* ec);
-void free_flatten_example(flat_example* fec);
+// TODO: make workspace and example const
+void flatten_features(VW::workspace& all, example& ec, features& fs);
 
 inline bool example_is_newline(const example& ec) { return ec.is_newline; }
 
@@ -194,13 +176,6 @@ void truncate_example_namespace(VW::example& ec, VW::namespace_index ns, const f
 void append_example_namespaces_from_example(VW::example& target, const VW::example& source);
 void truncate_example_namespaces_from_example(VW::example& target, const VW::example& source);
 }  // namespace details
-
-namespace model_utils
-{
-size_t read_model_field(io_buf& io, flat_example& fe, VW::label_parser& lbl_parser);
-size_t write_model_field(io_buf& io, const flat_example& fe, const std::string& upstream_name, bool text,
-    VW::label_parser& lbl_parser, uint64_t parse_mask);
-}  // namespace model_utils
 }  // namespace VW
 
 // Deprecated compat definitions
@@ -209,18 +184,6 @@ using polylabel VW_DEPRECATED("polylabel moved into VW namespace") = VW::polylab
 using polyprediction VW_DEPRECATED("polyprediction moved into VW namespace") = VW::polyprediction;
 using example VW_DEPRECATED("example moved into VW namespace") = VW::example;
 using multi_ex VW_DEPRECATED("multi_ex moved into VW namespace") = VW::multi_ex;
-using flat_example VW_DEPRECATED("flat_example moved into VW namespace") = VW::flat_example;
-
-VW_DEPRECATED("flatten_example moved into VW namespace")
-inline VW::flat_example* flatten_example(VW::workspace& all, VW::example* ec) { return VW::flatten_example(all, ec); }
-
-VW_DEPRECATED("flatten_sort_example moved into VW namespace")
-inline VW::flat_example* flatten_sort_example(VW::workspace& all, VW::example* ec)
-{
-  return VW::flatten_sort_example(all, ec);
-}
-VW_DEPRECATED("free_flatten_example moved into VW namespace")
-inline void free_flatten_example(VW::flat_example* fec) { return VW::free_flatten_example(fec); }
 
 VW_DEPRECATED("example_is_newline moved into VW namespace")
 inline bool example_is_newline(const VW::example& ec) { return VW::example_is_newline(ec); }

vowpalwabbit/core/include/vw/core/feature_group.h

@@ -510,6 +510,10 @@ class features
     return all_extents_complete;
   }
 };
+
+/// Both fs1 and fs2 must be sorted.
+/// Most often used with VW::flatten_features
+float features_dot_product(const features& fs1, const features& fs2);
 }  // namespace VW
 
 using feature_value VW_DEPRECATED("Moved into VW namespace. Will be removed in VW 10.") = VW::feature_value;

vowpalwabbit/core/src/example.cc

@@ -48,6 +48,9 @@ float VW::example::get_total_sum_feat_sq()
 
 float collision_cleanup(VW::features& fs)
 {
+  // Input must be sorted.
+  assert(std::is_sorted(fs.indices.begin(), fs.indices.end()));
+
   // This loops over the sequence of feature values and their indexes
   // when an index is repeated this combines them by adding their values.
   // This assumes that fs is sorted (which is the case in `flatten_sort_example`).
@@ -105,46 +108,23 @@ void vec_ffs_store(full_features_and_source& p, float fx, uint64_t fi)
 }
 namespace VW
 {
-flat_example* flatten_example(VW::workspace& all, example* ec)
-{
-  flat_example& fec = VW::details::calloc_or_throw<flat_example>();
-  fec.l = ec->l;
-  fec.tag = ec->tag;
-  fec.ex_reduction_features = ec->ex_reduction_features;
-  fec.example_counter = ec->example_counter;
-  fec.ft_offset = ec->ft_offset;
-  fec.num_features = ec->num_features;
 
+void flatten_features(VW::workspace& all, example& ec, features& fs)
+{
+  fs.clear();
   full_features_and_source ffs;
+  ffs.fs = std::move(fs);
   ffs.stride_shift = all.weights.stride_shift();
   if (all.weights.not_null())
-  {  // TODO:temporary fix. all.weights is not initialized at this point in some cases.
+  {
+    // TODO:temporary fix. all.weights is not initialized at this point in some cases.
     ffs.mask = all.weights.mask() >> all.weights.stride_shift();
   }
   else { ffs.mask = static_cast<uint64_t>(LONG_MAX) >> all.weights.stride_shift(); }
-  VW::foreach_feature<full_features_and_source, uint64_t, vec_ffs_store>(all, *ec, ffs);
-
-  std::swap(fec.fs, ffs.fs);
-
-  return &fec;
-}
-
-flat_example* flatten_sort_example(VW::workspace& all, example* ec)
-{
-  flat_example* fec = flatten_example(all, ec);
-  fec->fs.sort(all.parse_mask);
-  fec->total_sum_feat_sq = collision_cleanup(fec->fs);
-  return fec;
-}
-
-void free_flatten_example(flat_example* fec)
-{
-  // note: The label memory should be freed by by freeing the original example.
-  if (fec)
-  {
-    fec->fs.~features();
-    free(fec);
-  }
+  VW::foreach_feature<full_features_and_source, uint64_t, vec_ffs_store>(all, ec, ffs);
+  ffs.fs.sort(all.parse_mask);
+  ffs.fs.sum_feat_sq = collision_cleanup(ffs.fs);
+  fs = std::move(ffs.fs);
 }
 
 void return_multiple_example(VW::workspace& all, VW::multi_ex& examples)
@@ -213,42 +193,6 @@ void truncate_example_namespaces_from_example(VW::example& target, const VW::exa
   }
 }
 }  // namespace details
-
-namespace model_utils
-{
-size_t read_model_field(io_buf& io, flat_example& fe, VW::label_parser& lbl_parser)
-{
-  size_t bytes = 0;
-  lbl_parser.default_label(fe.l);
-  bytes += lbl_parser.read_cached_label(fe.l, fe.ex_reduction_features, io);
-  bytes += read_model_field(io, fe.tag);
-  bytes += read_model_field(io, fe.example_counter);
-  bytes += read_model_field(io, fe.ft_offset);
-  bytes += read_model_field(io, fe.global_weight);
-  bytes += read_model_field(io, fe.num_features);
-  bytes += read_model_field(io, fe.total_sum_feat_sq);
-  unsigned char index = 0;
-  bytes += ::VW::parsers::cache::details::read_cached_index(io, index);
-  bool sorted = true;
-  bytes += ::VW::parsers::cache::details::read_cached_features(io, fe.fs, sorted);
-  return bytes;
-}
-size_t write_model_field(io_buf& io, const flat_example& fe, const std::string& upstream_name, bool text,
-    VW::label_parser& lbl_parser, uint64_t parse_mask)
-{
-  size_t bytes = 0;
-  lbl_parser.cache_label(fe.l, fe.ex_reduction_features, io, upstream_name + "_label", text);
-  bytes += write_model_field(io, fe.tag, upstream_name + "_tag", text);
-  bytes += write_model_field(io, fe.example_counter, upstream_name + "_example_counter", text);
-  bytes += write_model_field(io, fe.ft_offset, upstream_name + "_ft_offset", text);
-  bytes += write_model_field(io, fe.global_weight, upstream_name + "_global_weight", text);
-  bytes += write_model_field(io, fe.num_features, upstream_name + "_num_features", text);
-  bytes += write_model_field(io, fe.total_sum_feat_sq, upstream_name + "_total_sum_feat_sq", text);
-  ::VW::parsers::cache::details::cache_index(io, 0);
-  ::VW::parsers::cache::details::cache_features(io, fe.fs, parse_mask);
-  return bytes;
-}
-}  // namespace model_utils
 }  // namespace VW
 
 namespace VW

vowpalwabbit/core/src/feature_group.cc

@@ -297,3 +297,28 @@ void VW::features::end_ns_extent()
     }
   }
 }
+
+float VW::features_dot_product(const features& fs1, const features& fs2)
+{
+  assert(std::is_sorted(fs1.indices.begin(), fs1.indices.end()));
+  assert(std::is_sorted(fs2.indices.begin(), fs2.indices.end()));
+
+  float dotprod = 0;
+  if (fs2.indices.empty()) { return 0.f; }
+
+  for (size_t idx1 = 0, idx2 = 0; idx1 < fs1.size() && idx2 < fs2.size(); idx1++)
+  {
+    uint64_t ec1pos = fs1.indices[idx1];
+    uint64_t ec2pos = fs2.indices[idx2];
+    if (ec1pos < ec2pos) { continue; }
+
+    while (ec1pos > ec2pos && ++idx2 < fs2.size()) { ec2pos = fs2.indices[idx2]; }
+
+    if (ec1pos == ec2pos)
+    {
+      dotprod += fs1.values[idx1] * fs2.values[idx2];
+      ++idx2;
+    }
+  }
+  return dotprod;
+}

vowpalwabbit/core/src/reductions/eigen_memory_tree.cc

@@ -11,6 +11,7 @@
 #include "vw/config/options.h"
 #include "vw/core/array_parameters.h"
 #include "vw/core/example.h"
+#include "vw/core/feature_group.h"
 #include "vw/core/learner.h"
 #include "vw/core/memory.h"
 #include "vw/core/model_utils.h"
@@ -75,14 +76,14 @@ emt_example::emt_example(VW::workspace& all, VW::example* ex)
   std::vector<std::vector<VW::namespace_index>> base_interactions;
 
   ex->interactions = &base_interactions;
-  auto* ex1 = VW::flatten_sort_example(all, ex);
-  for (auto& f : ex1->fs) { base.emplace_back(f.index(), f.value()); }
-  VW::free_flatten_example(ex1);
+  VW::features fs;
+  VW::flatten_features(all, *ex, fs);
+  for (auto& f : fs) { base.emplace_back(f.index(), f.value()); }
 
+  fs.clear();
   ex->interactions = full_interactions;
-  auto* ex2 = VW::flatten_sort_example(all, ex);
-  for (auto& f : ex2->fs) { full.emplace_back(f.index(), f.value()); }
-  VW::free_flatten_example(ex2);
+  VW::flatten_features(all, *ex, fs);
+  for (auto& f : fs) { full.emplace_back(f.index(), f.value()); }
 }
 
 emt_lru::emt_lru(uint64_t max_size) : max_size(max_size) {}