allow loading grammar from file

examples/common.cpp

@@ -394,6 +394,22 @@ bool gpt_params_parse(int argc, char ** argv, gpt_params & params) {
                 break;
             }
             params.grammar = argv[i];
+        } else if (arg == "--grammar-file") {
+            if (++i >= argc) {
+                invalid_param = true;
+                break;
+            }
+            std::ifstream file(argv[i]);
+            if (!file) {
+                fprintf(stderr, "error: failed to open file '%s'\n", argv[i]);
+                invalid_param = true;
+                break;
+            }
+            std::copy(
+                std::istreambuf_iterator<char>(file),
+                std::istreambuf_iterator<char>(),
+                std::back_inserter(params.grammar)
+            );
         } else {
             fprintf(stderr, "error: unknown argument: %s\n", arg.c_str());
             gpt_print_usage(argc, argv, default_params);
@@ -465,6 +481,7 @@ void gpt_print_usage(int /*argc*/, char ** argv, const gpt_params & params) {
     fprintf(stderr, "                        i.e. `--logit-bias 15043+1` to increase likelihood of token ' Hello',\n");
     fprintf(stderr, "                        or `--logit-bias 15043-1` to decrease likelihood of token ' Hello'\n");
     fprintf(stderr, "  --grammar GRAMMAR     BNF-like grammar (TODO explain) to constrain generations\n");
+    fprintf(stderr, "  --grammar-file FNAME  file to read grammar from\n");
     fprintf(stderr, "  -c N, --ctx-size N    size of the prompt context (default: %d)\n", params.n_ctx);
     fprintf(stderr, "  --ignore-eos          ignore end of stream token and continue generating (implies --logit-bias 2-inf)\n");
     fprintf(stderr, "  --no-penalize-nl      do not penalize newline token\n");

grammars/arithmetic.gbnf

@@ -0,0 +1,6 @@
+root  ::= (expr "=" ws term "\n")+
+expr  ::= term ([-+*/] term)*
+term  ::= ident | num | "(" ws expr ")" ws
+ident ::= [a-z] [a-z0-9_]* ws
+num   ::= [0-9]+ ws
+ws    ::= [ \t\n]*

grammars/chess.gbnf

@@ -0,0 +1,5 @@
+root    ::= "1. " move " " move "\n" ([1-9] [0-9]? ". " move " " move "\n")+
+move    ::= (pawn | nonpawn | castle) [+#]?
+nonpawn ::= [NBKQR] [a-h]? [1-8]? "x"? [a-h] [1-8]
+pawn    ::= ([a-h] "x")? [a-h] [1-8] ("=" [NBKQR])?
+castle  ::= "O-O" "-O"?

grammars/json.gbnf

@@ -0,0 +1,8 @@
+root ::= object | array
+value ::= object | array | string | number | boolean
+object ::= "{" ws (string ":" ws value ("," ws string ":" ws value)*)? "}"
+array ::= "[" ws (value ("," ws value)*)? "]"
+string ::= "\"" [ \t!#-\[\]-~]* "\"" ws
+number ::= [0-9]+ ws
+boolean ::= ("true" | "false") ws
+ws ::= [ \t\t] ws |