Update layer index parsing and allow tokenizer override

PiperOrigin-RevId: 788797948

gemma/model_store.cc

@@ -20,6 +20,7 @@
 #include <stdio.h>
 
 #include <array>
+#include <charconv>
 #include <cstdlib>
 #include <cstring>  // strcmp
 #include <string>
@@ -71,8 +72,15 @@ static std::string ReadTokenizer(BlobReader& reader,
     }
   }
 
+  // Read actual tokenizer from blob.
   if (!tokenizer.empty() && tokenizer != kMockTokenizer) {
-    return tokenizer;  // Read actual tokenizer from blob.
+    if (!tokenizer_path.Empty()) {
+      HWY_WARN("--weights has tokenizer but overriding with %s.",
+               tokenizer_path.path.c_str());
+      return ReadFileToString(tokenizer_path);
+    }
+
+    return tokenizer;
   }
 
   // No blob but user specified path to file: read it or abort.
@@ -163,22 +171,42 @@ class TypePrefix {
   std::array<size_t, kNumTypes> blobs_{0};
 };
 
+// Returns 0 if the blob does not seem to be a per-layer tensor, otherwise the
+// layer index.
+static size_t LayerIdxFromKey(const std::string& key) {
+  const auto parse_num = [&key](size_t begin, size_t end) -> size_t {
+    HWY_DASSERT(begin <= end);
+    HWY_DASSERT(end <= key.size());
+    size_t val = 0;
+    (void)std::from_chars(key.data() + begin, key.data() + end, val);
+    return val;
+  };
+
+  const size_t suffix_pos = key.rfind('_');
+  // If there is no digit after the last underscore, it is not a layer name.
+  if (suffix_pos == std::string::npos) return 0;
+  if (suffix_pos == key.size() - 1) return 0;
+
+  size_t layer_idx = parse_num(suffix_pos + 1, key.size());
+
+  HWY_ASSERT(layer_idx < 999);
+  return layer_idx;
+}
+
 // Returns the number of layers based on the largest blob name suffix seen.
 // This works with or without type prefixes because it searches for suffixes.
 static size_t DeduceNumLayers(const KeyVec& keys) {
+  // Built-in self-test.
+  {
+    HWY_ASSERT(LayerIdxFromKey("gr_conv_w_2") == 2);   // common case
+    HWY_ASSERT(LayerIdxFromKey("prefix_") == 0);       // no number
+    HWY_ASSERT(LayerIdxFromKey("c_embedding") == 0);   // per-model
+    HWY_ASSERT(LayerIdxFromKey("c_final_norm") == 0);  // per-model, two _
+  }
+
   size_t max_layer_idx = 0;
   for (const std::string& key : keys) {
-    const size_t suffix_pos = key.rfind('_');
-    if (suffix_pos == std::string::npos) continue;
-
-    char* end;
-    auto layer_idx = strtoul(key.c_str() + suffix_pos + 1, &end, 10);  // NOLINT
-    HWY_ASSERT(layer_idx < 999);  // Also checks for `ULONG_MAX` if out of range
-    // Ignore if not a suffix. Some names are prefixed with "c_" for historical
-    // reasons. In such cases, parsing layer_idx anyway returns 0.
-    if (end - key.c_str() != key.size()) continue;
-
-    max_layer_idx = HWY_MAX(max_layer_idx, layer_idx);
+    max_layer_idx = HWY_MAX(max_layer_idx, LayerIdxFromKey(key));
   }
   return max_layer_idx + 1;
 }