spec : add ngram-mod (#19164)

* spec : add ngram-mod

* cont : simplify + keep track of occupancy

* cont : cleanup

* cont : move initialization to common/speculative

* cont : cleanup

* cont : cleanup

* cont : fix

common/ngram-mod.h

@@ -0,0 +1,37 @@
+#pragma once
+
+#include <cstdint>
+#include <vector>
+
+//
+// common_ngram_mod
+// ref: https://github.com/ggml-org/llama.cpp/pull/19164
+//
+
+// basic n-gram hasher
+struct common_ngram_mod {
+    using entry_t = int32_t;
+
+    static constexpr entry_t EMPTY = -1;
+
+    common_ngram_mod(uint16_t n, size_t size);
+
+    size_t  idx(const entry_t * tokens) const;
+    void    add(const entry_t * tokens);
+    entry_t get(const entry_t * tokens) const; // return -1 if not found
+
+    void reset();
+
+    size_t get_n()    const;
+    size_t get_used() const;
+
+    size_t size()       const;
+    size_t size_bytes() const;
+
+private:
+    size_t n; // ngram size to hash
+
+    size_t used;
+
+    std::vector<entry_t> entries;
+};